Engine Controls - Theory & Operation: Other Dodge Neon II

POWERTRAIN CONTROL MODULE

The Powertrain Control Module (PCM) is located in the engine compartment. (Scheme 1)or (Scheme 2). 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.

Scheme 1

Scheme 2

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. Inlet Air/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. Air Conditioning Sense
2. Battery Voltage
3. Battery Temperature
4. Brake Switch
5. Engine Coolant Temperature
6. Engine Speed (Crankshaft Position Sensor)
7. Engine Run Time
8. Manifold Absolute Pressure
9. Power Steering Pressure Switch
10. Transmission Gear Selection (Park/Neutral Switch)
11. Throttle Position
12. Vehicle Distance (Speed)

The PCM adjusts ignition timing based on the following inputs.

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

The Automatic Shutdown (ASD) and fuel pump relays are mounted in the Power Distribution Center (PDC), 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 coil, 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 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 .

Note. Components are grouped into 2 categories. The first category, INPUT DEVICES , includes components that control or produce voltage signals monitored by PCM. The second category, OUTPUT SIGNALS , includes components controlled by PCM (this is accomplished by PCM grounding individual component circuits).

INPUT DEVICES

Note. To determine component location and input device usage on a specific model, see WIRING DIAGRAMS article.

Vehicles are equipped with different combinations of input devices. Not all devices are used on all models. Available input signals include

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 or stoplight switch.

The brake switch is located under the instrument panel, at the brake pedal arm. (Scheme 3) The brake switch has 3 internal switches controlling various functions of the vehicle. Its main function is to control operation of the vehicle's brakelights. 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 3

Camshaft Position Sensor

The Camshaft Position Sensor (CMP) is mounted to the rear of the cylinder head. (Scheme 4)or (Scheme 5). The CMP sensor also acts as a thrust plate to control camshaft endplay. The PCM sends approximately 8 volts to the Hall Effect sensor. This voltage is required to operate the Hall Effect chip and the electronics inside the sensor. A ground for the sensor is provided through the sensor return circuit. The input to the PCM occurs on a 5-volt output reference circuit. A target magnet attaches to the rear of the camshaft and indexes to the correct position. (Scheme 6) The target magnet has four different poles arranged in an asymmetrical pattern. As the target magnet rotates, the CMP sensor senses the change in polarity. (Scheme 7) The sensor output switch switches from high (5.0 volts) to low (0.5 volt) as the target magnet rotates. When the north pole of the target magnet passes under the sensor, the output switches high. The sensor output switches low when the south pole of the target magnet passes underneath.

Scheme 4

Scheme 5

Scheme 6

Scheme 7

Clutch Interlock/Upstop Switch

The clutch interlock/upstop switch is an assembly consisting of two switches: an engine starter inhibit switch (interlock) and a clutch pedal upstop switch. The switch assembly is located in the clutch/brake pedal bracket assembly. (Scheme 3) The clutch interlock switch prevents engine starter operation and inadvertent vehicle movement with the clutch pedal in the up position (not depressed), or under normal conditions with the clutch engaged and the transaxle in gear.

Crankshaft Position Sensor

The Crankshaft Position (CKP) sensor mounts to the engine block behind the generator, just above the oil filter. (Scheme 8)or (Scheme 9). The CKP sensor is a Hall Effect sensor. The second crankshaft counterweight has two sets of four timing reference notches including a 60 degree signature notch. (Scheme 10) The PCM sends approximately 8 volts to the Hall Effect sensor. This voltage is required to operate the Hall Effect chip and the electronics inside the CKP sensor. A ground for the sensor is provided through the sensor return circuit. The input to the PCM occurs on a 5-volt output reference circuit. The notches generate pulses from high to low in the CKP sensor output voltage. When a metal portion of the counterweight aligns with the CKP sensor, the sensor output voltage goes low (less than 0.5 volt). When a notch aligns with the sensor, voltage goes high (5.0 volts). As a group of notches pass under the sensor, the output voltage switches from low (metal) to high (notch) then back to low. If available, an oscilloscope can display the square wave patterns of each voltage pulses. From the width of the output voltage pulses, the PCM calculates engine speed. The width of the pulses represent the amount of time the output voltage stays high before switching back to low. The period of time the sensor output voltage stays high before switching back to low is referred to as pulse width. The faster the engine is operating, the smaller the pulse width on the oscilloscope.

Scheme 8

Scheme 9

Scheme 10

Cruise Control Switches (Neon)

Note. The cruise control system may also be referred to as the speed control system.

The speed control system is electronically controlled and vacuum operated. The electronic control is integrated into the Powertrain Control Module (PCM) which is located in the engine compartment. The controls are located on the steering wheel and consist of five switches. The ON, OFF and SET buttons are located on the left side of the air bag module. The RESUME, ACCEL, COAST and CANCEL buttons are located on the right side of the air bag module. The system is designed to operate at speeds above 25 MPH.

Cruise Control Switches (PT Cruiser)

The speed control system is electronically controlled and vacuum operated. The electronic control is integrated into the Powertrain Control Module (PCM) which is located in the engine compartment. The controls are located on a single stalk switch. The ON, OFF, RESUME, ACCEL, SET, COAST and CANCEL lever is located on the right side of the steering wheel. The system is designed to operate at speeds above 25 MPH.

Engine Coolant Temperature Sensor

On Neon, the Engine Coolant Temperature (ECT) sensor threads into the rear of the cylinder head, next to the camshaft position sensor. (Scheme 11) On PT Cruiser, the ECT sensor threads into the thermostat housing. (Scheme 12) On all models, the ECT Sensor is a Negative Thermal Coefficient (NTC) Sensor. The resistance of the ECT Sensor changes as coolant temperature changes. This results in different input voltages to the PCM. The PCM also uses the ECT Sensor input to operate the radiator cooling fan(s), and send a message over the PCI bus to the instrument cluster for temperature gauge operation. The ECT sensor provides an input to the PCM. As temperature increases, resistance of the sensor decreases. As coolant temperature varies, the ECT sensor resistance changes resulting in a different voltage value at the PCM ECT sensor signal circuit. The ECT sensor provides input for various PCM operations. The PCM uses the input to control air/fuel mixture, timing, and radiator fan on/off times. The PCM uses ECT sensor input to send messages over the PCI bus for temperature gauge operation.

Scheme 11

Scheme 12

Fuel Level Sensor

The fuel gauge sending unit (fuel level sensor) is attached to the side of the fuel pump module. The sending unit consists of a float, an arm, and a variable resistor track (card). Fuel level sensor is located on fuel pump module and can be replaced separately. The fuel pump module has 4 different circuits (wires). Two of these circuits are used for the fuel gauge sending unit for fuel gauge operation, and for certain OBD-II emission requirements. The other 2 wires are used for electric fuel pump operation.

1. For Fuel Gauge Operation As fuel level increases, the float and arm move up. This increases the sending unit resistance, causing the fuel gauge to read full. As fuel level decreases, the float and arm move down. This decreases the sending unit resistance causing the fuel gauge to read empty. After this fuel level signal is sent to the instrument cluster, the instrument cluster will transmit the data across the J1850 bus circuits to the PCM.
2. OBD-II Emission Monitor Requirements The PCM will monitor the voltage output sent from the resistor track on the sending unit to indicate fuel level. The purpose of this feature is to prevent the OBD-II system from recording/setting false misfire and fuel system monitor diagnostic trouble codes.
3. The feature is activated if the fuel level in the tank is less than approximately 15 percent of its rated capacity. If equipped with a Leak Detection Pump (EVAP system monitor), this feature will also be activated if the fuel level in the tank is more than approximately 85 percent of its rated capacity.

Heated Oxygen Sensor

The upstream Heated Oxygen Sensor (HO2S) threads into the outlet flange of the exhaust manifold. (Scheme 13)and (Scheme 14). The downstream heated oxygen sensor threads into the system depending on emission package. On models with Federal emission package, the HO2S is mounted after the catalytic converter. On models with Low Emission Vehicle (LEV) package, the HO2S is mounted mid catalytic converter. On models with, Ultra Low Emission Vehicle (ULEV) package, HO2S is mounted between catalytic converters.

A single sensor ground is used for all HO2S. As vehicle accumulates mileage, the catalytic converter deteriorates. The deterioration results in a less efficient catalyst. To monitor catalytic converter deterioration, the fuel injection system uses two heated oxygen sensors. One sensor upstream of the catalytic converter and one downstream of the converter. The PCM compares the reading from the sensors to calculate the catalytic converter oxygen storage capacity and converter efficiency. Also, the PCM uses the upstream heated oxygen sensor input when adjusting injector pulse width. When the catalytic converter efficiency drops below emission standards, the PCM stores a diagnostic trouble code and illuminates the Malfunction Indicator Light (MIL). The HO2S produce voltages from 0 to 1 volt, depending upon the oxygen content of the exhaust gas in the exhaust manifold. When a large amount of oxygen is present (caused by a lean air/fuel mixture, can be caused by misfire and exhaust leaks), the sensors produces a low voltage. When there is a lesser amount of oxygen present (caused by a rich air/fuel mixture, can be caused by internal engine problems) it produces a higher voltage. By monitoring the oxygen content and converting it to electrical voltage, the sensors act as a rich-lean switch. The oxygen sensors are equipped with a heating element that keeps the sensors at proper operating temperature during all operating modes. Maintaining correct sensor temperature at all times allows the system to enter into closed loop operation sooner. Also, it allows the system to remain in closed loop operation during periods of extended idle. In closed loop operation, the PCM monitors the HO2S input (along with other inputs) and adjusts the injector pulse width accordingly. During open loop operation, the PCM ignores the HO2S sensor input. The PCM adjusts injector pulse width based on preprogrammed (fixed) values and inputs from other sensors.

The Automatic Shutdown (ASD) relay supplies battery voltage to both the upstream and downstream oxygen sensors. The oxygen sensors are equipped with a heating element. The heating elements reduce the time required for the sensors to reach operating temperature. The PCM uses pulse width modulation to control the ground side of the heater to regulate the temperature on upstream O2 heater only. Downstream O2 heater does not use pulse width modulation.

1. Upstream Oxygen Sensor The input from the upstream HO2S tells the PCM the oxygen content of the exhaust gas. Based on this input, the PCM fine tunes the air/fuel ratio by adjusting injector pulse width. The sensor input switches from 0 to 1 volt, depending upon the oxygen content of the exhaust gas in the exhaust manifold. When a large amount of oxygen is present (caused by a lean air/fuel mixture), the sensor produces voltage as low as 0.1 volt. When there is a lesser amount of oxygen present (rich air/fuel mixture) the sensor produces a voltage as high as 1.0 volt. By monitoring the oxygen content and converting it to electrical voltage, the sensor acts as a rich-lean switch. The heating element in the sensor provides heat to the sensor ceramic element. Heating the sensor allows the system to enter into closed loop operation sooner. Also, it 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.
2. Downstream Oxygen Sensor The downstream HO2S input is used to detect catalytic converter deterioration. As the converter deteriorates, the input from the downstream sensor begins to match the upstream sensor input except for a slight time delay. By comparing the downstream HO2S input to the input from the upstream sensor, the PCM calculates catalytic converter efficiency. Downstream HO2S input is also used to establish the upstream O2 goal voltage (switching point).

Scheme 13

Scheme 14

Ignition Switch

The ignition sense signal input informs the Powertrain Control Module (PCM) that the ignition switch is in the CRANK or RUN position.

Inlet Air Temperature Sensor

The Inlet Air Temperature (IAT) sensor is located in the intake air duct. (Scheme 2)or (Scheme 15). The IAT sensor is a Negative Temperature Coefficient (NTC) sensor that provides information to the PCM regarding the temperature of the air entering the intake manifold. This information is used by PCM to adjust air/fuel mixture.

Scheme 15

Input Speed Sensor (A/T)

The input 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 16) 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 ratio.

Scheme 16

Knock Sensor

The knock sensor threads into the cylinder block in front of the starter. (Scheme 17) The knock sensor is designed to detect engine vibration that is caused by detonation. 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.

Note. Over or under tightening knock sensor affects knock sensor performance, possibly causing improper spark control.

Scheme 17

Manifold Absolute Pressure Sensor

The Manifold Absolute Pressure (MAP) sensor is mounted on the intake manifold. (Scheme 18)or (Scheme 19). The MAP serves as a PCM input, using a silicone 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. Like the camshaft and crankshaft position sensors, 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 18

Scheme 19

Output Speed Sensor (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 16) 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.

Park/Neutral Switch

Park/Neutral (P/N) switch may also be referred to as transmission range sensor and is available on vehicles equipped with an A/T only. See TRANSMISSION RANGE SENSOR (A/T) .

Power Steering Pressure Switch

The power steering pressure switch is mounted directly to the power steering gear. (Scheme 20) The power steering pressure switch is used to improve the vehicle's idle quality by causing a readjustment of the engine idle speed when increased fluid pressure is sensed in the power steering system. The pressure switch functions by signaling the Powertrain Control Module (PCM) that an increase in pressure of the power steering system is putting additional load on the engine. This type of condition exists when the front tires of the vehicle are turned while the vehicle is stationary and the engine is at idle speed. When the PCM receives the signal from the power steering pressure switch, it directs the engine to increase its idle speed. This increase in engine idle speed compensates for the additional load, thus maintaining the required engine idle speed and idle quality. The power steering pressure switch sends a signal to PCM when power steering pressure (load) rises to more than a predetermined pressure. When power steering load is high, such as during parking, PCM increases engine idle speed through IAC motor to prevent stalling.

Scheme 20

Sensor Return

The sensor return circuit (sensor ground circuit) provides a low electrical noise ground reference for all sensors. The sensor return circuit connects to the internal ground circuits within the powertrain control module.

Serial Communication Interface Receive

Serial Communication Interface (SCI) receive circuit is a serial communication link used when diagnosing vehicle using scan tool. PCM receives data and device activation commands from scan tool on this circuit.

Throttle Position Sensor

Throttle Position (TP) sensor is mounted on throttle body. (Scheme 21)or (Scheme 22). 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.

Scheme 21

Scheme 22

Transmission Range Sensor (A/T)

The Transmission Range Sensor (TRS) is mounted to the top of the valve body inside the transaxle and can only be serviced by removing the valve body. (Scheme 16) The electrical connector extends through the transaxle case. The TRS has four switch contacts that monitor shift lever position and send the information to the Transmission Control Module (TCM). The TRS also has an integrated temperature sensor (thermistor) that communicates transaxle temperature to the TCM and PCM. (Scheme 23) The TRS communicates Shift Lever Position (SLP) to the TCM as a combination of open and closed switches. Each shift lever position has an assigned combination of switch states (open/closed) that the TCM receives from four sense circuits. The TCM interprets this information and determines the appropriate transaxle gear position and shift schedule. Since there are four switches, there are 16 possible combinations of open and closed switches (codes). Seven of these codes are related to gear position and three are recognized as between gear codes. This results in six codes which should never occur. These are called invalid codes. An invalid code will result in a DTC, and the TCM will then determine the shift lever position based on pressure switch data. This allows reasonably normal transmission operation with a TRS failure.

Scheme 23

Transmission Temperature Sensor

The Transmission Range Sensor (TRS) has an integrated thermistor that the Transmission Control Module (TCM) uses to monitor the transmission's sump temperature. (Scheme 23) Since fluid temperature can affect transmission shift quality and converter lockup, the TCM requires this information to determine which shift schedule to operate in. The PCM also monitors this temperature data so it can energize the vehicle cooling fan(s) when a transmission overheat condition exists. If the thermistor circuit fails, the TCM will revert to calculated oil temperature usage.

Vehicle Speed Sensor (M/T)

Note. On models equipped with 4-speed A/T, Output Speed Sensor (OSS) supplies vehicle speed and distance inputs to PCM through Transmission Control Module (TCM). See OUTPUT SPEED SENSOR (A/T) .

The Vehicle Speed Sensor (VSS) is a pulse generator mounted to an adapter near the transmission output shaft. (Scheme 24) The sensor is driven through the adapter by a speedometer pinion gear. The VSS pulse signal to the speedometer/odometer is monitored by the PCM speed control circuitry to determine vehicle speed and to maintain speed control set speed.

Scheme 24

OUTPUT SIGNALS

Note. Each vehicle may be equipped with different combinations of computer-controlled components. The following components may NOT be used on all models. For theory and operation on each output component, refer to indicated system.

A/C Clutch Relay

See A/C CLUTCH RELAY under MISCELLANEOUS CONTROLS.

Auto Shutdown Relay

See AUTO SHUTDOWN RELAY under MISCELLANEOUS CONTROLS.

Distributorless Ignition System

See DISTRIBUTORLESS IGNITION SYSTEM under IGNITION SYSTEMS.

Electric EGR Transducer (PT Cruiser)

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

Evaporative Canister Purge Control Solenoid

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

Exhaust Gas Recirculation Solenoid (PT Cruiser)

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

Fuel Injectors

See FUEL CONTROL under FUEL SYSTEMS.

Fuel Pump Module

See FUEL DELIVERY under FUEL SYSTEMS.

Fuel Pump Relay

See FUEL DELIVERY under FUEL SYSTEMS.

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.

Manifold Tuning Valve (Neon)

See MANIFOLD TUNING VALVE under MISCELLANEOUS CONTROLS.

Fuel Filter/Pressure Regulator (Neon)

A combination fuel filter/fuel pressure regulator is used. Combination fuel filter/fuel pressure regulator is located on the top of the fuel pump module. (Scheme 25) Fuel filter/fuel pressure regulator contains a diaphragm, calibrated springs and a fuel return valve. (Scheme 26) The internal fuel filter is also part of the assembly. Fuel filter/fuel pressure regulator is a mechanical device that is calibrated to maintain fuel system operating pressure of 53-63 psi (365-434 kPa) at fuel injectors.

Fuel is supplied to fuel filter/pressure regulator by the electric fuel pump through an opening at the bottom of filter/regulator. The fuel pump module contains a check valve to maintain some fuel pressure when the engine is not operating. This will help to start the engine. If fuel pressure at the pressure regulator exceeds approximately 58 psi, an internal diaphragm closes and excess fuel pressure is routed back into the tank through the pressure regulator. A separate fuel return line is not used.

Scheme 25

Scheme 26

Fuel Pressure Regulator (PT Cruiser)

The fuel pressure regulator is mounted in the fuel pump module which is mounted in top of fuel tank. (Scheme 27) The fuel system uses a nonadjustable pressure regulator that maintains fuel system pressure at approximately 53-63 psi (365-434 kPa). The fuel pressure regulator contains a diaphragm, calibrated spring and a fuel return valve. The spring pushes down on the diaphragm and closes off the fuel return port. System fuel pressure reflects the amount of fuel pressure required to open the return port. The pressure regulator is a mechanical device that is not controlled by the PCM or engine vacuum.

Scheme 27

Fuel Pump

Fuel pump is part of fuel pump module. See FUEL PUMP MODULE .

The electric fuel pump is located in and is part of the fuel pump module. (Scheme 25)or (Scheme 28). Fuel pump is a positive displacement, gerotor type, immersible pump with a permanent magnet electric motor. The fuel pump module is suspended in fuel in the fuel tank. The pump draws fuel through a strainer and pushes it through the motor to the outlet. The pump contains a check valve. The check valve in the pump outlet, maintains pump pressure during engine off conditions, for a short while. It is normal for fuel pressure to drop to zero after cool down. The check valve maintains volume of fuel in the rail and lines, not pressure. The fuel pump relay provides voltage to the fuel pump. The fuel pump has a maximum deadheaded pressure output of approximately 130 psi (880 kPa). The regulator adjusts fuel system pressure to approximately 53-63 psi (365-434 kPa).

Scheme 28

The fuel pump relay is located in the Power Distribution Center (PDC). (Scheme 29)or (Scheme 30). PDC is located in engine compartment. (Scheme 15)or (Scheme 31). The fuel pump relay supplies battery voltage to the fuel pump. A buss bar in the PDC 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 PDC and the relay. The fuse is located in the PDC. See WIRING DIAGRAMS article. 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.

Scheme 29

Scheme 30

Scheme 31

Auto Shutdown (ASD) relay provides voltage to fuel injectors. See AUTO SHUTDOWN RELAY 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 SPEED

Note. DO NOT attempt to correct a high idle speed condition by turning factory sealed throttle body throttle plate set screw. This will not change idle speed of warm engine but may cause cold start problems due to restricted airflow.

Idle Air Control (IAC) motor is mounted on throttle body. (Scheme 21)or (Scheme 22). 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).

Throttle Body

The throttle body mounts to the intake manifold. The throttle position sensor and the Idle Air Control (IAC) motor attach to the throttle body. (Scheme 21)or (Scheme 22). 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 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.

IGNITION SYSTEMS

The PCM completely controls ignition system. During crank/start mode, PCM will set a fixed amount of spark advance for an efficient engine start. Amount of spark advance or retard is determined by inputs that PCM receives from ECT sensor, engine vacuum and engine RPM. During engine operation, PCM can supply an infinite number of advance curves to ensure proper engine operation.

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

All vehicles use a maintenance free EVAP charcoal canister. (Scheme 32)or (Scheme 33). 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 32

Scheme 33

EVAP Canister Purge Solenoid

All vehicles use a proportional purge solenoid. (Scheme 20) 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.

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

Note. Neon is not equipped with an EGR system.

PT Cruiser

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)and (Scheme 34). 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 34) 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 34

LEAK DETECTION PUMP SYSTEM

The Evaporative (EVAP) emission system is designed to prevent the escape of fuel vapors from the fuel system. (Scheme 35) 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 33)or (Scheme 36). 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 37) 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 35

Scheme 36

Scheme 37

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 38)

Scheme 38

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 39)

Scheme 39

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 40) 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 40

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

Section 1 - P1495 Leak Detection Pump Solenoid Circuit

Section 1 refers to section 1 in the EVAP LDP Test Sequence. (Scheme 41) 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 41

Section 2 - P1494 Leak Detection Pump Switch Or Mechanical Fault

Section 2 refers to section 2 in the EVAP LDP Test Sequence. (Scheme 41) 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 41) 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.

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 42) 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 thru the recirculation tube.

Scheme 42

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 the PCV valve 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 5)and (Scheme 43).

When the engine is not operating, or during an engine backfire, the spring forces the plunger back against the seat. (Scheme 44) 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 45) 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 46)

Scheme 43

Scheme 44

Scheme 45

Scheme 46

DATA LINK CONNECTOR

The Data Link Connector (DLC) is a 16-pin connector located below driver's side of instrument panel. (Scheme 47)or (Scheme 48). 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 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.

Scheme 47

Scheme 48

PROGRAMMABLE COMMUNICATION INTERFACE BUS SYSTEM

Note. For additional information on Programmable Communication Interface (PCI) system communications, see appropriate BODY CONTROL MODULES article in ACCESSORIES & EQUIPMENT.

The PCI bus is a single wire multiplexed network capable of supporting binary encoded messages shared between modules. 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 vehicle's on-board diagnostic systems and Diagnostic Readout Box (DRBIII(R)). Problems with operation of bus or DRBIII(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.

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

The Malfunction Indicator Light (MIL) is located in the instrument cluster. 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.

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»(ref-142543-S30126393772002081000000) .
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 FAILED 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
7. EGR Monitor (If Equipped)

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 inches of water. 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 .020" 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.

EGR Monitor (If Equipped)

The Powertrain Control Module (PCM) performs an on-board diagnostic check of the EGR system. The EGR monitor is used to test whether the EGR system is operating within specifications. The diagnostic check activates only during selected engine/driving conditions. When the conditions are met, the EGR is turned off (solenoid energized) and the O2 sensor compensation control is monitored. Turning off the EGR shifts the air/fuel ratio in the lean direction. The O2 sensor data should indicate an increase in the O2 concentration in the combustion chamber when the exhaust gases are no longer recirculated. While this test does not directly measure the operation of the EGR system, it can be inferred from the shift in the O2 sensor data whether the EGR system is operating correctly. Because the O2 sensor is being used, the O2 sensor test must pass its test before the EGR test. Also looks at EGR linear potentiometer for feedback.

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 condition.

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 strictly considered part of engine performance system, some controlled devices can adversely affect driveability if they malfunction.

The A/C clutch relay is located in the Power Distribution Center (PDC). (Scheme 29)or (Scheme 30). PDC is located in engine compartment. (Scheme 15)or (Scheme 31). 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. 22 (Neon) or fuse No. 11 (PT Cruiser) in PDC. 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 Automatic Shutdown (ASD) relay is located in Power Distribution Center (PDC). (Scheme 29)or (Scheme 30). PDC is located in engine compartment. (Scheme 15)or (Scheme 31). The ASD relay supplies battery voltage to the fuel injectors, ignition coil, generator field winding, EGR solenoid (if equipped) and the heating elements in the oxygen sensors. The PCM controls the ASD 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 ON or START, the PCM monitors the crankshaft and camshaft position sensor signals to determine engine speed and ignition timing. If the PCM does not receive crankshaft and camshaft position sensor signals when the ignition switch is in the RUN position, it will de-energize the ASD relay.

Neon

The Manifold Tuning Valve (MTV) connects both passages of the intake manifold plenum. It is an electric motor. The PCM controls MTV solenoid. The MTV optimizes acoustical tuning of intake system during wide open throttle operation throughout RPM range.