Engine Controls - Theory & Operation: Other Pontiac Aztek I

Speed Density

Engine is equipped with a Mass Airflow (MAF) sensor, a Manifold Absolute Pressure (MAP) and an Intake Air Temperature (IAT) sensor.

The MAF sensor is an air flow meter that measures the amount of air entering the engine. The PCM uses the MAF sensor signal to provide the correct fuel delivery for all engine speeds and loads. A small quantity of air entering the engine indicates a deceleration or idle condition. A large quantity of air entering the engine indicates an acceleration or high load condition. The MAF sensor has an ignition 1 voltage circuit, a ground circuit and a signal circuit. PCM applies a voltage to the sensor on the signal circuit. The sensor uses the voltage to produce a frequency based on the inlet air flow through the sensor bore. The frequency varies within a range of about 2000 Hz at idle to about 10,000 Hz at maximum engine load. If PCM detects the frequency signal is less than the possible range of a correctly operating MAF sensor, a related DTC may set. To diagnose, go to SELF-DIAGNOSTICS - 3.4L AZTEK & RENDEZVOUS article.

The MAP sensor responds to pressure changes in the intake manifold. The pressure changes occur based on the engine load. The MAP sensor has a 5-volt reference circuit, low reference circuit and a MAP sensor signal circuit. The PCM supplies 5 volts to the MAP sensor on the 5-volt reference circuit. PCM also provides a ground on the low reference circuit. The MAP sensor provides a signal to PCM on the MAP sensor signal circuit which is relative to the pressure changes in the manifold. The PCM should detect a low signal voltage at a low MAP, such as during an idle or a deceleration. The PCM should detect a high signal voltage at a high MAP, such as when ignition is on, with engine off, or at a Wide Open Throttle (WOT). The MAP sensor is also used in order to determine the Barometric Pressure (BARO). This occurs when the ignition switch is turned on, with engine off. The BARO reading may also be updated whenever engine is operated at WOT. The PCM monitors the MAP sensor signal for voltage outside of normal range. If PCM detects a MAP sensor signal voltage that is excessively low, a related DTC may set. To diagnose, go to SELF-DIAGNOSTICS - 3.4L AZTEK & RENDEZVOUS article.

The IAT sensor is a variable resistor. The IAT sensor has a signal circuit and a low reference circuit. The IAT sensor measures the temperature of the air entering engine. The PCM supplies 5 volts to the IAT signal circuit and a ground for the IAT low reference circuit. When IAT sensor is cold, sensor resistance is high. When air temperature increases, sensor resistance decreases. With high sensor resistance, PCM detects a high voltage on IAT signal circuit. With lower sensor resistance, PCM detects a lower voltage on IAT signal circuit. If PCM detects an intermittent IAT signal voltage, a related DTC may set. To diagnose, go to SELF-DIAGNOSTICS - 3.4L AZTEK & RENDEZVOUS article.

COMPUTERIZED ENGINE CONTROLS

The computerized engine control system monitors and controls a variety of engine/vehicle functions. The computerized engine control system is primarily an emission control system designed to maintain a 14.7:1 air/fuel ratio under most operating conditions. When the ideal air/fuel ratio is maintained, the Three-Way Catalytic (TWC) converter can control Oxides Of Nitrogen (NOx), Hydrocarbon (HC) and Carbon Monoxide (CO) emissions.

The computerized engine control system consists of engine PCM, input devices (sensor and switch input signals) and output signals.

POWERTRAIN CONTROL MODULE

The Powertrain Control Module (PCM) has electronic controls to reduce exhaust emissions while maintaining excellent driveability and fuel economy. PCM is located near air filter box. (Scheme 1) The PCM is the control center of this system. PCM monitors numerous engine and vehicle functions. PCM constantly looks at the information from various sensors and other inputs, and controls the systems that affect vehicle performance and emissions. PCM also performs the diagnostic tests on various parts of the system. PCM can recognize operational problems and alert the driver via the Malfunction Indicator Light (MIL). When PCM detects a malfunction, it stores a Diagnostic Trouble Code (DTC). The problem area is identified by the particular DTC that is set. The control module supplies a buffered voltage to various sensors and switches. Review components and wiring diagrams in order to determine which systems are controlled by the PCM.

The following are some of the functions that the PCM controls

1. Engine fueling.
2. Ignition Control (IC).
3. Knock Sensor (KS) system.
4. EVAP emission system.
5. EGR system.
6. Automatic transmission functions.
7. Generator.
8. A/C clutch control.
9. Cooling fan control.

Scheme 1

INPUT DEVICES

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

A/C On/Request Signal

The A/C power switch is mounted in the instrument panel. This switch provides a simple "on" (A/C request) signal, which is monitored by the PCM. The PCM uses this signal to determine control of the A/C clutch relay (if equipped) and to adjust idle speed when A/C compressor clutch is engaged. Control unit also activates radiator cooling fan when this signal is present. If this signal is not present on A/C equipped vehicles, engine may idle rough when A/C compressor cycles.

Battery Voltage

Battery voltage is monitored by PCM. If battery voltage swings low, a weak spark or improper fuel control may result. To compensate for low battery voltage, PCM may increase idle speed, advance ignition timing, increase ignition dwell or richen the air/fuel mixture. If voltage swings high, PCM may set a charging system fault code and turn on Malfunction Indicator Light (MIL). If voltage signal swings excessively low (less than 9 volts) or excessively high (16 volts, most models), PCM shuts down for as long as condition exists. If condition is short-term, MIL flickers and vehicle may stumble. Vehicle stalls if condition persists.

Brake Switch Feedback

On models equipped with cruise control systems, PCM may monitor the brake switch circuit to determine when to engage and disengage cruise control. One circuit of brake switch is in series with power supply for TCC solenoid located in automatic transmission.

Camshaft Position Sensor

The Camshaft Position (CMP) sensor is located at top of the timing cover, behind the water pump. (Scheme 2) As camshaft sprocket turns, a magnet activates Hall Effect switch in CMP sensor. This signal is generated whenever cylinder No. 1 is at Top Dead Center (TDC) of its compression stroke.

This signal is used by PCM to indicate the position of No. 1 piston during its intake stroke. This allows the PCM to calculate true Sequential Fuel Injection (SFI) mode of operation. If sensor should fail while engine is running, engine will continue to run using the last calculated CMP sensor signal to maintain SFI mode. Upon restart, engine will run as long as fault is present with a 1-in-6 chance of injector sequence being correct.

Scheme 2

Cranking Signal

Cranking signal is a 12-volt signal monitored by the PCM. Signal is present when ignition switch is in the START position. The PCM uses this signal to determine the need for starting enrichment. PCM also cancels diagnostics until engine is running and 12-volt signal is no longer present.

Crankshaft Position Sensor

The Crankshaft Position (CKP) sensor utilizes a pick-up coil type sensor. The 7X CKP sensor is located on side of engine block and the 24X CKP sensor is mounted behind the harmonic balancer. (Scheme 3)or (Scheme 4). The CKP sensor monitors crankshaft position and sends signals to ignition control module. These signals provide PCM with a Top Dead Center (TDC) position reference for each piston, as well as supplying an engine speed (RPM) signal. This allows PCM to fire appropriate ignition coil at the proper time, determine triggering of the fuel injectors, and to compute crankshaft position and RPM. CKP sensor signal may also used to detect a cylinder misfire by monitoring changes in crankshaft speed.

Scheme 3

Scheme 4

Engine Coolant Temperature Sensor

The Engine Coolant Temperature (ECT) sensor is a thermistor (temperature sensitive resistor) located in an engine coolant passage, near thermostat housing. (Scheme 5) The PCM supplies and monitors a 5-volt signal to ECT sensor. This monitored 5-volt signal is then modified by resistance of the ECT sensor. When coolant temperatures are low, ECT sensor resistance is high and the PCM sees a high monitored voltage signal. When coolant temperatures are high, ECT sensor resistance is low and the PCM sees a low monitored voltage. When fully warmed, ECT sensor should reflect a temperature of at least 185°F (85°C).

Coolant temperature input is used in the control of fuel delivery, ignition timing, idle speed, emission control devices and Torque Converter Clutch (TCC) application. An ECT sensor which is out of calibration will not set a DTC, but can cause fuel delivery and driveability problems. An ECT sensor circuit problem should set a related DTC.

Scheme 5

EGR Pintle Position Sensor

This sensor is mounted inside the linear EGR valve and informs PCM of EGR pintle movement. PCM uses this information to control EGR flow. (Scheme 6)

Scheme 6

Fuel Pump Feedback

PCM monitors fuel pump circuit between fuel pump relay/oil pressure switch and fuel pump. This enables the PCM to determine if fuel pump is being energized by fuel pump relay or back-up oil pressure switch. A failure in this monitored circuit results in the setting of a related diagnostic trouble code in PCM memory.

Fuel Tank Pressure Sensor

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

Gear Switches

Gear switches are located inside automatic transmission. Switches may be normally open or closed, and change status depending upon internal hydraulic pressures. PCM uses high gear switch information in controlling emission components and engagement of Torque Converter Clutch (TCC).

Intake Air Temperature Sensor

The Intake Air Temperature (IAT) sensor is a thermistor (temperature sensitive resistor) and is mounted in fresh air intake tube. Low intake air temperature produces high internal sensor resistance, while high temperature causes low internal sensor resistance. The PCM supplies and monitors a 5-volt signal to sensor through a pull-down resistor in PCM.

IAT sensor allows PCM to determine intake air temperature. PCM uses signal to delay exhaust gas recirculation until intake air temperature reaches about 40°F (5°C). If intake air temperature becomes excessively high, PCM compensates by slightly retarding ignition timing. After a vehicle has cooled overnight, IAT and ECT sensor signals (resistance and temperature) should be close to the same reading. Failure in IAT sensor circuit should set a related DTC.

Knock Sensor

The Knock Sensor (KS) is a piezoelectric device which detects abnormal engine vibrations (spark knock) in the engine. KS is located on side of engine block, above oil filter. (Scheme 7) This vibration results in the production of a very low AC signal, which is sent from the KS to the KS module (integral to PCM). The PCM then retards ignition timing until the engine knock ceases. Two KS are used on some models.

A fault in the KS circuit may set a DTC. See SELF-DIAGNOSTICS - 3.4L AZTEK & RENDEZVOUS article.

Scheme 7

Manifold Absolute Pressure Sensor

The Manifold Absolute Pressure (MAP) sensor measures changes in manifold pressure. MAP sensor is mounted on top of engine, near PCV. (Scheme 8) Changes in manifold pressure result from engine load and speed changes. The MAP sensor converts these changes in manifold pressure into a voltage output signal to PCM (1.5 volts at idle to about 4.5 volts at wide open throttle). The PCM can monitor these signals and adjust air/fuel ratio and ignition timing under various operating conditions.

If MAP sensor fails, the PCM substitutes a fixed MAP value, and uses the TP sensor to control fuel delivery. A fault in the MAP circuit should set a related diagnostic trouble code. If a related DTC is not present and MAP sensor is suspected of causing a driveability problem, perform functional check of MAP sensor. See SYSTEM & COMPONENT TESTING - 3.4L AZTEK & RENDEZVOUS article.

Scheme 8

Heated Oxygen Sensor

The Heated Oxygen Sensor (HO2S) is mounted in exhaust system and monitors oxygen content of exhaust gases. The oxygen content causes the Zirconia/Platinum-tipped O2S to produce a voltage signal which is proportional to exhaust gas oxygen concentration (0-3 percent) compared to outside oxygen (20-21 percent). This voltage signal is low (about 0.1 volt) when a lean mixture is present and high (about one volt) when a rich mixture is present. As PCM compensates for a lean or rich condition, this voltage signal constantly fluctuates between high and low, crossing a .45-volt reference voltage supplied by PCM on the O2S signal line. This is referred to as "cross counts".

Once engine has entered closed loop mode, a fault in the HO2S circuit (cooled-down sensor or open or shorted HO2S circuit) will return engine to open loop mode. A problem in the HO2S circuit should set a related DTC.

Park/Neutral Position Switch

The Park/Neutral Position (PNP) switch is connected to transmission gear selector and signals PCM when transmission is in Park or Neutral. PCM uses this information for determining control of ignition timing, Torque Converter Clutch (TCC) and idle speed.

Throttle Position Sensor

The TP sensor is a variable mechanical resistor connected directly to throttle shaft linkage. (Scheme 9) TP sensor has 3 wires connected to it. One is connected to a 5-volt reference voltage supply from PCM, another is connected to PCM ground and third is a signal return which is monitored by PCM. Voltage signal from TP sensor varies from closed throttle (0.5-1.0 volt) to wide open throttle (4.5-5.0 volts). PCM uses this signal to determine control of fuel, idle speed, spark timing and Torque Converter Clutch (TCC). A problem in TP sensor circuit may set a related DTC.

Scheme 9

Vehicle Speed Sensor

The Vehicle Speed Sensor (VSS) is a magnetic inductive pickup that relays vehicle speed information to PCM. The PCM uses this information in order to control shift timing, line pressure, and TCC apply and release. The VSS mounts in the case extension at the vehicle speed sensor reluctor wheel, which is pressed onto the final drive carrier assembly. The sensor consists of a permanent magnet surrounded by a coil of wire. As vehicle speed sensor reluctor wheel on the final drive carrier assembly rotates, an AC signal is produced by the VSS. This AC signal consists of a voltage and frequency that changes based on vehicle speed. The PCM uses the frequency portion of this signal to determine vehicle speed. Higher vehicle speeds induce a higher frequency and a higher voltage measurement at the sensor. The voltage portion of the signal is used in diagnostic procedures. Sensor resistance should be 1650-2200 ohms at 68°F (20°C). Output voltage will vary with vehicle speed from a minimum of 0.5 volt AC at 100 RPM to 200 volts at 6000 RPM.

A/C Clutch Relay

See MISCELLANEOUS PCM CONTROLS .

Cruise Control Stepper Motor

See MISCELLANEOUS PCM CONTROLS .

EGR System

See EMISSION SYSTEMS .

Electronic Ignition

See IGNITION SYSTEMS .

EVAP Purge Solenoid

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

EVAP Vent Solenoid

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

Fuel Injectors

See FUEL CONTROL under FUEL SYSTEMS.

Fuel Pump & Fuel Pump Relay

See FUEL DELIVERY under FUEL SYSTEMS.

Idle Air Control Valve

See IDLE SPEED under FUEL SYSTEMS.

Malfunction Indicator Light

See SELF-DIAGNOSTIC SYSTEM .

Self-Diagnostics

See SELF-DIAGNOSTIC SYSTEM .

Serial Data

See SELF-DIAGNOSTIC SYSTEM .

Shift Solenoids (Electronic Transmission)

See MISCELLANEOUS CONTROLS .

Fuel Pressure Regulator

Fuel pressure regulator is a diaphragm-operated relief valve with injector pressure on one side and manifold pressure (vacuum) on the other. Pressure regulator maintains a pressure of 52-59 psi (358-405 kPa) under all operating conditions. Pressure regulator compensates for engine load by increasing fuel pressure when low manifold vacuum is experienced.

Fuel Pump

An in-tank, electric fuel pump delivers fuel to injector(s) through an in-line fuel filter. The pump is designed to supply fuel pressure in excess of vehicle requirements. The pressure relief valve controls maximum fuel pump pressure.

When ignition switch is turned to ON position, PCM turns on electric fuel pump by energizing fuel pump relay. PCM keeps pump on if engine is running or cranking (PCM is receiving reference pulses from ignition module). If there are no reference pulses, PCM turns pump off.

Fuel Pump Relay

When ignition switch is turned to ON position, PCM turns electric fuel pump on by energizing fuel pump relay. PCM keeps relay energized if engine is running or cranking (PCM is receiving reference pulses from ignition module). If there are no reference pulses, PCM turns pump off.

For additional information on fuel pump activation, see SYSTEM & COMPONENT TESTING - 3.4L AZTEK & RENDEZVOUS article.

FUEL CONTROL

The PCM, using input signals, determines adjustments to the air/fuel mixture to provide the optimum ratio for proper combustion under all operating conditions. Fuel control systems can operate in the open loop or closed loop mode.

Closed Loop Mode

When HO2S reaches operating temperature, coolant temperature reaches a preset temperature and a specific period of time has passed since engine start-up, PCM operates in closed loop mode. In closed loop mode, PCM controls air/fuel ratio based upon HO2S signals (in addition to other input parameters) to maintain as close to a 14.7:1 air/fuel ratio as possible. If HO2S cools off (due to excessive idling) or a fault occurs in HO2S circuit, vehicle will re-enter open loop mode.

Fuel System Operating Modes

Internal PCM calibration controls fuel delivery during starting, clear flood mode, deceleration and heavy acceleration.

1. Starting With ignition switch in the ON position, before engaging the starter, the PCM energizes the fuel pump relay for 2 seconds allowing the fuel pump to build up pressure. The PCM first tests speed density, then switches to the MAF sensor. PCM also uses the ECT, TP and MAP sensors to determine the proper air/fuel ratio for starting. PCM controls amount of fuel delivered in the starting mode by changing the pulse width of the injectors. This is done by pulsing the injectors for very short times.
2. Clear Flood If the engine floods, clear engine by pressing the accelerator pedal down to the floor and then crank engine. When TP sensor is at Wide-Open Throttle (WOT), the PCM reduces the injector pulse width in order to increase the air/fuel ratio. The PCM holds this injector rate as long as the throttle stays wide open and engine speed is below a predetermined RPM. If throttle is not held wide open, the PCM returns to the starting mode.
3. Run Mode The run mode has 2 conditions called Open Loop and Closed Loop. When engine is first started and engine speed is above a predetermined RPM, the system begins Open Loop operation. The PCM ignores the signal from the HO2S and calculates the air/fuel ratio based on inputs from the ECT, MAF, MAP and TP sensors. The system stays in Open Loop until both HO2S have varying voltage output, showing that they are hot enough to operate properly (his depends upon engine temperature), ECT sensor is above a specified temperature. and a specific amount of time has elapsed after starting the engine. Specific values for the mentioned conditions exist for each different engine, and are stored in the Electrically Erasable Programmable Read-Only Memory (EEPROM). The system begins Closed Loop operation after reaching these values. In Closed Loop, the PCM calculates the air/fuel ratio (injector on-time) based upon the signal from various sensors, but mainly from the HO2S. This allows the air/fuel ratio to stay very close to 14.7:1.
4. Acceleration When the driver pushes on the accelerator pedal, air flow into the cylinders increases rapidly, while fuel flow tends to lag behind. To prevent possible hesitation, PCM increases the pulse width to the injectors to provide extra fuel during acceleration. PCM determines the amount of fuel required based upon the throttle position, coolant temperature, manifold air pressure, mass airflow, and engine speed.
5. Deceleration When the driver releases the accelerator pedal, air flow into the engine is reduced. The PCM reads the corresponding changes in throttle position, manifold air pressure, and mass airflow. The PCM shuts OFF fuel completely if the deceleration is very rapid or for long periods, such as long, closed-throttle coast-down. The fuel shuts OFF in order to protect the catalytic converters.
6. Battery Voltage Correction PCM compensates for low battery voltage by increasing injector pulse width and increasing idle RPM. PCM is able to perform these commands because of a built-in memory/learning function.
7. Fuel Cut-Off When ignition is turned off, injectors are de-energized to prevent dieseling. Injectors are not energized if RPM reference pulses are not received by the PCM, even with ignition on. This prevents flooding before starting. Fuel cut-off also occurs at high engine RPM or excessive vehicle speed to prevent internal damage to engine. Some models may also cut off fuel injector signals during periods of sudden, closed throttle deceleration (when fuel is not needed).

Open Loop Mode

When engine is cold and engine speed is greater than 400 RPM, PCM operates in open loop mode. In open loop mode, PCM calculates air/fuel ratio based upon coolant temperature and MAP or MAF sensor readings. Engine remains in open loop mode until O2S reaches operating temperature, coolant temperature reaches a preset temperature and a specific period of time has elapsed after engine starts.

Sequential Fuel Injection System

Fuel injectors on Sequential Fuel Injection (SFI) system are pulsed sequentially in spark plug firing order. Main differences between sequential and simultaneous systems are injectors, wiring and the PCM. Constant fuel pressure is maintained to the injectors. Air/fuel mixture is regulated by amount of time injector stays open (pulse width). Various sensors provide information to the PCM to control pulse width.

IDLE SPEED

PCM controls engine idle speed depending upon engine operating conditions. PCM senses engine operating conditions and determines best idle speed.

The engine idle speed is controlled by the Idle Air Control (IAC) valve. The IAC valve is on the throttle body. (Scheme 9) The IAC valve pintle moves in and out of an idle air passage bore to control air flow around the throttle plate. The IAC valve consists of a movable pintle, driven by a gear attached to an electric motor called a stepper motor. The stepper motor is capable of highly accurate rotation, or of movement, called steps. The stepper motor has 2 separate windings that are called coils. Each coil is supplied current by 2 circuits from the PCM. When PCM changes polarity of a coil, the stepper motor moves one step. The PCM uses a predetermined number of counts to determine the IAC pintle position. Observe IAC counts with a scan tool. The IAC counts will increment up or down as the PCM attempts to change the IAC valve pintle position. An IAC Reset will occur when the ignition key is turned OFF. First, the PCM will seat the IAC pintle in the idle air passage bore. Second, the PCM will retract the pintle a predetermined number of counts to allow for efficient engine start-up. If the engine idle speed is out of range for a calibrated period of time, an idle speed DTC will set.

ELECTRONIC IGNITION SYSTEM

The Electronic Ignition (EI) system is responsible for producing and controlling a high energy secondary spark. This spark is used to ignite the compressed air/fuel mixture at precisely the correct time. This provides optimal performance, fuel economy, and control of exhaust emissions. This ignition system uses one coil for each pair of cylinders. Each pair of cylinders that are at Top Dead Center (TDC) at the same time are known as companion cylinders. The cylinder that is at TDC of the compression stroke is called the event cylinder. The cylinder that is at TDC of the exhaust stroke is called the waste cylinder. When the coil is triggered both companion cylinder spark plugs fire at the same time, completing a series circuit. Because the lower pressure inside the waste cylinder offers very little resistance, the event cylinder uses most of the available voltage to produce a very high energy spark. This is known as waste spark ignition. The EI system consists of the Crankshaft Position (CKP) sensor, Camshaft Position (CMP) sensor, Ignition Control Module (ICM) and PCM.

1. Camshaft Position Sensor See «CAMSHAFT POSITION SENSOR»(ref-152284-S26805024992003012500000) under INPUT DEVICES.
2. Crankshaft Position Sensor «CRANKSHAFT POSITION SENSOR»(ref-152284-S18123347172003012500000) under INPUT DEVICES.
3. Ignition Control Module & Ignition Coils Three dual tower ignition coils are mounted to the Ignition Control Module (ICM) and are serviced individually. (Scheme 10) The ICM performs the following functions: ICM receives and processes the signals from the CKP sensor "B". ICM determines the correct direction of the crankshaft rotation, and cuts spark and fuel delivery to prevent damage from backfiring if reverse rotation is detected. ICM determines the correct coil triggering sequence, based on the 7X CKP signal. This coil sequencing occurs at start-up, and is remembered by the ICM. After engine is running, ICM will continue to trigger the coils in the correct sequence. ICM produces and inputs 3X reference signals to PCM. The ICM contains the coil driver circuits that command the coils to operate.

Scheme 10

IGNITION TIMING CONTROL

Ignition spark timing and ignition dwell time are controlled entirely by the PCM. The PCM monitors information from various engine sensors, computes the desired spark timing and dwell, and firing of the ignition coil via ignition control circuit to the coil driver.

CATALYTIC CONVERTER

A Three-Way-Catalytic (TWC) converter is used to reduce exhaust emissions. This type of converter can reduce Hydrocarbons (HC), Carbon Monoxide (CO) and Oxides Of Nitrogen (NOx).

EVAPORATIVE EMISSION SYSTEM

The Evaporative Emission (EVAP) control system limits fuel vapors from escaping into the atmosphere. (Scheme 11) Fuel tank vapors are allowed to move from the fuel tank, due to pressure in the tank, through the vapor pipe, into the EVAP canister. Carbon in the canister absorbs and stores the fuel vapors. Excess pressure is vented through the vent line and EVAP vent solenoid to atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them. At an appropriate time, the control module will command the EVAP purge solenoid ON, open, allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent solenoid OFF, open, fresh air will be drawn through the solenoid and vent line to the EVAP canister. Fresh air is drawn through the canister, pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through the EVAP purge pipe and EVAP purge solenoid into the intake manifold to be consumed during normal combustion. The control module uses several tests to determine if the EVAP system is leaking.

Scheme 11

EVAP Canister

The canister is filled with carbon pellets used to absorb and store fuel vapors. Fuel vapor is stored in the canister until the control module determines that the vapor can be consumed in the normal combustion process.

The EVAP purge solenoid controls the flow of vapors from the EVAP system to the intake manifold. Solenoid is located near throttle body. (Scheme 12) This normally closed solenoid is Pulse Width Modulated (PWM) by the control module to precisely control the flow of fuel vapor to the engine. The solenoid will also be opened during some portions of the EVAP testing, allowing engine vacuum to enter the EVAP system.

Scheme 12

The EVAP vent solenoid controls fresh air flow into the EVAP canister. The solenoid is normally open. The control module will command the solenoid closed during some EVAP tests, allowing the system to be tested for leaks.

The Fuel Tank Pressure (FTP) sensor measures the difference between the pressure or vacuum in the fuel tank and outside air pressure. The control module provides a 5-volt reference and a ground to the FTP sensor. The FTP sensor provides a signal voltage back to the control module that can vary between 0.1-4.9 volts. As FTP increases, FTP sensor voltage decreases, high pressure equals low voltage. As FTP decreases, FTP voltage increases, low pressure or vacuum equals high voltage.

EXHAUST GAS RECIRCULATION

The Exhaust Gas Recirculation (EGR) system is designed to reduce Oxides Of Nitrogen (NOx) emissions by lowering combustion temperatures. A metered amount of exhaust gas is recirculated into the intake manifold and mixed with the air/fuel mixture. A linear EGR valve is used on all engines.

EGR valve includes electric motor to raise and lower EGR valve pintle and internal EGR valve pintle position sensor. EGR valve pintle is used to control EGR flow. PCM controls pintle based on engine temperature, engine RPM and EGR valve pintle position sensor inputs.

POSITIVE CRANKCASE VENTILATION

The Positive Crankcase Ventilation (PCV) system provides effective evacuation of crankcase vapors. Fresh air from the air filter housing is supplied to the crankcase, where it is mixed with blow-by gases and passed through the PCV valve and into the intake manifold. This mixture is then passed into the combustion chamber and burned.

The PCV valve provides primary control in this system by metering the flow (according to manifold vacuum) of the blow-by vapors. When manifold vacuum is high (at idle), the PCV valve restricts the flow to maintain a smooth idle.

Under conditions in which abnormal amounts of blow-by gases are produced (such as worn cylinders or rings), system is designed to allow excess gases to flow back through crankcase vent hose into air inlet.

Spring pressure holds PCV valve closed when engine is not running. This prevents hydrocarbon fumes from collecting in the intake manifold, a condition which could result in hard starting.

During engine operation, manifold vacuum pulls the valve closed against spring pressure. As vacuum decreases with increased engine load, spring pressure begins to overpower vacuum strength. This allows PCV valve to open proportional to engine load and evacuation requirements. Should the engine backfire, the PCV valve closes to prevent ignition of fumes in the crankcase.

As a bulb and system check, the MIL will illuminate when ignition switch is turned to ON position and engine is not running. When engine is started, MIL should go out. If MIL does not go out, a malfunction has been detected in the computerized engine control system or MIL circuit is faulty. MIL may be used on some models to display a stored DTC. To access DTCs, see SELF-DIAGNOSTICS - 3.4L AZTEK & RENDEZVOUS article.

PCM has a serial data line. Serial data is a stream of electrical impulses which can be exchanged between control modules. Serial data can be interpreted using a special scan tool. Access serial data by connecting a scan tool to DLC. Update intervals and information contained within data stream vary with model application.

MISCELLANEOUS CONTROLS

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

A/C CLUTCH

PCM regulates operation of the A/C clutch through a relay. The PCM disengages the A/C compressor when compressor load on engine may cause driveability problems (i.e., during hot restart, idle, low speed steering maneuvers and wide open throttle operation) or if A/C refrigerant pressure drops to less than or rises to greater than normal operating levels.

Refrigerant pressure is sensed through the monitoring of high and/or low pressure switch(es) or a pressure sensor which registers either high or low pressure levels. Hot restart is monitored through the Engine Coolant Temperature (ECT) sensor. For component application and related wiring, see appropriate A/C COMPRESSOR CLUTCH CONTROLS article in AIR CONDITIONING & HEATING.

A/C Pressure Switches

A/C high and low pressure switches may be used in the A/C compressor clutch or compressor clutch relay circuit. Switches are normally closed, completing the circuit which energizes the compressor clutch. When system refrigerant pressure increases beyond a certain point, high side switch opens, causing compressor clutch to disengage.

If system refrigerant level decreases (causing freon pressure to drop), low side pressure switch opens, preventing compressor damage by causing compressor clutch to disengage.

COOLING FAN CONTROL

The engine cooling fan system is controlled by the Body Control Module (BCM) and Powertrain Control Module (PCM). The BCM performs the calculations as to how long, when and what speed the cooling fans should turn on. The BCM then sends a Class 2 message to the PCM to engage the cooling fan relays. If there is a malfunction with the BCM, the PCM will control the engine cooling fans independently. The engine cooling system consists of 2 electric cooling fans and 3 fan relays. The relays are arranged in a series/parallel configuration that allows the PCM to operate both fans together at low or high speeds. The cooling fans and fan relays receive battery positive voltage from the underhood accessory wiring junction block.

During low speed operation, PCM supplies the ground path for the low speed fan relay through the low speed cooling fan relay control circuit. This energizes the cooling fan 1 relay coil, closes the relay contacts, and supplies battery positive voltage from the cool fan 1 fuse through the cooling fan motor supply voltage circuit to the left cooling fan. The ground path for the left cooling fan is through the cooling fan 2 relay and the right cooling fan. The result is a series circuit with both fans running at low speed.

During high speed operation the PCM supplies the ground path for the cooling fan 1 relay through the low speed cooling fan relay control circuit. After a 3-second delay, the PCM supplies a ground path for the cooling fan 3 relay and the cooling fan 2 relay through the high speed cooling fan relay control circuit. This energizes the cooling fan 2 relay coil, closes the relay contacts, and provides a ground path for the left cooling fan. At the same time the cooling fan 2 relay coil is energized closing the relay contacts and provides battery positive voltage from the cool fan 2 fuse on the cooling fan motor supply voltage circuit to the right cooling fan. During high speed fan operation, both engine cooling fans have there own ground path. The result is a parallel circuit with both fans running at high speed.

CRUISE CONTROL

On models equipped with cruise control, the system is operated by the PCM. PCM receives inputs from Vehicle Speed Sensor (VSS), servo diaphragm position sensor, cruise control switch and brake release switch. Based on these inputs, PCM controls position of cruise control stepper motor. PCM prevents system engagement at speeds of less than 25 MPH. A system fault is stored as a DTC in PCM memory.

Electronic Transmission

PCM controls transmission and other vehicle functions. PCM monitors a number of engine/vehicle functions and uses data to control shift solenoid "A", shift solenoid "B", TCC solenoid and the force motor. PCM also regulates TCC engagement, upshift pattern, downshift pattern and line pressure (shift quality).

1. 1-2 & 2-3 Shift Solenoids The shift solenoid valves are 2 identical, normally open, electronic exhaust valves that control upshifts and downshifts in all forward gear ranges. These shift solenoid valves work together in a combination of ON and OFF sequences in order to control the positions of the 1-2 and 2-3 shift valve trains. PCM monitors numerous inputs in order to determine the appropriate solenoid state combination and transmission gear for the vehicle operating conditions. PCM energizes the shift solenoids by providing a ground to the solenoid's electrical circuit. This sends a current through the coil winding of the solenoid, thereby creating a magnetic field. The magnetic field repels the plunger inside the solenoid and seats the solenoid metering ball against the fluid inlet port. This action prevents the exhaust of fluid through the solenoid and provides an increase in fluid pressure at the end of the shift valves. This fluid pressure initiates an upshift by moving the shift valves. Shift solenoid resistance should measure 19-24 ohms when measured at 68°F (20°C) and 24-31 ohms when measured at 190°F (88°C). The shift solenoid valves should energize when voltage is greater than 7.5 volts and de-energize when the voltage is less than one volt.
2. Transmission Fluid Pressure Manual Valve Position Switch The automatic Transmission Fluid Pressure (TFP) switch is attached to the valve body and consists of one fluid pressure switch that monitors TCC release pressure. This switch is used as a diagnostic tool to confirm that the TCC is actually OFF when it has been commanded OFF by the PCM. The TCC release switch is a normally closed pressure switch. A normally closed switch allows current to flow from the positive contact through the switch to ground when no fluid is present. Fluid pressure moves the diaphragm to disconnect the positive and ground contacts, opening the switch and stopping current flow. This change in switch status electronically signals the PCM that the TCC is released.
3. TCC Pulse Width Modulation Solenoid The TCC Pulse Width Modulation Solenoid (PWM) solenoid valve is a normally closed (hydraulically) used to control the apply and release of the converter clutch. PCM operates the solenoid with a negative duty cycle at a fixed frequency of 32 Hz in order to control the rate of TCC apply/release. The solenoid's ability to ramp the TCC apply and release pressures results in smoother TCC operation. When vehicle's operating conditions are appropriate to apply the TCC, the PCM immediately increases the duty cycle to about 22 percent. The PCM then ramps the duty cycle up to a maximum of 98 percent in order to achieve full TCC apply pressure. The rate at which the PCM increases the duty cycle controls the TCC apply. Similarly, the PCM also ramps down the TCC solenoid duty cycle in order to control TCC release. Some operating conditions prevent or enable TCC apply under various conditions. Also, if PCM receives a zero voltage signal from the TCC brake switch, signaling that the brake pedal has been depressed, the PCM immediately releases the TCC. TCC duty cycle for Electronically Controlled Capacity Clutch (ECCC) equipped vehicles is 22 percent for minimum apply pressure and 43 percent for maximum apply pressure. The TCC PWM solenoid valve will typically be 40-60 percent at full apply. Your results may vary. The TCC PWM solenoid valve resistance should measure 10-12 ohms at 68°F (20°C) and 13-15 ohms when measured at 190°F (88°C).
4. Transmission Fluid Temperature Sensor The automatic Transmission Fluid Temperature (TFT) sensor is a negative temperature coefficient thermistor (temperature sensitive resistor) that provides information to the PCM regarding the transmission fluid temperature. The TFT sensor clips on to the valve body and is replaced as a separate component and not as a part of the AT wiring harness assembly. The TFT sensor monitors non-pressurized fluid in the sump in order to determine operating temperature of the transmission fluid. The internal electrical resistance of the sensor varies in relation to the operating temperature of the transmission fluid. The PCM sends a 5-volt reference signal to the TFT sensor and measures voltage drop in the electrical circuit. A lower fluid temperature creates a higher resistance in the TFT sensor, which produces a higher voltage signal. PCM uses this input in order to help determine proper line pressure, shift schedules and TCC apply. When transmission fluid temperature reaches 266°F (130°C), PCM enters Hot Mode. Temperature higher than this will modify transmission shift schedules and the TCC apply in an attempt to reduce the fluid temperature by reducing the transmission heat generation. During Hot Mode, PCM applies the TCC at all times in 3rd and 4th gears. Also, PCM performs the 2-3 and 3-4 shifts earlier in order to help reduce fluid heat generation. PCM stays in Hot Mode until fluid temperature drops less than 248°F (120°C).
5. Transmission Pressure Control Solenoid The transmission Pressure Control (PC) solenoid is a precision electronic pressure regulator that controls transmission line pressure based on current flow through its coil windings. As current flow is increased, magnetic field which is produced by the coil moves the solenoid's plunger further away from the exhaust port. Opening the exhaust port decreases the output fluid pressure, which is regulated by the PC solenoid. This ultimately decreases line pressure. The PCM controls the PC solenoid valve based upon various inputs including throttle position, fluid temperature, MAP sensor and gear state. PCM controls the PC solenoid valve on a positive duty cycle at a fixed frequency of 585 Hz (cycles per second). Duty cycle is defined as the percentage of time when current flows through the solenoid coil during each cycle. A higher duty cycle provides a greater current flow through the solenoid. The high (positive) side of the PC solenoid valve electrical circuit at the PCM controls the PC solenoid valve operation. The PCM provides a ground path for the circuit, monitors average current, and continuously varies the PC solenoid valve duty cycle in order to maintain the correct average current flowing through the PC solenoid valve. The PC solenoid valve resistance should be 3-5 ohms at 68°F (20°C).