Engine Controls - Theory & Operation Chrysler Town & Country IV

INTRODUCTION

This article covers basic description and operation of engine performance-related systems and components. Read this article before diagnosing vehicles or systems with which you are not completely familiar.

Note. References to California models apply to California emission vehicles, which may be verified by underhood Emission Control label. California emissions may be available in other states.

POWERTRAIN CONTROL MODULE

The Powertrain Control Module (PCM) is located on left side of engine compartment, near front of battery. (Scheme 1) The PCM is a digital computer containing a microprocessor. The PCM receives input signals from various switches and sensors referred to as PCM inputs. Based on these inputs, the PCM adjusts various engine and vehicle operations through devices referred to as PCM outputs. See INPUT DEVICES and OUTPUT SIGNALS.

Based on inputs the PCM receives, the PCM adjusts fuel injector pulse width, idle speed, ignition timing and canister purge operation. The PCM regulates the cooling fans, A/C and speed control systems. The PCM changes generator charge rate by adjusting the generator field. The PCM adjusts injector pulse width (air/fuel ratio) based on the following inputs.

1. Battery Voltage
2. Intake Air Temperature Sensor
3. Engine Coolant Temperature
4. Engine Speed (Crankshaft Position Sensor)
5. Exhaust Gas Oxygen Content (Heated Oxygen Sensors)
6. Manifold Absolute Pressure
7. Throttle Position

The PCM adjusts engine idle speed through the idle air control motor based on the following inputs.

1. Brake Switch
2. Engine Coolant Temperature
3. Engine Speed (Crankshaft Position Sensor)
4. Park/Neutral (Transmission Gear Selection)
5. Transaxle Gear Engagement
6. Throttle Position
7. Vehicle Speed (From Transmission Control Module)

The PCM adjusts ignition timing based on the following inputs.

1. Intake Air Temperature
2. Engine Coolant Temperature
3. Engine Speed (Crankshaft Position Sensor)
4. Knock Sensor
5. Manifold Absolute Pressure
6. Park/Neutral (Transmission Gear Selection)
7. Transaxle Gear Engagement
8. Throttle Position

The Automatic Shutdown (ASD) and fuel pump relays are mounted externally, but are turned on and off by the PCM through the same circuit. The camshaft and crankshaft signals are sent to the PCM. If the PCM does not receive both signals within approximately one second of engine cranking, it deactivates the ASD and fuel pump relays. When these relays are deactivated, power is shutoff to the fuel injectors, ignition coils, fuel pump and the heating element in each oxygen sensor. The PCM contains a voltage converter that changes battery voltage to a regulated 8.0 volts. The 8.0 volts powers the camshaft position sensor, crankshaft position sensor and vehicle speed sensor. The PCM also provides a 5.0-volt supply for the engine coolant temperature sensor, intake air temperature sensor, manifold absolute pressure sensor and throttle position sensor.

The PCM contains a self-diagnostic system which stores a Diagnostic Trouble Code (DTC) if an electronic control system failure exists. DTC may be retrieved from PCM for system diagnosis by using a scan tool. See SELF-DIAGNOSTIC SYSTEM .

Scheme 1

Note. Components are grouped into 2 categories. The first category covers INPUT DEVICES , which control or produce voltage signals monitored by the PCM. The second category covers OUTPUT SIGNALS , which are components controlled by the PCM.

INPUT DEVICES

Vehicles are equipped with different combinations of input devices. Not all devices are used on all models. To determine the input device usage on a specific model, see WIRING DIAGRAMS article. The available input signals include the following

A/C Pressure Transducer

A/C pressure transducer may also be referred to as A/C pressure sensor. The A/C pressure transducer is a switch that is installed on a fitting located on the refrigerant liquid line between the filter-drier and the expansion valve in the right rear corner of the engine compartment. (Scheme 2) An internally threaded hex fitting on the transducer connects it to the externally threaded Schrader-type fitting on the liquid line. A rubber "O" ring seals the connection between the transducer and the liquid line fitting. Three terminals within a molded plastic connector receptacle on the top of the transducer connects it to the vehicle electrical system through a take out and connector of the headlight and dash wiring harness. The A/C pressure transducer cannot be adjusted or repaired. If faulty or damaged, the transducer must be replaced.

The A/C pressure transducer monitors the pressures in the high side of the refrigerant system. The transducer will change its internal resistance in response to the pressures it monitors. The PCM provides a 5.0-volt reference signal and a sensor ground to the transducer, then monitors the output voltage of the transducer on a sensor return circuit to determine refrigerant pressure. The PCM is programmed to respond to this and other sensor inputs by controlling the operation of the A/C compressor clutch and the radiator cooling fan to help optimize A/C system performance, and to protect the system components from damage. The A/C pressure transducer input to the PCM will also prevent the A/C compressor clutch from engaging when ambient temperatures are below 50°F (10°C) due to the pressure/temperature relationship of the refrigerant. The Schrader-type valve in the liquid line fitting permits the A/C pressure transducer to be removed or installed without disturbing the refrigerant in the system.

Scheme 2

Auto Shutdown Relay

See AUTO SHUTDOWN RELAY under RELAYS under MISCELLANEOUS CONTROLS.

Scheme 3

Battery Voltage

Powertrain Control Module (PCM) monitors battery voltage to determine fuel injector pulse width and generator field control. If battery voltage is not within specified range, PCM will increase fuel injector pulse width.

Brake Switch

Note. The brake switch may also be referred to as brakelight switch.

The brake switch is located under the instrument panel, at the brake pedal arm. (Scheme 4) The brake switch has 3 internal switches controlling various functions of the vehicle. Its main function is to control operation of the vehicle's brake lights. Other functions include speed control deactivation, brake sense for the anti-lock brake system and brake sense for the brake transmission shift interlock. The switch can only be adjusted once. That is during the initial installation of the switch. If the switch is not adjusted properly or has been removed for some service, a new switch must be installed and adjusted.

Scheme 4

Camshaft Position Sensor (2.4L)

Camshaft Position (CMP) sensor is located on rear of cylinder head, just below valve cover. (Scheme 5) CMP sensor uses a target magnet attached to rear of camshaft and uses locating dowels for proper positioning. (Scheme 6)

As target magnet rotates, CMP sensor senses the change in the polarity. The polarity change causes CMP sensor output voltage to switch from high-voltage of 5 volts (at magnet north pole) to a low-voltage of .3 volt (at magnet south pole). CMP sensor delivers input signal to Powertrain Control Module (PCM). PCM uses this input signal along with input signal from Crankshaft Position (CKP) sensor for determining crankshaft position for fuel injection synchronization and cylinder identification.

Scheme 5

Scheme 6

Camshaft Position Sensor (3.3L & 3.8L)

Camshaft Position (CMP) sensor is located on top of timing chain cover, just below thermostat housing. (Scheme 7) End of CMP sensor is located directly above camshaft sprocket. A notched ring is mounted on front of camshaft sprocket.

As camshaft rotates, CMP sensor generates pulses due to notch areas on the notched ring. When solid metal area on notched ring passes below CMP sensor, voltage decreases to less than .5 volt. When notch on notched ring aligns with bottom of CMP sensor, voltage increases to 5 volts. Switching of the voltage generates pulses. These pulses provide an input signal to Powertrain Control Module (PCM). PCM uses input signal along with Crankshaft Position (CKP) sensor input signal for determining fuel injection synchronization and cylinder identification.

Scheme 7

Crankshaft Position Sensor (2.4L)

Crankshaft Position (CKP) sensor is located on cylinder block, behind generator, near oil filter. (Scheme 8) Crankshaft contains a crankshaft pulse ring which contains 2 sets of notches ground into the No. 2 counterweight.

As crankshaft rotates, CKP sensor generates pulses due to notch areas on the No. 2 counterweight. When solid area on crankshaft pulse ring aligns with bottom of CKP sensor, voltage decreases to less than .3 volt.

When notch area on crankshaft ring aligns with bottom of CKP sensor, voltage increases to 5 volts. Switching of the voltage generates pulses. These pulses provide an input signal to the Powertrain Control Module (PCM). PCM uses input signal to determine crankshaft position. PCM uses input signal along with other various input signals for controlling fuel injection synchronization and ignition system.

Scheme 8

Crankshaft Position Sensor (3.3L & 3.8L)

Crankshaft Position (CKP) sensor is located on top of transaxle housing, above flywheel, near corner of rear exhaust manifold. (Scheme 9)

Flywheel contains 3 sets of slots areas near starter ring. Each slot area contains 4 slots in each set. As crankshaft rotates, CKP sensor generates pulses due to slot areas on the flywheel.

When solid area on flywheel aligns with bottom of CKP sensor, voltage decreases to less than .3 volt. When slot area on flywheel aligns with bottom of CKP sensor, voltage increases to 5 volts. Switching of the voltage generates pulses. These pulses provide an input signal to the Powertrain Control Module (PCM). PCM uses input signal to determine crankshaft position. PCM uses input signal along with other various input signals for controlling fuel injection synchronization and ignition system.

Scheme 9

Cruise Control System Switches

Note. Cruise control system may also be referred to as speed control system.

There are 2 separate switch pods that operate the cruise control system and are located on the steering wheel. The cruise control system has 5 separate resistance switches that provide a single multiplexed (MUX) voltage input to the PCM. The switch names are: On, Off, Set, Coast, Resume, Accel, Tap-up, Coast and Cancel. Based on conditions when the buttons are pushed (and released), the 5 voltage ranges provided to the PCM result in the following functions: On, Off, Set, Coast, Resume, Accel, Tap-Up, Tap-Down, Coast and Cancel. The PCM also receives an input from the brake switch to sense whether the brake pedal has been depressed. When the PCM receives the brake depressed input, it turns off power to the cruise control servo and disengages cruise control. Also the power to the servo is supplied thru the brake switch, which opens the circuit when the brake pedal is depressed. The individual switches cannot be repaired. If one switch fails, the entire switch module must be replaced.

Downstream Oxygen Sensor

Downstream oxygen sensor is a heated oxygen sensor mounted on exhaust pipe behind catalytic converter. (Scheme 10) Heater is used on downstream oxygen sensor so sensor reaches operating temperature quicker and maintains sensor at operating temperature during all operating modes. Heater on downstream oxygen sensor receives voltage from Auto Shutdown (ASD) relay. For operation of ASD relay, see RELAYS under MISCELLANEOUS CONTROLS.

Downstream oxygen sensor produces a small electrical voltage input signal to Powertrain Control Module (PCM) depending on amount of oxygen content in the exhaust gas. Downstream oxygen sensor produces a low voltage when a large amount of oxygen exists in exhaust gas (lean condition) and a higher voltage when a lesser amount of oxygen exists in exhaust gas (rich condition).

PCM uses input signal to monitor catalytic converter. As catalytic converter deteriorates, input signal from downstream oxygen sensor begins to match input signal from upstream oxygen sensor except for a slight time delay. PCM compares input signals from upstream and downstream oxygen sensors to calculate efficiency of catalytic converter. If efficiency of catalytic converter becomes less than specified value, a diagnostic trouble code will be stored in PCM.

Scheme 10

Engine Coolant Temperature Sensor (All Models)

Engine Coolant Temperature (ECT) sensor delivers input signal to Powertrain Control Module (PCM) to indicate engine coolant temperature. PCM uses input signal for controlling air/fuel ratio, ignition system, idle speed and cooling fan operation. ECT sensor is located on or near the thermostat housing. (Scheme 11)and (Scheme 12).

Scheme 11

Scheme 12

Inlet Air Temperature Sensor

Inlet Air Temperature (IAT) sensor delivers input signal to Powertrain Control Module (PCM) to indicate temperature of incoming intake air. PCM uses input signal for controlling fuel injector pulse width and spark advance. Inlet air temperature sensor is mounted to inlet air ducting. (Scheme 13)and (Scheme 14).

Scheme 13

Scheme 14

Knock Sensor

The knock sensor threads into the cylinder block. (Scheme 15)and (Scheme 16). When the knock sensor detects a knock in one of the cylinders, it sends an input signal to the PCM. In response, the PCM retards ignition timing for all cylinders by a scheduled amount. Knock sensors contain a piezoelectric material which constantly vibrates and sends an input voltage signal to the PCM while the engine operates. As the intensity of the crystal's vibration increases, the knock sensor output voltage also increases. The voltage signal produced by the knock sensor increases with the amplitude of vibration. The PCM receives the knock sensor voltage signal as an input. If the signal increases above a predetermined level, the PCM will store that value in memory and retard ignition timing to reduce engine knock. If the knock sensor voltage exceeds a preset value, the PCM retards ignition timing for all cylinders. It is not a selective cylinder retard.

The PCM ignores knock sensor input during engine idle conditions. Once the engine speed exceeds a specified value, knock retard is allowed. Knock retard uses its own short term and long term memory program. Long term memory stores previous detonation information in its battery-backed RAM. The maximum authority that long term memory has over timing retard can be calibrated. Short term memory is allowed to retard timing up to a preset amount under all operation conditions (as long as RPM is above the minimum RPM) except at wide open throttle. The PCM, using short term memory, can respond quickly to retard timing when engine knock is detected. Short term memory is lost anytime the ignition key is turned off.

Scheme 15

Scheme 16

Manifold Absolute Pressure Sensor

The Manifold Absolute Pressure (MAP) sensor is mounted on the intake manifold. (Scheme 17)and (Scheme 18). The MAP serves as a PCM input, using a silicon based sensing unit, to provide data on the manifold vacuum that draws the air/fuel mixture into the combustion chamber. The PCM requires this information to determine injector pulse width and spark advance. When MAP equals barometric pressure, the pulse width will be at maximum. A 5-volt reference is supplied to the MAP sensor from the PCM and returns a voltage signal to the PCM that reflects manifold pressure. The zero pressure reading is 0.5 volt and full scale is 4.5 volts. For a pressure swing of 0-15 psi, the voltage changes 4.0 volts. The sensor is supplied a regulated 4.8-5.1 volts to operate the sensor. Ground is provided through the sensor return circuit. The MAP sensor input is the number one contributor to pulse width. The most important function of the MAP sensor is to determine barometric pressure. The PCM needs to know if the vehicle is at sea level or is it 5000 feet above sea level, because the air density changes with altitude. It will also help to correct for varying weather conditions. If a hurricane was coming through the pressure would be very, very low or there could be a real fair weather, high pressure area. This is important because as air pressure changes the barometric pressure changes. Barometric pressure and altitude have a direct inverse correlation, as altitude goes up barometric pressure goes down. The first thing that happens as the ignition key is turned on, before reaching the crank position, the PCM powers up, and looks at the MAP voltage. Based upon the voltage it sees, it knows the current barometric pressure relative to altitude. Once the engine starts, the PCM looks at the voltage again, continuously every 12 milliseconds, and compares the current voltage to what it was at key on. The difference between current and what it was at key on is manifold vacuum.

During key On (engine not running) the sensor reads (updates) barometric pressure. A normal range can be obtained by monitoring known-good sensor in your work area. As the altitude increases the air becomes thinner (less oxygen). If a vehicle is started and driven to a very different altitude than where it was at key on, the barometric pressure needs to be updated. Anytime the PCM sees Wide Open throttle, based upon TP sensor angle and RPM, it will update barometric pressure in the MAP memory cell. With periodic updates, the PCM can make its calculations more effectively. The PCM uses the MAP sensor to aid in calculating the following

1. Barometric pressure.
2. Engine load.
3. Manifold pressure.
4. Injector pulse-width.
5. Spark-advance programs.
6. Shift-point strategies (via the PCI bus).
7. Idle speed.
8. Decel fuel shutoff.

The PCM recognizes a decrease in manifold pressure by monitoring a decrease in voltage from the reading stored in the barometric pressure memory cell. The MAP sensor is a linear sensor; as pressure changes, voltage changes proportionately. The range of voltage output from the sensor is usually between 4.5 volts at sea level to as low as 0.3 volt at 26 in. Hg. Barometric pressure is the pressure exerted by the atmosphere upon an object. At sea level on a standard day, no storm, barometric pressure is 29.92 in. Hg. For every 100 feet of altitude barometric pressure drops .10 in. Hg. If a storm goes through it can either add, high pressure, or decrease, low pressure from what should be present for that altitude. You should make a habit of knowing what the average pressure and corresponding barometric pressure is for your area.

Scheme 17

Scheme 18

Park/Neutral Switch (A/T Models)

On 3-speed models, park/neutral switch is located on side of transaxle. (Scheme 19) On 4-speed models, park/neutral switch is located on side of transaxle and may be referred to as Transmission Range (TR) sensor. (Scheme 20) On all models, park/neutral switch delivers input signal to Powertrain Control Module (PCM) to indicate transaxle gear selection. PCM uses input signal for controlling idle speed and ignition system.

Scheme 19

Scheme 20

Throttle Position Sensor

Throttle Position (TP) sensor is mounted on throttle body. The sensor connects to the throttle blade shaft. The TP sensor is a variable resistor that provides the Powertrain Control Module (PCM) with an input voltage signal. The signal represents throttle blade position. As the position of the throttle blade changes, the resistance of the TP sensor changes. The PCM supplies approximately 5 volts to the TP sensor. The TP sensor output voltage (input signal to the PCM) represents throttle blade position. The TP sensor output voltage to the PCM varies from approximately 0.6 volt at minimum throttle opening (idle) to a maximum of 4.5 volts at wide open throttle. Along with inputs from other sensors, the PCM uses the TP sensor input to determine current engine operating conditions. The PCM also adjusts fuel injector pulse width and ignition timing based on these inputs.

Transmission Control Module (4-Speed A/T)

See TRANSMISSION CONTROL MODULE (4-SPEED A/T) under TRANSMISSION under MISCELLANEOUS CONTROLS.

Transmission Input Speed Sensor (4-Speed A/T)

The input speed sensor is a two-wire magnetic pick-up device that generates an AC voltage signal as rotation occurs. It is threaded into the transaxle case and sealed with an "O" ring. (Scheme 20) The sensor is considered a primary input to the Transmission Control Module (TCM). The input speed sensor provides information on how fast the input shaft is rotating. As the teeth of the input clutch hub pass by the sensor coil, an AC voltage signal is generated and sent to the TCM. The TCM interprets this information as input shaft RPM. The TCM compares the input speed signal with the output speed signal to determine transmission gear ratio, speed ratio error detection and clutch volume index. The TCM also compares the input speed signal and the engine speed signal to determine torque converter clutch slippage and torque converter element speed ration.

Transmission Output Speed Sensor (4-Speed A/T)

The output speed sensor is a two-wire magnetic pickup device that generates an AC voltage signal as rotation occurs. It is threaded into the transaxle case and sealed with an "O" ring. (Scheme 20) The sensor is considered a primary input to the Transmission Control Module (TCM). The TCM converts this signal into a pulse per mile signal and sends it to the PCM. The PCM, in turn, sends the vehicle speed message across the communication bus to the Body Control Module (BCM). The BCM sends this signal to the instrument cluster to display vehicle speed to the driver. The vehicle speed signal pulse is roughly 8000 pulses per mile.

Transmission Range Sensor (4-Speed A/T)

Transmission Range (TR) sensor may also be referred to as park/neutral switch. See PARK/NEUTRAL SWITCH (A/T MODELS) .

Torque Converter Clutch Solenoid (3-Speed A/T)

See TORQUE CONVERTER CLUTCH SOLENOID (3-SPEED A/T) under TRANSMISSION under MISCELLANEOUS CONTROLS.

Upstream Oxygen Sensor

Upstream oxygen sensor is a heated oxygen sensor mounted on the exhaust manifold. (Scheme 21)and (Scheme 22). Heater is used on upstream oxygen sensor so sensor reaches operating temperature quicker and maintains sensor at operating temperature during all operating modes. Heater on upstream oxygen sensor receives voltage from Auto Shutdown (ASD) relay. For operation of ASD relay, see RELAYS under MISCELLANEOUS CONTROLS.

Upstream oxygen sensor produces a small electrical voltage input signal to Powertrain Control Module (PCM) depending on amount of oxygen content in the exhaust gas. Upstream oxygen sensor produces a low voltage when a large amount of oxygen exists in exhaust gas (lean condition) and a higher voltage when a lesser amount of oxygen exists in exhaust gas (rich condition). The heating element in the sensor provides heat to the sensor ceramic element. Heating the sensor allows the system to enter closed-loop operation sooner. It also allows the system to remain in closed-loop operation during periods of extended idle. In closed-loop, the PCM adjusts injector pulse width based on the upstream heated oxygen sensor input along with other inputs. In open-loop, the PCM adjusts injector pulse width based on preprogrammed (fixed) values and inputs from other sensors.

Scheme 21

Scheme 22

Vehicle Speed Sensor (3-Speed A/T)

Vehicle Seed Sensor (VSS) is mounted on extension housing for passenger's side axle shaft at firewall side of transaxle. (Scheme 23) VSS delivers an input signal to Powertrain Control Module (PCM) to indicate transaxle output shaft speed. PCM uses input signal from VSS, along with TP sensor closed throttle input signal to determine if a closed throttle deceleration or normal idle condition exists. During deceleration, PCM controls Idle Air Control (IAC) motor to maintain a desired MAP value. During idle conditions, PCM controls IAC motor to maintain a desired idle speed.

Scheme 23

OUTPUT SIGNALS

Note. Vehicles are equipped with different combinations of computer-controlled components. Not all components listed below are used on every vehicle. For theory and operation on each output component, refer to the system indicated after component.

A/C Clutch Relay

See A/C CLUTCH RELAY under RELAYS under MISCELLANEOUS CONTROLS.

See AUTO SHUTDOWN RELAY under RELAYS under MISCELLANEOUS CONTROLS.

Data Link Connector

See DATA LINK CONNECTOR under SELF-DIAGNOSTIC SYSTEM.

Distributorless Ignition System

See DISTRIBUTORLESS IGNITION SYSTEM under IGNITION SYSTEMS.

EGR Transducer Solenoid

See EXHAUST GAS RECIRCULATION SYSTEM under EMISSION SYSTEMS & SUB-SYSTEMS.

EVAP Purge Solenoid

See EVAPORATIVE EMISSIONS SYSTEM under EMISSION SYSTEMS & SUB-SYSTEMS.

Fuel Injectors

See FUEL INJECTORS under FUEL CONTROL under FUEL SYSTEMS.

Fuel Pump

See FUEL PUMP under FUEL DELIVERY under FUEL SYSTEMS.

Fuel Pump Relay

See FUEL PUMP RELAY under FUEL DELIVERY under FUEL SYSTEMS.

Generator Field

See GENERATOR under MISCELLANEOUS CONTROLS.

Idle Air Control Motor

See IDLE SPEED under FUEL SYSTEMS.

Leak Detection Pump

See LEAK DETECTION PUMP SYSTEM under EMISSION SYSTEMS & SUB-SYSTEMS.

Malfunction Indicator Light

See MALFUNCTION INDICATOR LIGHT under SELF-DIAGNOSTIC SYSTEM.

Radiator Fan Control Relay

See RADIATOR FAN CONTROL RELAY under RELAYS under MISCELLANEOUS CONTROLS.

Serial Communications Interface Transmit

See SERIAL COMMUNICATIONS INTERFACE under SELF-DIAGNOSTIC SYSTEM.

Starter Relay

See STARTER RELAY under RELAYS under MISCELLANEOUS CONTROLS.

Tachometer

See TACHOMETER under MISCELLANEOUS CONTROLS.

FUEL SYSTEMS

Note. The 3.3L is available in 2 models, low-emission model and flex-fuel model. Low-emission model may also be referred to as Low Emission Vehicle (LEV) model and is used for California emission vehicles. Flex-fuel model may also be referred to as Flex-Fuel Vehicle (FFV) model and is used in all vehicles except California emission vehicles. California emission vehicles may be verified by underhood Emission Control label and may be available in other states. Flex-fuel model may be also referred to as an E85 application and may be identified by label attached to inside of gasoline filler door. On flex-fuel models, if test procedure or specification differs from low-emission models, test procedure or specification will be listed separately. If test procedure or specification does not vary, flex-fuel model will be included with low emission model.

FUEL DELIVERY

Note. Fuel system is a returnless design. Fuel pressure regulator is mounted on fuel pump module.

Fuel pump is an in-tank pump mounted at the bottom of fuel pump module, located in top of fuel tank. Fuel pump module also contains fuel pressure regulator and fuel level sensor for fuel gauge. (Scheme 24) Fuel pump is a positive displacement, immersible pump with a permanent magnet electric motor. Fuel is drawn through inlet strainer on bottom of fuel pump module, through fuel pressure regulator, fuel filter, and delivered to fuel rail and fuel injectors. Fuel pump contains an internal check valve in the pump outlet. Check valve is used to maintain fuel pressure when engine and fuel pump are not operating.

The fuel pump has a maximum deadheaded pressure output of about 130 psi (9.14 kg/cm 2 ). The fuel pressure regulator adjusts fuel system pressure to about 53-63 psi (3.73-4.43 kg/cm 2 ). Fuel pump is operated by voltage supplied from fuel pump relay. If the fuel pump or electrical wiring harness requires service, fuel pump module must be replaced.

Scheme 24

The fuel pump relay is located in Intelligent Power Module (IPM) in engine compartment, near the battery. (Scheme 1)and (Scheme 3).

The fuel pump relay supplies battery voltage to the fuel pump. A buss bar in the IPM supplies voltage to the solenoid side and contact side of the relay. The fuel pump relay power circuit contains a fuse between the buss bar in the IPM and the relay. The fuse is located in the IPM (fuse No. 17). The PCM controls the fuel pump relay by switching the ground path for the solenoid side of the relay on and off. The PCM turns the ground path off when the ignition switch is in the Off position. When the ignition switch is in the On position, the PCM energizes the fuel pump. If the crankshaft position sensor does not detect engine rotation, the PCM de-energizes the relay after approximately one second.

Fuel Pressure Regulator

Fuel pressure regulator is a nonadjustable in-tank pressure regulator mounted on the fuel pump module on top of fuel tank. (Scheme 24) Fuel pressure regulator maintains fuel system pressure at about 53-63 psi (365-434 kPa). The fuel pressure regulator consists of a diaphragm, calibrated spring and a fuel return valve. Calibrated spring pushes downward on the diaphragm and closes off the fuel return port. System fuel pressure reflects the amount of fuel pressure required to open the fuel return port. When fuel return valve opens, excess fuel is returned to the fuel tank. The fuel pressure regulator is a mechanical device that is not controlled by the PCM or engine vacuum.

Auto Shutdown (ASD) relay provides voltage to fuel injectors. See AUTO SHUTDOWN RELAY under RELAYS under MISCELLANEOUS CONTROLS.

The fuel injectors are positioned in the intake manifold or cylinder head with the nozzle ends directly above the intake valve port. The fuel injectors are 12-volt electrical solenoids. The injector contains a pintle that closes off an orifice at the nozzle end. When electric current is supplied to the injector, the armature and needle move a short distance against a spring, allowing fuel to flow out the orifice. Because the fuel is under high pressure, a fine spray is developed in the shape of a hollow cone or two streams. The spraying action atomizes the fuel, adding it to the air entering the combustion chamber. Fuel injectors are not interchangeable between engines. The PCM provides battery voltage to each injector through the Auto Shutdown (ASD) relay. Injector operation is controlled by a ground path provided for each injector by the PCM. Injector on-time (pulse width) is variable, and is determined by the PCM processing all the data previously discussed to obtain the optimum injector pulse width for each operating condition. The pulse width is controlled by the duration of the ground path provided.

Idle Air Control (IAC) motor is mounted on throttle body. (Scheme 13)and (Scheme 25). The Powertrain Control Module (PCM) controls idle speed by use of IAC motor to compensate for engine load, coolant temperature or barometric pressure changes. The throttle body has an air by-pass passage that provides air for the engine during closed throttle idle. The IAC motor pintle protrudes into the air by-pass passage and regulates air flow through it. The PCM adjusts engine idle speed by moving the IAC motor pintle in and out of the by-pass passage. The adjustments are based on inputs the PCM receives. The inputs are from the throttle position sensor, crankshaft position sensor, coolant temperature sensor, MAP sensor, vehicle speed sensor and various switch operations (brake, park/neutral, A/C). When engine RPM is above idle speed, the IAC motor is used for off-idle dashpot, deceleration air flow control and A/C compressor load control (also opens the passage slightly before the compressor is engaged so that the engine RPM does not dip down when the compressor engages).

Scheme 25

Throttle Body

The throttle body is located on the intake manifold. Fuel does not enter the intake manifold through the throttle body. Fuel is sprayed into the manifold by the fuel injectors. Filtered air from the air cleaner enters the intake manifold through the throttle body. The throttle body contains an air control passage controlled by an Idle Air Control (IAC) motor. The air control passage is used to supply air for idle conditions. A throttle valve (plate) is used to supply air for above idle conditions. Certain sensors are attached to the throttle body. The accelerator pedal cable, speed control cable and transmission control cable (when equipped) are connected to the throttle body linkage arm. A (factory adjusted) set screw is used to mechanically limit the position of the throttle body throttle plate. Never attempt to adjust the engine idle speed using this screw. All idle speed functions are controlled by the PCM.

The distributorless ignition system used on these engines is referred to as the Direct Ignition System (DIS). The system's three main components are the coil pack, crankshaft position sensor, and the camshaft position sensor. The crankshaft position sensor and camshaft position sensor are Hall Effect devices. The camshaft position sensor and crankshaft position sensor generate pulses that are inputs to the PCM. The PCM determines engine position from these sensors. The PCM calculates injector sequence and ignition timing from crankshaft and camshaft position. For a description of both crankshaft and camshaft position sensors, see INPUT DEVICES under COMPUTERIZED ENGINE CONTROLS.

EVAPORATIVE EMISSIONS SYSTEM

The evaporation control system prevents the emission of fuel tank vapors into the atmosphere. When fuel evaporates in the fuel tank, the vapors pass through vent hoses or tubes to a charcoal filled evaporative canister. The canister temporarily holds the vapors. See EVAP CANISTER . The Powertrain Control Module (PCM) allows intake manifold vacuum to draw vapors into the combustion chambers during certain operating conditions. All engines use a proportional purge solenoid system. The PCM controls vapor flow by operating the purge solenoid. See EVAP CANISTER PURGE SOLENOID . A Leak Detection Pump (LDP) is used as part of the evaporative system for OBD-II requirements. See LEAK DETECTION PUMP SYSTEM . Vehicle is also equipped with On-Board Refueling Vapor Recovery (ORVR). See ON-BOARD REFUELING VAPOR RECOVERY . The evaporative system uses specially manufactured lines/hoses. If replacement becomes necessary, only use fuel resistant, low permeation hose.

EVAP Canister

Two maintenance free EVAP charcoal canisters are used. (Scheme 26) The vacuum and vapor tubes connect to the top of the canister. Fuel tank vapors vent into the canister. The canister temporarily holds the fuel vapors until the intake manifold vacuum draws them into the combustion chamber. The Powertrain Control Module (PCM) purges the canister through the proportional purge solenoid. The PCM purges the canister at predetermined intervals and engine conditions. Purge-free memory cells are used to identify the fuel vapor content of the EVAP canister. Since the EVAP canister is not purged 100 percent of the time, the PCM stores information about the EVAP canister's vapor content in a memory cell.

The purge-free cells are constructed similar to certain purge-normal cells. The purge-free cells can be monitored by using the DRBIII(R) scan tool. The only difference between the purge-free cells and normal adaptive cells is that in purge-free, the purge is completely turned off. This gives the PCM the ability to compare purge and purge-free operation.

Scheme 26

EVAP Canister Purge Solenoid

All vehicles use a proportional purge solenoid. (Scheme 27) The solenoid regulates the rate of vapor flow from the EVAP canister to the throttle body. The Powertrain Control Module (PCM) controls the solenoid. During the cold start warm-up period and the hot start time delay, the PCM does not energize the solenoid. When de-energized, no vapors are purged. The purge solenoid operates at a frequency of 200 Hz and is controlled by an engine controller circuit that senses the current being applied to the purge solenoid and then adjusts that current to achieve the desired purge flow. The purge solenoid controls the purge rate of fuel vapors from the vapor canister and fuel tank to the engine intake manifold.

Scheme 27

Fuel Filler Cap

The plastic fuel tank filler cap is threaded/quarter turn onto the end of the fuel filler tube. Its purpose is to retain vapors and fuel in the fuel tank. The fuel filler cap incorporates a two-way relief valve that is closed to atmosphere during normal operating conditions. The relief valve is calibrated to open when a pressure of 2.5 psi or vacuum of 0.6 in. Hg occurs in the fuel tank. When the pressure or vacuum is relieved, the valve returns to the normally closed condition. Remove filler cap before servicing any fuel system component to relieve tank pressure. If equipped with a California emissions package and a Leak Detection Pump (LDP), the cap must be tightened securely. If cap is left loose, a Diagnostic Trouble Code (DTC) may be set.

EXHAUST GAS RECIRCULATION SYSTEM

The EGR system consists of EGR tube (connects a passage in the intake manifold to the exhaust port in the cylinder head), EGR valve, electric EGR transducer and connecting hoses. (Scheme 5), (Scheme 28) and (Scheme 29). The EGR system reduces oxides of nitrogen (NOx) in engine exhaust and helps prevent detonation (engine knock). Under normal operating conditions, engine cylinder temperature can reach more than 3000°F. Formation of NOx increases proportionally with combustion temperature. To reduce the emission of these oxides, the cylinder temperature must be lowered. The system allows a predetermined amount of hot exhaust gas to recirculate and dilute the incoming air/fuel mixture. The diluted air/fuel mixture reduces peak flame temperature during combustion. The electric EGR transducer contains an electrically operated solenoid and a back-pressure transducer. (Scheme 29) The Powertrain Control Module (PCM) operates the solenoid and determines when to energize the solenoid. Exhaust system back-pressure controls the transducer.

When the PCM energizes the solenoid, vacuum does not reach the transducer. Vacuum flows to the transducer when the PCM de-energizes the solenoid. When exhaust system back-pressure becomes high enough, it fully closes a bleed valve in the transducer. When the PCM de-energizes the solenoid and back-pressure closes the transducer bleed valve, vacuum flows through the transducer to operate the EGR valve. De-energizing the solenoid, but not fully closing the transducer bleed hole (because of low back-pressure), varies the strength of vacuum applied to the EGR valve. Varying the strength of the vacuum changes the amount of EGR supplied to the engine. This provides the correct amount of exhaust gas recirculation for different operating conditions. This system does not allow EGR at idle. A failed or malfunctioning EGR system can cause engine spark knock, sags or hesitation, rough idle, engine stalling and increased emissions.

Scheme 28

Scheme 29

LEAK DETECTION PUMP SYSTEM

The Evaporative (EVAP) emission system is designed to prevent the escape of fuel vapors from the fuel system. (Scheme 30) Leaks in the system, even small ones, can allow fuel vapors to escape into the atmosphere. Government regulations require on-board testing to make sure that the EVAP system is functioning properly. The leak detection system tests for EVAP system leaks and blockage. It also performs self-diagnostics. During self-diagnostics, the Powertrain Control Module (PCM) first checks the Leak Detection Pump (LDP) for electrical and mechanical faults. The LDP is located below driver's side of steering gear, above engine cradle. (Scheme 31) If the first checks pass, the PCM then uses the LDP to seal the vent valve and pump air into the system to pressurize it. If a leak is present, the PCM will continue pumping the LDP to replace the air that leaks out. The PCM determines the size of the leak based on how fast/long it must pump the LDP as it tries to maintain pressure in the system. EVAP leak detection system components are described as follows.

1. Service Port Used with special tools like the Miller Evaporative Emissions Leak Detector (EELD) to test for leaks in the system.
2. EVAP Purge Solenoid The PCM uses the EVAP purge solenoid to control purging of excess fuel vapors stored in the EVAP canister. It remains closed during leak testing to prevent loss of pressure.
3. EVAP Canister The EVAP canister stores fuel vapors from the fuel tank for purging.
4. EVAP Purge Orifice Limits purge volume.
5. EVAP System Air Filter Provides air to the LDP for pressurizing the system. It filters out dirt while allowing a vent to atmosphere for the EVAP system.

The main purpose of the LDP is to pressurize the fuel system for leak checking. It closes the EVAP system vent to atmospheric pressure so the system can be pressurized for leak testing. The diaphragm is powered by engine vacuum. It pumps air into the EVAP system to develop a pressure of about 7.5 in. H2O (1/4 psi). A reed switch in the LDP allows the PCM to monitor the position of the LDP diaphragm. The PCM uses the reed switch input to monitor how fast the LDP is pumping air into the EVAP system. This allows detection of leaks and blockage. The LDP assembly consists of several parts. (Scheme 32) The solenoid is controlled by the PCM and it connects the upper pump cavity to either engine vacuum or atmospheric pressure. A vent valve closes the EVAP system to atmosphere, sealing the system during leak testing. The pump section of the LDP consists of a diaphragm that moves up and down to bring air in through the air filter and inlet check valve, and pump it out through an outlet check valve into the EVAP system. The diaphragm is pulled up by engine vacuum, and pushed down by spring pressure as the LDP solenoid turns on and off. The LDP also has a magnetic reed switch to signal diaphragm position to the PCM. When the diaphragm is down, the switch is closed, which sends a 12-volt (system voltage) signal to the PCM. When the diaphragm is up, the switch is open, and there is no voltage sent to the PCM. This allows the PCM to monitor LDP pumping action as it turns the LDP solenoid on and off.

Scheme 30

Scheme 31

Scheme 32

LDP At Rest (Not Powered)

When the LDP is at rest (no electrical/vacuum) the diaphragm is allowed to drop down if the internal (EVAP system) pressure is not greater than the return spring. The LDP solenoid blocks the engine vacuum port and opens the atmospheric pressure port connected through the EVAP system air filter. The vent valve is held open by the diaphragm. This allows the canister to see atmospheric pressure. (Scheme 33)

Scheme 33

Diaphragm Upward Movement

When the PCM energizes the LDP solenoid, the solenoid blocks the atmospheric port leading through the EVAP air filter and at the same time opens the engine vacuum port to the pump cavity above the diaphragm. The diaphragm moves upward when vacuum above the diaphragm exceeds spring force. This upward movement closes the vent valve. It also causes low pressure below the diaphragm, unseating the inlet check valve and allowing air in from the EVAP air filter. When the diaphragm completes its upward movement, the LDP reed switch turns from closed to open. (Scheme 34)

Scheme 34

Diaphragm Downward Movement

Based on reed switch input, the PCM de-energizes the LDP solenoid, causing it to block the vacuum port, and open the atmospheric port. This connects the upper pump cavity to atmosphere through the EVAP air filter. The spring is now able to push the diaphragm down. The downward movement of the diaphragm closes the inlet check valve and opens the outlet check valve pumping air into the evaporative system. The LDP reed switch turns from open to closed, allowing the PCM to monitor LDP pumping (diaphragm up/down) activity. (Scheme 35) During the pumping mode, the diaphragm will not move down far enough to open the vent valve. The pumping cycle is repeated as the solenoid is turned on and off. When the evaporative system begins to pressurize, the pressure on the bottom of the diaphragm will begin to oppose the spring pressure, slowing the pumping action. The PCM watches the time from when the solenoid is de-energized, until the diaphragm drops down far enough for the reed switch to change from opened to closed. If the reed switch changes too quickly, a leak may be indicated. The longer it takes the reed switch to change state, the tighter the evaporative system is sealed. If the system pressurizes too quickly, a restriction somewhere in the EVAP system may be indicated.

Scheme 35

Pumping Action

During portions of this test, the PCM uses the reed switch to monitor diaphragm movement. The solenoid is only turned on by the PCM after the reed switch changes from open to closed, indicating that the diaphragm has moved down. At other times during the test, the PCM will rapidly cycle the LDP solenoid on and off to quickly pressurize the system. During rapid cycling, the diaphragm will not move enough to change the reed switch state. In the state of rapid cycling, the PCM will use a fixed time interval to cycle the solenoid. If the system does not pass the EVAP Leak Detection Test, the following DTCs may be set

1. P0442: EVAP Leak Monitor Medium Leak Detected
2. P0455: EVAP Leak Monitor Large Leak Detected
3. P0456: EVAP Leak Monitor Small Leak Detected
4. P1486: EVAP Leak Monitor Pinched Hose Found
5. P1494: Leak Detection Pump Sw Or Mechanical Fault
6. P1495: Leak Detection Pump Solenoid Circuit

EVAP Leak Detection Test Enabling Conditions

1. Cold start with ambient temperature (obtained from modeling the inlet air temperature sensor) between 40°F (4°C) and 90°F (32°C) for 0.040" leak. Between 40°F (4°C) and 85°F (29°) for 0.020" leak.
2. Engine coolant temperature is within 10 to 18° F (-8 to -12°C) of ambient.
3. Battery voltage is between 10 and 15 volts.
4. Low fuel warning light off (fuel level must be between 15 and 85 percent.
5. MAP sensor reading is 22 in. Hg or above. This is the manifold absolute pressure, not vacuum.
6. No engine stall during test.

Note. If battery voltage drops below 10 volts for more than 5 seconds during engine cranking, the EVAP leak detection test will not run

A DTC will not be set is a one trip fault is set or is the MIL is illuminated for any of the following

1. Purge Solenoid Electrical Fault
2. All TPS Faults
3. All Engine Controller Self Test Faults
4. LDP Pressure Switch Fault
5. All Camshaft and/or Crankshaft Sensor Fault
6. EGR Solenoid Electrical Fault
7. All MAP Sensor Faults
8. All Injector Faults
9. Ambient Temperature Sensor Electrical Faults
10. BARO Out of Range
11. Vehicle Speed Faults
12. All Coolant Sensor Faults
13. LDP Solenoid Circuit

Note. If battery temperature is not within range, or if the engine coolant temperature is not within a specified range of the battery temperature, the PCM will not run tests for DTC P0441, P0442, P0455, P1486 or P1494. These temperature calibrations may be different between models.

Section 1 - P1495 Leak Detection Pump Solenoid Circuit

Section 1 refers to section 1 in the EVAP LDP Test Sequence. (Scheme 36) When the ignition key is turned on, the LDP diaphragm should be in the down position and the LDP reed switch should be closed. If the EVAP system has residual pressure, the LDP diaphragm may be up. This could result in the LDP reed switch being open when the key is turned on and a DTC P1494 fault could be set because the PCM is expecting the reed switch to be closed. After the ignition key is turned on, the PCM immediately tests the LDP solenoid circuit for electrical faults. If a fault is detected, DTC P1495 will set, the MIL will illuminate, and the remaining EVAP Leak Detection Test is canceled.

Scheme 36

Section 2 - P1494 Leak Detection Pump Switch Or Mechanical Fault

Section 2 refers to section 2 in the EVAP LDP Test Sequence. (Scheme 36) If DTC P1495 is not set, the PCM will check for DTC P1494. If the LDP reed switch was closed when the ignition key was turned on, the PCM energizes the LDP solenoid for up to 8 seconds and monitors the LDP switch. As the LDP diaphragm is pulled up by engine vacuum, the LDP reed switch should change from closed to open. If it does not, the PCM sets a temporary fault (DTC P1494) in memory, and waits until the next time the Enabling Conditions are met to run the test again. If this is again detected, DTC P1494 is stored and the MIL is illuminated. If the problem is not detected during the next enabling cycle, the temporary fault will be cleared. However, if the PCM detects the reed switch open when the ignition key is turned on, the PCM must determine if this condition is due to residual pressure in the EVAP system, or an actual fault. The PCM stores information in memory on EVAP system purging from previous engine run or drive cycles. If little or no purging took place, residual pressure could be holding the LDP diaphragm up, causing the LDP switch to be open. Since this is not a malfunction, the PCM cancels the EVAP Leak Detection Test without setting the temporary fault. If there was sufficient purging during the previous cycle to eliminate EVAP system pressure, the PCM judges that this is a malfunction and sets a temporary fault in memory. The next time that the Enabling Conditions are met, the test will run again. If the fault is again detected, the MIL will illuminate and DTC P1494 will be stored. If the fault is not detected, the temporary fault will be cleared.

Section 3 - P1486 EVAP Leak Monitor Pinched Hose Found

Section 3 refers to section 3 in the EVAP LDP Test Sequence. (Scheme 36) If no fault has been detected so far, the PCM begins testing for possible blockage in the EVAP system between the LDP and the fuel tank. This is done by monitoring the time required for the LDP to pump air into the EVAP system during two to three pump cycles. If no blockage is present, the LDP diaphragm is able to quickly pump air out of the LDP each time the PCM turns off the LDP solenoid. If a blockage is present, the PCM detects that the LDP takes longer to complete each pump cycle. If the pump cycles take longer than expected (approximately 6 to 10 seconds) the PCM will suspect a blockage. On the next drive when Enabling Conditions are met, the test will run again. If blockage is again detected, DTC P1486 is stored, and the MIL is illuminated.

Section 4 - No DTC Can Be Set During This Time

Section 4 refers to section 4 in the EVAP LDP Test Sequence. (Scheme 36) After the LDP blockage tests are completed, the PCM then tests for EVAP system leakage. First, the PCM commands the LDP to rapidly pump for 20 to 50 seconds (depending on fuel level) to build pressure in the EVAP system. This evaluates the system to see if it can be sufficiently pressurized. This evaluation (rapid pump cycling) may occur several times prior to leak checking. The LDP reed switch does not close and open during rapid pumping because the diaphragm does not travel through its full range during this part of the test.

Section 5 - DTC P0456, P0442, P0455 EVAP Leak Monitor & Leak Detected

Section 5 refers to section 5 in the EVAP LDP Test Sequence. (Scheme 36) Next, the PCM performs one or more test cycles by monitoring the time required for the LDP reed switch to close (diaphragm to drop) after the LDP solenoid is turned off. If the switch does not close, or closes after a long delay, it means that the system does not have any significant leakage and the EVAP Leak Detection Test is complete. However, if the LDP reed switch closes quickly, there may be a leak or the fuel level may be low enough that the LDP must pump more to finish pressurizing the EVAP system. In this case, the PCM will rapidly pump the LDP again to build pressure in the EVAP system, and follow that by monitoring the time needed for several LDP test cycles. This process of rapid pumping followed by several LDP test cycles may repeat several times before the PCM judges that a 1eak is present. When leaks are present, the LDP test cycle time will be inversely proportional to the size of the leak. The larger the leak, the shorter the test cycle time. The smaller the leak, the longer the test cycle time. DTCs may be set when a leak as small as.020" (0.5 mm) diameter is present. If the system detects a leak, a temporary fault will be stored in PCM memory. The time it takes to detect a.020",.040", or larger leak is based on calibrations that vary from model to model. The important point to remember is if a leak is again detected on the next EVAP Leak Detection Test, the MIL will illuminate and a DTC will be stored based on the size of leak detected. If no leak is detected during the next test, the temporary fault will be cleared.

Diagnostic Tips

During diagnosis, you can compare the LDP solenoid activity with the monitor sequence. (Scheme 36) If the PCM detects a problem that could set a DTC, the testing is halted and LDP solenoid activity will stop. As each section of the test begins, it indicates that the previous section passed successfully. By watching to see which tests complete, you can see if any conditions are present that the PCM considers abnormal. For example, if the LDP solenoid is energized for the test cycles to test for blockage (DTC P1486), it means that the LDP has already passed its test for DTC P1494. Then, if the PCM detects a possible blockage, it will set a temporary fault without turning on the MIL and continue the leak portion of the test. However, the PCM will assume that the system is already pressurized and skip the rapid pump cycles. Always diagnose leaks, if possible, before disconnecting connections. Disconnecting connections may mask a leak condition. Keep in mind that if the purge solenoid seat is leaking, it could go undetected since the leak would end up in the intake manifold. Disconnect the purge solenoid at the manifold when leak checking. In addition, a pinched hose fault (DTC P1486) could set if the purge solenoid does not purge the fuel system properly (blocked seat). The purge solenoid must vent the fuel system prior to the LDP system test. If the purge solenoid cannot properly vent the system, the LDP cannot properly complete the test for DTC P1486 and this fault can set due to pressure being in the EVAP system during the test sequence. Multiple actuation's of the DRB III(R) Leak Detection Pump Monitor Test can hide a.020" leak because of excess vapor generation. Additionally, any source for additional vapor generation can hide a small leak in the EVAP system. Excess vapor generation can delay the fall of the LDP diaphragm thus hiding the small leak. An example of this condition could be bringing a cold vehicle into a warm shop for testing or high ambient temperatures. Fully plugged and partially plugged underhood vacuum lines have been known to set MIL conditions. DTCs P0456 and P1494 can be set for this reason. Always, thoroughly check hoses for pinches or blockage before condemning components.

Test Equipment

The Miller Evaporative Emission Leak Detector (8404) is capable of visually detecting leaks in the evaporative system and will take the place of the Ultrasonic Leak Detector (6917A). The EELD utilizes shop air and a smoke generator to visually detect leaks down to .020" or smaller. The food grade oil used to make the smoke includes an ultraviolet trace dye that will leave telltale signs of the leak under a Black light. This is helpful when components have to be removed to determine the exact leak location. For detailed test instructions, follow the operators manual packaged with the EELD.

ON-BOARD REFUELING VAPOR RECOVERY

The ORVR (On-Board Refueling Vapor Recovery) system consists of a unique fuel tank, fuel control valve, one-way check valve and vapor canister. (Scheme 37) The ORVR system is used to remove excess fuel tank vapors. This is done while the vehicle is being refueled. Fuel flowing into the fuel filler tube (approx. 1" I.D.) creates an aspiration effect drawing air into the fuel fill tube. During refueling, the fuel tank is vented to the EVAP canister to capture escaping vapors. With air flowing into the filler tube, there are no fuel vapors escaping to the atmosphere. Once the refueling vapors are captured by the EVAP canister, the vehicle's computer controlled purge system draws the vapor out of the canister for the engine to burn. The vapor flow is metered by the purge solenoid so that there is no, or minimal impact on driveability or tailpipe emissions. As fuel starts to flow through the fuel fill tube, it opens the normally closed check valve and enters the fuel tank. Vapor or air is expelled from the tank through the control valve and on to the vapor canister. Vapor is absorbed in the EVAP canister until vapor flow in the lines stops, either following shutoff or by having the fuel level in the tank rise high enough to close the control valve. The control valve contains a float that rises to seal the large diameter vent path to the canister. At this point in the fueling of the vehicle, the tank pressure increases, the check valve closes (preventing tank fuel from spitting back at the operator), and fuel then rises up the filler tube to shutoff the dispensing nozzle.

If the engine is shutoff while the On-Board diagnostics test is running, low level tank pressure can be trapped in the fuel tank and fuel cannot be added to the tank until the pressure is relieved. This is due to the LDP closing the vapor outlet from the top of the tank and the one-way check valve not allowing the tank to vent through the fill tube to atmosphere. Therefore, when fuel is added, it will back-up in the fill tube and shutoff the dispensing nozzle. The pressure can be eliminated in 2 ways: Vehicle purge must be activated for a long enough period to eliminate the pressure. Removing the fuel cap and allowing enough time for the system to vent through the recirculation tube.

Scheme 37

PRESSURE RELIEF/ROLLOVER VALVE

Fuel tank is equipped with a pressure relief/rollover valve. Dual-function valve relieves fuel tank pressure and prevents fuel flow through fuel tank vent hoses in case of a vehicle rollover. Valve consists of a plunger, spring, orifice and guide plate. Valve is normally open, allowing fuel vapors to vent to EVAP canister where they are stored.

If bottom of plunger is contacted by sloshing fuel in fuel tank, plunger seats in guide plate, preventing liquid fuel from reaching EVAP canister. In a vehicle rollover, valve is inverted. This forces plunger against guide plate and fuel is prevented from flowing through valve orifice and into fuel tank vent tube.

POSITIVE CRANKCASE VENTILATION

Positive Crankcase Ventilation (PCV) system is used to remove crankcase blow-by gases from crankcase by using a PCV valve and manifold vacuum. Crankcase blow-by gases are drawn through PCV and then returned back into the intake manifold with incoming air/fuel mixture. The PCV valve contains a spring loaded plunger. The plunger meters the amount of crankcase vapors routed into the combustion chamber based on intake manifold vacuum. (Scheme 38)and (Scheme 39).

When the engine is not operating, or during an engine backfire, the spring forces the plunger back against the seat. (Scheme 40) This will prevent vapors from flowing through the valve.

During periods of high manifold vacuum, such as idle or cruising speeds, vacuum is sufficient to completely compress the spring. It will then pull the plunger to the top of the valve. (Scheme 41) In this position there is minimal vapor flow through the valve. During periods of moderate manifold vacuum, the plunger is only pulled part way back from inlet. This results in maximum vapor flow through the valve. (Scheme 42)

Scheme 38

Scheme 39

Scheme 40

Scheme 41

Scheme 42

ON-BOARD DIAGNOSTICS

The PCM has been programmed to monitor many different circuits of the fuel injection system. This monitoring is called on-board diagnosis. Certain criteria, or arming conditions, must be met for a trouble code to be entered into the PCM memory. The criteria may be a range of engine RPM, engine temperature, and/or input voltage to the PCM. If a problem is sensed with a monitored circuit, and all of the criteria or arming conditions are met, then a trouble code will be stored in the PCM. It is possible that a trouble code for a monitored a circuit may not be entered into the PCM memory even though a malfunction has occurred. This may happen because one of the trouble code criteria have not been met. The PCM compares input signal voltages from each input device with specifications (the established high and low limits of the range) that are programmed into it for that device. If the input voltage is not within specifications and other trouble code criteria are met, a trouble code will be stored in the PCM memory. The On Board Diagnostics have evolved to the second Generation of Diagnostics referred to as OBD-II. These OBD-II diagnostics control the functions necessary to meet the requirements of California OBD-II and Federal OBD regulation. These requirements specify the inclusion of a Malfunction Indicator Light (MIL). See MALFUNCTION INDICATOR LIGHT .

The Data Link Connector (DLC) is a 16-pin connector located below driver's side of instrument panel. (Scheme 43) The Powertrain Control Module (PCM) contains a self-diagnostic system which stores a Diagnostic Trouble Code (DTC) if an electronic control system failure exists. DTC may be retrieved from PCM for system diagnosis by using Data Link Connector (DLC) and a scan tool. The DLC also provides a means to communicate with various vehicle control modules, check system operating conditions and to operate various system components. For additional information on using DLC and scan tool usage, see SELF-DIAGNOSTICS - CARAVAN, TOWN & COUNTRY, & VOYAGER article.

Scheme 43

PCI BUS SYSTEM

Note. For additional information on PCI system communications, see BODY CONTROL MODULES - CARAVAN, TOWN & COUNTRY, & VOYAGER article in ACCESSORIES & EQUIPMENT.

The Programmable Communication Interface (PCI bus) multiplex system consists of a single wire. The diagnostic junction port acts as a splice to connect each module and DLC. Diagnostic junction port is located below instrument panel to left of steering column. Each module uses its local ground as bus reference. If more than one module is trying to access the PCI bus at one time, the code being sent determines which message has higher priority, and is then allowed to access bus first. Communication over the bus is essential to proper operation of vehicles on-board diagnostic systems and Diagnostic Readout Box (DRB-III(R)). Problems with operation of bus or DRB-III(R) must be corrected before proceeding with diagnostic testing.

SERIAL COMMUNICATIONS INTERFACE

Serial Communications Interface (SCI) circuit is used by PCM to send data to and receive data and sensor activation signals from scan tool. Scan tool uses signals sent on SCI to display fault messages or diagnostic trouble codes, sensor voltages and device states (On/Off). Scan tool uses SCI to send solenoid and switch activation commands to PCM so that devices and circuits can be tested. SCI is also used to reset Service Reminder Indicator (SRI) light on some vehicles and to write SRI mileage to PCM.

STATE DISPLAY TEST MODE

The switch inputs to the Powertrain Control Module (PCM) have two recognized states; HIGH and LOW. For this reason, the PCM cannot recognize the difference between a selected switch position versus an open circuit, a short circuit, or a defective switch. If the State Display screen shows the change from HIGH to LOW or LOW to HIGH, assume the entire switch circuit to the PCM functions properly. Connect the DRBIII(R) scan tool to the data link connector and access the State Display screen. Then access either State Display Inputs and Outputs or State Display Sensors.

CIRCUIT ACTUATION TEST MODE

The Circuit Actuation Test Mode checks for proper operation of output circuits or devices the Powertrain Control Module (PCM) may not internally recognize. The PCM attempts to activate these outputs and allow an observer to verify proper operation. Most of the tests provide an audible or visual indication of device operation (click of relay contacts, fuel spray, etc.). Except for intermittent conditions, if a device functions properly during testing, assume the device, its associated wiring, and driver circuit work correctly. Connect the DRBIII(R) scan tool to the data link connector and access the Actuators screen.

TASK MANAGER

The PCM is responsible for efficiently coordinating the operation of all the emissions related components. The PCM is also responsible for determining if the diagnostic systems are operating properly. The software designed to carry out these responsibilities is referred to as the "Task Manager". The Task Manager determines which tests happen when and which functions occur when. Many of the diagnostic steps required by OBD-II must be formed under specific operating conditions. The Task Manager software organizes and prioritizes the diagnostic procedures. The job of the Task Manager is to determine if conditions are appropriate for tests to be run, monitor the parameters for a trip for each test, and record the results of the test. The following are the responsibilities of the Task Manager software.

1. Test Sequence
2. MIL Illumination
3. Diagnostic Trouble Codes
4. Trip Indicator
5. Freeze Frame Data Storage
6. Similar Conditions Window

TEST SEQUENCE

In many instances, emissions systems must fail diagnostic tests more than once before the PCM illuminates the Malfunction Indicator Light (MIL). These tests are know as "two-trip monitors". Other tests that turn the MIL on after a single failure are known as "one-trip monitors". A trip is defined as start the vehicle and operate it to meet the criteria necessary to run the given monitor. Many of the diagnostic tests must be performed under certain operating conditions. However, there are times when tests cannot be run because another test is in progress (conflict), another test has failed (pending) or the Task Manager has set a fault that may cause a failure of the test (suspend).

1. Pending Under some situations the Task Manager will not run a monitor if the MIL is illuminated and a fault is stored from another monitor. In these situations, the Task Manager postpones monitors pending resolution of the original fault. The Task Manager does not run the test until the problem is remedied. For example, when the MIL is illuminated for an Oxygen Sensor fault, the Task Manager does not run the Catalyst Monitor until the Oxygen Sensor fault is remedied. Since the Catalyst Monitor is based on signals from the Oxygen Sensor, running the test would produce inaccurate results.
2. Conflict There are situations when the Task Manager does not run a test if another monitor is in progress. In these situations, the effects of another monitor running could result in an erroneous failure. If this conflict is present, the monitor is not run until the conflicting condition passes. Most likely the monitor will run later after the conflicting monitor has passed. For example, if the Fuel System Monitor is in progress, the Task Manager does not run the EGR Monitor. Since both tests monitor changes in air/fuel ratio and adaptive fuel compensation, the monitors will conflict with each other.
3. Suspend Occasionally the Task Manager may not allow a two-trip fault to mature. The Task Manager will suspend the maturing of a fault if a condition exists that may induce an erroneous failure. This prevents illuminating the MIL for the wrong fault and allows more precise diagnosis. For example, if the PCM is storing a one-trip fault for the Oxygen Sensor and the EGR monitor, the Task Manager may still run the EGR Monitor but will suspend the results until the Oxygen Sensor Monitor either passes or fails. At that point the Task Manager can determine if the EGR system is actually failing or if an Oxygen Sensor is failing.

The Malfunction Indicator Light (MIL) may also be referred to as the SERVICE ENGINE SOON light. MIL is located in the instrument cluster or the message center (if equipped) on top of driver's side of instrument panel. MIL is displayed as an engine icon. When ignition is first turned on, MIL should come to verify bulb and circuit operation.

The PCM Task Manager carries out the illumination of the MIL. The Task Manager triggers MIL illumination upon test failure, depending on monitor failure criteria. The Task Manager Screen shows both a Requested MIL state and an Actual MIL state. When the MIL is illuminated upon completion of a test for a third trip, the Requested MIL state changes to OFF. However, the MIL remains illuminated until the next key cycle. On some vehicles, the MIL will actually turn OFF during the third key cycle. During the key cycle for the third good trip, the Requested MIL state is OFF, while the Actual MIL state is ON. After the next key cycle, the MIL is not illuminated and both MIL states read OFF.

DIAGNOSTIC TROUBLE CODE PRIORITY

With OBD-II, different DTC faults have different priorities according to regulations. As a result, the priorities determine MIL illumination and DTC erasure. DTCs are entered according to individual priority. DTCs with a higher priority overwrite lower priority DTCs.

1. Priority 0 This is a non-emissions related DTC.
2. Priority 1 One trip failure of a two-trip DTC for non-fuel system and non-misfire conditions.
3. Priority 2 One trip failure of a two-trip DTC for fuel system rich or fuel system lean condition, or misfire condition.
4. Priority 3 Two trip failure for non-fuel system or non-misfire condition, or a matured one-trip comprehensive component fault.
5. Priority 4 Two trip failure for matured fuel system rich or fuel system lean and misfire condition, or a one-trip catalyst damaging misfire.

Non-emission related failures have no priority. One trip failures of two-trip faults have low priority. Two trip failures or matured faults have higher priority. One and two trip failures of fuel system and misfire monitor take precedence over non-fuel system and non-misfire failures.

DTC SELF ERASURE

With one-trip components or systems, the MIL is illuminated upon test failure and DTCs are stored. Two-trip monitors are components requiring failure in two consecutive trips for MIL illumination. Upon failure of the first test, the Task Manager enters a maturing code. If the component fails the test for a second time the code matures and a DTC is set. After three good trips, the MIL is extinguished and the Task Manager automatically switches the trip counter to a warm-up cycle counter. DTCs are automatically erased following 40 warm-up cycles if the component does not fail again. For misfire and fuel system monitors, the component must pass the test under a Similar Conditions Window in order to record a good trip. A Similar Conditions Window is when engine RPM is within +/-375 RPM and load is within +/-10 percent of when the fault occurred. It is important to understand that a component does not have to fail under a similar window of operation to mature. It must pass the test under a Similar Conditions Window when it failed to record a Good Trip for DTC erasure for misfire and fuel system monitors. DTCs can be erased anytime with a DRB III. Erasing the DTC with the DRBIII(R) erases all OBD-II information. The DRBIII(R) scan tool automatically displays a warning that erasing the DTC will also erase all OBD-II monitor data. This includes all counter information for warm-up cycles, trips and Freeze Frame.

TRIP INDICATOR

The trip is essential for running monitors and turning off the Malfunction Indicator Light (MIL). A trip is defined as a set of vehicle operating conditions that must be met for a specific monitor to run. All trips begin with an ignition key cycle. Good trip counters are: global good trip, fuel system good trip, misfire good trip and alternate good trip (appears as Global Good Trip on DRBIII(R)).

1. Global Good Trip To increment a Global Good Trip, the oxygen sensor and catalyst efficiency monitors must have run and passed, and engine run time must be more than 2 minutes.
2. Fuel System Good Trip To count a good trip (3 required) and turn off the MIL, the following conditions must be met. Engine must be in closed loop, must be operating in similar conditions window and short term multiplied by long term must be less than threshold value. For more information on similar conditions window, see «SIMILAR CONDITIONS WINDOW»(/chrysler/town-country/iv-2000-2005/remont/theory-operation/#engine-controls-theory-operation__similar-conditions-window) .
3. Misfire Good Trip If operating in similar conditions window and 1000 engine revolutions have occurred with no misfires, the PCM will count one good trip (3 required) in order to turn off MIL.
4. Alternate Good Trip Alternate Good Trips are used in place of Global Good Trips for Comprehensive Components and Major Monitors. If the Task Manager cannot run a Global Good Trip because a component fault is stopping the monitor from running, it will attempt to count an Alternate Good Trip. The Task Manager counts an Alternate Good Trip for Comprehensive components when the following conditions are met: Two minutes of engine run time, idle or driving, and no other faults occur. The Task Manager counts an Alternate Good Trip for a Major Monitor when the monitor runs and passes. Only the Major Monitor that failed needs to pass to count an Alternate Good Trip.
5. Warm-Up Cycles Once the MIL has been turned off by the Good Trip Counter, the PCM will automatically switch to a Warm-Up Cycle Counter that can be viewed by the DRBIII(R) scan tool. Warm-up cycles are used to clear DTCs and freeze frame data from PCM memory. Forty warm-up cycles are necessary to clear DTCS and freeze frame data. A warm-up cycle is defined as the engine is started, an increase of 40°F (4°C) in engine coolant temperature exists after engine is started and engine coolant temperature reaches at least 160°F (71°C).

FREEZE FRAME DATA STORAGE

Once a failure occurs, the Task Manager records several engine operating conditions and stores it in a Freeze Frame. The Freeze Frame is considered one frame of information taken by an on-board data recorder. When a fault occurs, the PCM stores the input data from various sensors so that technicians can determine under what vehicle operating conditions the failure occurred. The data stored in Freeze Frame is usually recorded when a system fails the first time for two trip faults. Freeze Frame data will only be overwritten by a different fault with a higher priority. Erasing DTCs, either with the DRBIII(R) or by disconnecting the battery also clears all Freeze Frame data.

SIMILAR CONDITIONS WINDOW

The similar conditions window displays information about the engine operation during a monitor. Manifold absolute pressure (engine load) and engine RPM information is stored in this window when a failure occurs. There are 2 different similar conditions windows, fuel system and misfire.

Fuel System

1. Fuel System Similar Conditions Window An indicator that ABSOLUTE MAP WHEN FUEL SYS FAIL and RPM WHEN FUEL SYS FAIL are all in the same range when failure occurred. This is indicated by switching from NO to YES.
2. Absolute MAP When Fuel Sys Fail Stored MAP reading at time of failure. Tells at what engine load the failure occurred.
3. Absolute MAP A real-time reading of engine load to aid the user in accessing the Similar Conditions Window.
4. RPM When Fuel Sys Fail Stores the engine RPM at time of failure. Informs the user at what engine RPM the failure occurred.
5. Engine RPM A real-time reading of engine RPM to aid the user in accessing the Similar Conditions Window.
6. Adaptive Memory Factor The PCM uses both Short Term Compensation and Long Term Adaptive to calculate Adaptive Memory Factor for total fuel correction.
7. Upstream O2S Volts A real-time reading of O2 sensor to indicate performance of sensor. For example, stuck lean, stuck rich, etc.
8. Similar Conditions Window (SCW) Time In Window The PCM uses this timer to indicate that after all Similar Conditions have been met, if engine has been running good enough in SCW without failure detected. This timer is used to increment a Good Trip.
9. Fuel System Good Trip Counter This trip counter is used to turn off the MIL for fuel system DTCs. A Fuel System Good Trip increment is when engine is in Similar Conditions Window, Adaptive Memory Factor must be less than calibrated threshold and Adaptive Memory Factor must stay below that threshold for a specific amount of time.
10. Test Done This Trip Indicates the monitor has already run and completed during the current trip.

Misfire

1. Same Misfire Warm-Up State Indicates if misfire occurred when engine was warmed-up more than 160°F (71°C).
2. In Similar Misfire Window Indicates that Absolute MAP When Misfire Occurred and RPM When Misfire Occurred are all in the same range when failure occurred. Indicated by switching from NO to YES.
3. Absolute MAP When Misfire Occurred Stored MAP reading at time of failure. Tells what engine load the failure occurred.
4. Absolute MAP A real-time reading of engine load to aid the user in accessing the Similar Conditions Window.
5. RPM When Misfire Occurred Stores the engine RPM at time of failure.
6. Engine RPM A real-time reading of engine RPM to aid the user in accessing the Similar Conditions Window.
7. Adaptive Memory Factor The PCM uses both Short Term Compensation and Long Term Adaptive to calculate Adaptive Memory Factor for total fuel correction.
8. 200 Rev Counter Counts 0-100 720 degree cycles.
9. Similar Conditions Window (SCW) Cat 200 Rev Counter Counts when in Similar Conditions.
10. Similar Conditions Window (SCW) FTP 1000 Rev Counter Counts 0-4 when in Similar Conditions.
11. Misfire Good Trip Counter Counts up to 3 to turn off MIL.
12. Misfire Data Data collected during test.
13. Test Done This Trip Indicates YES when test is done.

MONITORS

There are electronic circuit monitors that check fuel, emission, engine and ignition performance. These monitors use information from various sensor circuits to indicate the overall operation of the fuel, engine, ignition and emission systems and thus the emissions performance of the vehicle. The fuel, engine, ignition and emission systems monitors do not indicate a specific component problem. They do indicate that there is an implied problem within one of the systems and that a specific problem must be diagnosed. If any of these monitors detect a problem affecting vehicle emissions, the Malfunction Indicator Light (MIL) will be illuminated. These monitors generate Diagnostic Trouble Codes that can be displayed with the MIL or a scan tool. The following is a list of the system monitors

1. Misfire Monitor
2. Fuel System Monitor
3. Oxygen Sensor Monitor
4. Oxygen Sensor Heater Monitor
5. Catalyst Monitor
6. Leak Detection Pump Monitor

All these system monitors require two consecutive trips with the malfunction present to set a fault. For diagnostic procedures, see appropriate SELF-DIAGNOSTICS article.

Misfire Monitor

Excessive engine misfire results in increased catalyst temperature and causes an increase in HC emissions. Severe misfires could cause catalyst damage. To prevent catalytic converter damage, the Powertrain Control Module (PCM) monitors engine misfire. The PCM monitors for misfire during most engine operating conditions (positive torque) by looking at changes in the crankshaft speed. If a misfire occurs, the speed of the crankshaft will vary more than normal.

Fuel System Monitor

To comply with clean air regulations, vehicles are equipped with a catalytic converters. These converters reduce the emission of hydrocarbons, oxides of nitrogen and carbon monoxide. The catalyst works best when the air/fuel ratio is at or near the optimum of 14.7 to 1. The PCM is programmed to maintain the optimum air/fuel ratio of 14.7 to 1. This is done by making short term corrections in the fuel injector pulse width based on the Oxygen (O2) sensor output. The programmed memory acts as a self calibration tool that the engine controller uses to compensate for variations in engine specifications, sensor tolerances and engine fatigue over the life span of the engine. By monitoring the actual air/fuel ratio with the O2 sensor (short term) and multiplying that with the program long-term (adaptive) memory and comparing that to the limit, it can be determined whether it will pass an emissions test. If a malfunction occurs such that the PCM cannot maintain the optimum air/fuel ratio, then the MIL will be illuminated.

Oxygen Sensor Monitor

Effective control of exhaust emissions is achieved by an oxygen feedback system. The most important element of the feedback system is the Oxygen (O2) sensor. The O2 sensor is located in the exhaust path. Once it reaches operating temperature 572-662°F (300-350°C), the sensor generates a voltage that is inversely proportional to the amount of oxygen in the exhaust. The information obtained by the sensor is used to calculate the fuel injector pulse width. This maintains a 14.7 to 1 air/fuel ratio. At this mixture ratio, the catalyst works best to remove hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxide (NOx) from the exhaust.

The O2 sensor is also the main sensing element for the Catalyst and Fuel Monitors. The O2 sensor can fail in any or all of the following manners.

1. Slow Response Rate
2. Reduced Output Voltage
3. Dynamic Shift
4. Shorted Or Open Circuits

Response rate is the time required for the sensor to switch from lean to rich once it is exposed to a richer than optimum air/fuel mixture or vice versa. As the sensor starts malfunctioning, it could take longer to detect the changes in the oxygen content of the exhaust gas. The output voltage of the O2 sensor ranges from 0 to 1 volt. A good sensor can easily generate any output voltage in this range as it is exposed to different concentrations of oxygen. To detect a shift in the air/fuel mixture (lean or rich), the output voltage has to change beyond a threshold value. A malfunctioning sensor could have difficulty changing beyond the threshold value.

Oxygen Sensor Heater Monitor

If there is an Oxygen (O2) sensor shorted to voltage DTC, as well as a O2 sensor heater DTC, the O2 sensor fault must be repaired first. Before checking the O2 sensor fault, verify that the heater circuit is operating correctly. Effective control of exhaust emissions is achieved by an oxygen feedback system. The most important element of the feedback system is the O2 sensor. The O2 sensor is located in the exhaust path. Once it reaches operating temperature 572-662°F (300-350°C), the sensor generates a voltage that is inversely proportional to the amount of oxygen in the exhaust. The information obtained by the sensor is used to calculate the fuel injector pulse width. This maintains a 14.7 to 1 air/fuel ratio. At this mixture ratio, the catalyst works best to remove hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxide (NOx) from the exhaust. The voltage readings taken from the O2 sensor are very temperature sensitive. The readings are not accurate below 572°F (300°C). Heating of the O2 sensor is done to allow the engine controller to shift to closed loop control as soon as possible. The heating element used to heat the O2 sensor must be tested to ensure that it is heating the sensor properly. The O2 sensor circuit is monitored for a drop in voltage. The sensor output is used to test the heater by isolating the effect of the heater element on the O2 sensor output voltage from the other effects.

Catalyst Monitor

To comply with clean air regulations, vehicles are equipped with catalytic converters. These converters reduce the emission of hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxide (NOx). Normal vehicle miles or engine misfire can cause a catalyst to decay. This can increase vehicle emissions and deteriorate engine performance, driveability and fuel economy. The catalyst monitor uses dual Oxygen (O2) sensors to monitor the efficiency of the converter. The dual O2 sensor's strategy is based on the fact that as a catalyst deteriorates, its oxygen storage capacity and its efficiency are both reduced. By monitoring the oxygen storage capacity of a catalyst, its efficiency can be indirectly calculated. The upstream O2 sensor is used to detect the amount of oxygen in the exhaust gas before the gas enters the catalytic converter. The PCM calculates the air/fuel mixture from the output of the O2 sensor. A low voltage indicates high oxygen content (lean mixture). A high voltage indicates a low content of oxygen (rich mixture). When the upstream O2 sensor detects a lean condition, there is an abundance of oxygen in the exhaust gas. A functioning converter would store this oxygen so it can use it for the oxidation of HC and CO. As the converter absorbs the oxygen, there will be a lack of oxygen downstream of the converter. The output of the downstream O2 sensor will indicate limited activity in this condition. As the converter loses the ability to store oxygen, the condition can be detected from the behavior of the downstream O2 sensor. When the efficiency drops, no chemical reaction takes place. This means the concentration of oxygen will be the same downstream as upstream. The output voltage of the downstream O2 sensor copies the voltage of the upstream sensor. The only difference is a time lag (seen by the PCM) between the switching of the O2 sensor's. To monitor the system, the number of lean-to-rich switches of upstream and downstream O2 sensor's is counted. The ratio of downstream switches to upstream switches is used to determine whether the catalyst is operating properly. An effective catalyst will have fewer downstream switches than it has upstream switches i.e., a ratio closer to zero. For a totally ineffective catalyst, this ratio will be one-to-one, indicating that no oxidation occurs in the device. The system must be monitored so that when catalyst efficiency deteriorates and exhaust emissions increase to over the legal limit, the MIL will be illuminated.

Leak Detection Pump Monitor

The leak detection assembly incorporates two primary functions. It must detect a leak in the evaporative system and it must seal the evaporative system so the leak detection test can be run. The primary components within the assembly are: a three port solenoid that activates both of the functions listed above; a pump which contains a switch, two check valves and a spring/diaphragm, and a canister vent valve seal which contains a spring loaded vent seal valve. Immediately after a cold start, between predetermined temperature thresholds limits, the three port solenoid is briefly energized. This initializes the pump by drawing air into the pump cavity and also closes the vent seal. During non test conditions, the vent seal is held open by the pump diaphragm assembly which pushes it open at the full travel position. The vent seal will remain closed while the pump is cycling due to the reed switch triggering of the three port solenoid that prevents the diaphragm assembly from reaching full travel. After the brief initialization period, the solenoid is de-energized allowing atmospheric pressure to enter the pump cavity, thus permitting the spring to drive the diaphragm which forces air out of the pump cavity and into the vent system.

When the solenoid is energized and de-energized, the cycle is repeated creating flow in typical diaphragm pump fashion. The pump is controlled in the 2 following modes.

1. Pump Mode The pump is cycled at a fixed rate to achieve a rapid pressure build in order to shorten the overall test time.
2. Test Mode The solenoid is energized with a fixed duration pulse. Subsequent fixed pulses occur when the diaphragm reaches the switch closure point. The spring in the pump is set so that the system will achieve an equalized pressure of about 7.5 in. H2O. The cycle rate of pump strokes is quite rapid as the system begins to pump up to this pressure. As the pressure increases, the cycle rate starts to drop off. If there is no leak in the system, the pump will eventually stop pumping at the equalized pressure. If there is a leak, it will continue to pump at a rate representative of the flow characteristic of the size of the leak. From this information it can be determined if the leak is larger than the required detection limit (currently set at .040" orifice). If a leak is revealed during the leak test portion of the test, the test is terminated at the end of the test mode and no further system checks will be performed. After passing the leak detection phase of the test, system pressure is maintained by turning on the leak detection pump's solenoid until the purge system is activated. Purge activation in effect creates a leak. The cycle rate is again interrogated and when it increases due to the flow through the purge system, the leak check portion of the diagnostic is complete. The canister vent valve will unseal the system after completion of the test sequence as the pump diaphragm assembly moves to the full travel position. Evaporative system functionality will be verified by using the stricter EVAP purge flow monitor. At an appropriate warm idle, the leak detection pump will be energized to seal the canister vent. The purge flow will be clocked up from some small value in an attempt to see a shift in the O2 sensor control system. If fuel vapor, indicated by a shift in the O2 sensor control is present, the test is passed. If not, it is assumed that the purge system is not functioning in some respect. The leak detection pump is again turned off and the test is ended.

NON-MONITORED CIRCUITS

The PCM does not monitor all circuits, systems and conditions that could cause a malfunction or driveability problem. However, problems with these systems may cause the PCM to store diagnostic trouble codes for other systems or components. For example, a fuel pressure problem will not register a fault directly, but could cause a rich/lean condition or misfire. This could cause the PCM to store an oxygen sensor or misfire diagnostic trouble code. The major non-monitored circuits are identified as follows.

1. Cylinder Compression The PCM cannot detect uneven, low or high cylinder compression. Low compression lowers oxygen content in the exhaust which leads to fuel system, oxygen sensor or misfire detection fault.
2. Excessive Oil Consumption Although the PCM monitors engine exhaust oxygen content when the system is in closed loop, it cannot determine excessive oil consumption.
3. Exhaust System The PCM cannot detect a plugged, restricted or leaking exhaust system, although it may set a diagnostic trouble code for EGR, oxygen sensor or fuel system fault.
4. Fuel Injector Mechanical Malfunctions The PCM cannot determine if a fuel injector is clogged, needle is sticking or if the wrong injector is installed. However, these could result in a rich or lean condition causing the PCM to store a diagnostic trouble code for a misfire, oxygen sensor or fuel system.
5. Fuel Pressure The fuel pressure regulator controls fuel system pressure. The PCM cannot detect a clogged fuel pump inlet filter, clogged in-line fuel filter, or a pinched fuel supply. However, these could result in a rich or lean condition causing the PCM to store an oxygen sensor, or fuel system diagnostic trouble code.
6. PCM Connector Engagement The PCM may not be able to determine spread or damaged connector pins. However, it might store diagnostic trouble codes as a result of spread connector pins.
7. PCM System Grounds The PCM cannot determine a poor system ground. However, one or more diagnostic trouble codes may be generated as a result of this condition. The module should be mounted to the body at all times, also during diagnostics.
8. Secondary Ignition Circuit The PCM cannot detect an inoperative ignition coil, fouled or worn spark plugs, ignition cross firing or open spark plug cables. The misfire will however, increase the oxygen content in the exhaust, deceiving the PCM into thinking the fuel system is too lean.
9. Throttle Body Airflow The PCM cannot detect a clogged or restricted air cleaner inlet or filter element.
10. Vacuum Assist The PCM cannot detect leaks or restrictions in the vacuum circuits of vacuum assisted engine control system devices. However, these could cause the PCM to store a MAP sensor diagnostic trouble code and cause a high idle condition.

HIGH & LOW LIMITS

The PCM compares input signal voltages from each input device with established high and low limits for the device. If the input voltage is not within limits and other criteria are met, the PCM stores a diagnostic trouble code in memory. Other diagnostic trouble code criteria might include engine RPM limits or input voltages from other sensors or switches that must be present before verifying a diagnostic trouble.

TRIP DEFINITION

A "Trip" means vehicle operation (following an engine-off period) of duration and driving mode such that all components and systems are monitored at least once by the diagnostic system. The monitors must successfully pass before the PCM can verify that a previously malfunctioning component is meeting the normal operating conditions of that component. For misfire or fuel system malfunction, the MIL may be extinguished if the fault does not recur when monitored during 3 subsequent sequential driving cycles in which conditions are similar to those under which the malfunction was first determined. Anytime the MIL is illuminated, a DTC is stored. The DTC can self erase only after the MIL has been extinguished. Once the MIL is extinguished, the PCM must pass the diagnostic test for the most recent DTC for 40 warm-up cycles (80 warm-up cycles for the Fuel System Monitor and the Misfire Monitor). A warm-up cycle can best be described by the following

1. The engine must be running.
2. A rise of 40°F (4.4°C) in engine temperature must occur from the time when the engine was started.
3. Engine coolant temperature must increase to more than 160°F (71°C).
4. A "driving cycle" that consists of engine start-up and engine shutoff.

Once the above conditions occur, the PCM is considered to have passed a warm-up cycle. Due to the conditions required to extinguish the MIL and erase the DTC, it is most important that after a repair has been made, all DTCs be erased and the repair verified by running 1-good trip.

MISCELLANEOUS CONTROLS

Note. Although not considered true engine performance-related systems, some controlled devices may affect driveability if they malfunction.

The A/C clutch relay is located in Intelligent Power Module (IPM) in engine compartment, near the battery. (Scheme 1)and (Scheme 3). The Powertrain Control Module (PCM) controls cycling of A/C compressor clutch by controlling ground circuit on A/C clutch relay. A/C clutch relay receives constant battery voltage at one side of relay from fuse No. 18 (20-amp) in IPM. When ignition is on, the A/C clutch relay receives voltage at the other side of the relay from ignition switch.

The PCM controls the ground circuit for A/C clutch relay. When ignition is on, PCM completes ground circuit for A/C clutch relay and A/C clutch relay provides voltage to A/C compressor clutch. When PCM senses low idle speed or full throttle, PCM opens the ground circuit and A/C clutch relays opens, shutting off A/C compressor clutch.

The Auto Shutdown (ASD) relay is located in Intelligent Power Module (IPM) in engine compartment, near the battery. (Scheme 1)and (Scheme 3). The ASD sense circuit informs the PCM when the ASD relay energizes. A 12-volt signal at this input indicates to the PCM that the ASD has been activated. This input is used only to sense that the ASD relay is energized. When energized, the ASD relay supplies battery voltage to the fuel injectors, ignition coils and the heating element in each oxygen sensor. When energized, the ASD relay provides power to operate the injectors, ignition coil, generator field, oxygen sensor heaters, EGR solenoid and also provides a sense circuit to the PCM for diagnostic purposes. If the PCM does not receive 12 volts from this input after grounding the ASD relay, it sets a Diagnostic Trouble Code (DTC). The PCM energizes the ASD anytime there is a crankshaft position sensor signal that exceeds a predetermined value. The ASD relay can also be energized after the engine has been turned off to perform an O2 sensor heater test.

Radiator fan control relay may be referred to as radiator fan control module. Relay is a solid state relay. Relay controls operation of radiator cooling fans. Radiator cooling fans will operate at variable speeds depending on engine coolant temperature and the A/C system pressure. PCM pulses ground (control) circuit to signal requested fan speed. Radiator fan control relay is located behind front grille, riveted to front bumper brace, in front of condenser. (Scheme 44)

Radiator fan control relay receives constant battery voltage from fuse No. 27 (40-amp) in Intelligent Power Module (IPM). The Powertrain Control Module (PCM) controls the ground circuit for radiator fan control relay by using a pulsed (time on) ground. This pulsed ground is known as Pulse Width Modulated (PWM) ground. The PWM ground causes radiator cooling fan relay to produce a proportional output voltage for operating radiator cooling fans at different speeds.

Radiator cooling fans operate at high speed when engine coolant temperature is approximately 230°F (110°C) and then reduces to low speed when engine coolant temperature decreases to approximately 220°F (104°C). Radiator cooling fans will turn off when engine coolant temperature decreases to approximately 214°F (101°C).

When the A/C pressure sensor closes at approximately 300 psi (21.1 kg/cm 2 ), radiator cooling fans operate at high speed. When A/C pressure decreases to approximately 250 psi (17.6 kg/cm 2 ), A/C pressure sensor opens and radiator cooling fans operate at low speed. Radiator cooling fans will turn off when A/C pressure decreases to approximately 248 psi (17.4 kg/cm 2 ).

Radiator cooling fans operate at high speed when transaxle fluid temperature is approximately 232°F (111°C) and then reduces to low speed when transaxle fluid temperature decreases to approximately 204°F (96°C). Radiator cooling fans will turn off when transaxle fluid temperature decreases to approximately 192°F (89°C).

Scheme 44

Starter relay is located in Intelligent Power Module (IPM) in engine compartment, near the battery. (Scheme 1)and (Scheme 3). Starter relay provides battery voltage to solenoid on starter motor for starter operation. A double-start override feature is used on the starting system to prevent starter from operating if engine is already running.

Starter relay receives constant battery voltage at one side of relay from fuse No. 9 (40-amp) in IPM. When ignition switch is in START position, starter relay receives voltage at the other side of the relay from ignition switch. The Powertrain Control Module (PCM) provides the ground circuit to energize starter relay. When starter relay is energized, PCM monitors engine RPM and will remove the ground circuit once engine RPM has reached a predetermined RPM. This prevents starter from operating if engine is already running.

GENERATOR

The Powertrain Control Module (PCM) uses various input signals and an internal electronic voltage regulator to maintain charging system voltage at 12.9-15.0 volts. Internal electronic voltage regulator controls charging system voltage by cycling the ground circuit on the generator field. Battery voltage is supplied to generator through a fusible link and the Auto Shutdown (ASD) relay. For operation of ASD relay, see AUTO SHUTDOWN RELAY . If a failure exists in the charging system, a diagnostic trouble code will be stored in the PCM. For additional charging system information, see GENERATORS & REGULATORS - CARAVAN, TOWN & COUNTRY, & VOYAGER article in STARTING & CHARGING SYSTEMS.

CRUISE CONTROL SYSTEM

The speed control system is electronically controlled and vacuum operated. The electronic control is integrated into the Powertrain Control Module (PCM). The controls are located on the steering wheel. The On/Off, and Set buttons are located on the left side of the air bag module. The Resume/Accel, Cancel and Coast buttons are located on the right side of the air bag module. The system is designed to operate at speeds above 30 MPH. When speed control is activated by depressing the On switch, the PCM allows a set speed to be stored in RAM for speed control. To store a set speed, depress and release the SET switch while the vehicle is moving at a speed between 25 and 85 MPH. In order for the speed control to engage, the brakes cannot be applied, nor can the gear selector be indicating the transmission is in Park or Neutral. The speed control can be disengaged manually by stepping on the brake pedal, depressing the Off switch, depressing the Cancel switch, depressing the clutch pedal or operating in 1st or 2nd gear (autostick, if equipped).

For added safety, the speed control system is programmed to disengage for any of the following conditions

1. An indication of Park or Neutral.
2. A rapid increase RPM (indicates that the clutch has been disengaged).
3. Excessive engine RPM (indicates that the transmission may be in a low gear).
4. The speed signal increases at a rate of 10 MPH per second (indicates that the co-efficient of friction between the road surface and tires is extremely low).
5. The speed signal decreases at a rate of 10 MPH per second (indicates that the vehicle may have decelerated at an extremely high rate).
6. If the actual speed is greater than 20 MPH over the set speed.
7. Autostick shifts into 1st or 2nd gear (autostick, if equipped).

Once the speed control has been disengaged, depressing the Resume switch when speed is greater than 20 MPH allows the vehicle to resume control to the target speed that was stored in the PCM. While the speed control is engaged, the driver can increase the vehicle speed by depressing the Accel switch. The new target speed is stored in the PCM when the Accel switch is released. The PCM also has a "tap-up" feature in which target speed increases by 2 MPH for each momentary switch activation of the Accel switch. The PCM also provides a means to decelerate to a new lower target speed without disengaging speed control. Depress and hold the Coast switch until the desired speed is reached, then release the switch. The PCM also has a "Tap Down" feature in which target speed decreases at 1 MPH for each momentary switch activation of the coast switch.

The powertrain control module supplies engine RPM to tachometer on instrument panel through the PCI Bus.

The powertrain control module and Transmission Control Module (TCM) supply various information between each other through the PCI bus. This information includes engine speed and vehicle load, and is used for operating the transmission and electronic engine controls. The TCM is located behind left front fender and is mounted to frame rail. (Scheme 45)

Scheme 45

Torque converter clutch operation is controlled by powertrain control module. Torque converter clutch is engaged only in 3rd gear. The PCM uses various input signals to control torque converter clutch solenoid. Torque converter clutch solenoid is located on transaxle valve body with connector located on side of transaxle. (Scheme 19) If a failure exists in torque converter clutch solenoid or wiring circuit, a diagnostic trouble code will be stored in the PCM.

See also:
• WIRING DIAGRAMS
• INPUT DEVICES
• OUTPUT SIGNALS
• SELF-DIAGNOSTIC SYSTEM
• PARK/NEUTRAL SWITCH (A/T MODELS)
• EXHAUST GAS RECIRCULATION SYSTEM
• EVAPORATIVE EMISSIONS SYSTEM
• GENERATOR
• LEAK DETECTION PUMP SYSTEM
• SERIAL COMMUNICATIONS INTERFACE
• EVAP CANISTER
• EVAP CANISTER PURGE SOLENOID
• ON-BOARD REFUELING VAPOR RECOVERY
• SIMILAR CONDITIONS WINDOW