Engine Control System - Theory & Operation — Buick LeSabre VIII (2000-2005)

INTRODUCTION

This article covers basic description and operation of engine performance-related systems and components. Read this article before diagnosing vehicles or systems with which you are not completely familiar. For component locations, see COMPONENT LOCATIONS in appropriate SELF-DIAGNOSTICS article.

Mass Airflow Sensor

The Mass Airflow (MAF) sensor measures flow of air entering the engine in grams per second. This measurement of airflow is a reflection of engine load (throttle opening and air volume), similar to the relationship of engine load to MAP or vacuum sensor signal. Mass Airflow (MAF) signal should remain relatively constant at cruise, gradually changing with throttle angle, and rapidly changing on sudden acceleration or deceleration. The PCM uses MAF information to control fuel delivery. Sensor produces a frequency signal which cannot be easily measured in testing (1000-12,000 Hertz). This varying signal is proportional to airflow.

Speed Density

The speed density system is only needed when there is a Mass Air Flow (MAF) sensor malfunction. If the PCM detects a malfunction with the MAF sensor circuit, the PCM will default to the speed density fuel management.

Three sensors provide the PCM with the basic information for the fuel management portion of its operation. That is, 3 specific signals to the PCM establish the engine speed and the air density factors. The engine speed signal comes from the ignition system. Air density is derived from the IAT and the MAP sensor inputs. The IAT sensor measures the air temperature that is entering the engine. The IAT signal works in conjunction with the MAP sensor to determine air density. As the intake manifold pressure increases, the air density in the intake manifold also increases and additional fuel is required. This information from the IAT and MAP sensors is used by the PCM to control injector pulse width.

SUPERCHARGER (3.8L VIN 1)

The supercharger is a positive displacement pump that consists of two counter-rotating rotors in a housing with an inlet port and an outlet port. The rotors are designed with three lobes and a helical twist. An air bypass circuit is built into the housing. The rotors in the supercharger are designed to run at a minimal clearance, not in contact with each other or the housing. The rotors are timed to each other by a pair of precision spur gears which are pressed onto the rotor shafts. The forward end of the rotors are held in position by deep-groove ball bearings. The back end of the rotors are supported by sealed roller bearings.

The gears and ball bearings are lubricated by synthetic oil. The oil reservoir is self-contained in the supercharger and does not rely on engine oil for lubrication.

The cover on the supercharger contains the input shaft which is supported by two, deep-groove ball bearings and is coupled to the rotor drive gears. The pulley is pressed and keyed onto the input shaft. These bearings are lubricated by the synthetic oil contained in the same reservoir as the gears and rotor bearings.

The supercharger is designed to pump more air than the engine would normally use. This excess air creates a boost pressure in the intake manifold. Maximum boost can range from 7 to 9 psi (48 to 63 kPa). Because the supercharger is a positive displacement pump and is directly driven from the engine drive belt system, boost pressure is available at all driving conditions.

When boost is not desired, such as during idle and light throttle cruising, the excess air that the supercharger is producing is routed through the bypass passage between the intake manifold and the supercharger inlet. This bypass circuit is regulated by a bypass valve which is similar to a throttle plate. The bypass valve is controlled by a vacuum actuator which is connected to the vacuum signal between the throttle and the supercharger inlet. Spring force from the actuator holds the valve closed to create boost, and vacuum pulls the valve open when the throttle closes to decrease boost. The open bypass valve reduces pumping loss thereby increasing fuel efficiency.

The solenoid valve attached to the bypass actuator is an electronically controlled, three-way valve. This valve, controlled by the PCM, determines whether pressure from the manifold is routed to the bypass actuator or closed off. The valve allows pressure from the manifold to open the bypass valve and regulate boost pressure during specific driving conditions.

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 which is designed to maintain a 14.7:1 air/fuel ratio under most operating conditions. When the ideal air/fuel ratio is maintained, the 3-way catalytic converter can control oxides of nitrogen (NOx), hydrocarbon (HC) and carbon monoxide (CO) emissions.

The computerized engine control system consists of the master controller (PCM or ECM), input devices (sensors and switches) and output signals.

POWERTRAIN CONTROL MODULE

For exact location of Powertrain Control Module (PCM), see PCM LOCATION in appropriate SELF-DIAGNOSTICS article or COMPONENT LOCATIONS in appropriate SYSTEM & COMPONENT TESTING article. On some models, the PCM may be referred to as an Electronic Control Module (ECM). Although the 2 units may process different signals, the 2 terms are interchangeable.

The PCM is the control center of the vehicle. It controls the following

Fuel metering system.
Transmission shifting.
Ignition timing.
On-board diagnostics for powertrain functions.

The PCM constantly looks at the information from various sensors and controls the systems that affect vehicle performance and emissions. The PCM also performs the diagnostic functions for those systems. It can recognize operational problems and will alert the driver through the Malfunction Indicator Lamp (MIL) when a malfunction has occurred. When the PCM detects a malfunction, it stores a DTC which will help identify problem areas. This is done to aid the technician in making repairs.

The PCM supplies either 5 or 12 volts to power various sensors and switches. This is done through transistors in the PCM. The circuits have very high impedance and typically will not illuminate a test lamp when connected to the circuit. In some cases, even an ordinary shop voltmeter will not give an accurate reading because its resistance is too low in comparison with the input impedance of the circuit being probed. Therefore, a DMM with at least 10 megohms input impedance is required to ensure accurate voltage readings.

The PCM controls output circuits such as the injectors, cooling fan relays, etc. by controlling the ground or the power feed circuit through transistors or a device called an Output Driver Module (ODM).

Torque Management

Some vehicles are programmed with a Torque Management function. The Torque Management is a function of the PCM that reduces engine power under certain conditions. Torque Management is performed for the following reasons

To prevent overstress of the powertrain components.
To reduce engine power during certain Throttle Actuator Control (TAC) system faults.
To limit the engine power when the brakes are applied more than approximately 40 percent.
To prevent damage to the vehicle during certain abusive maneuvers.

The PCM monitors the following sensors and engine parameters to calculate engine output torque

Air/Fuel ratio.
Mass Air Flow (MAF).
Manifold Absolute Pressure (MAP).
Intake Air Temperature (IAT).
Spark Advance.
Engine Speed Engine.
Engine Coolant Temperature (ECT).
A/C Clutch Status.

The PCM monitors the torque converter status, the transmission gear ratio, and the extended brake switch input in order to determine if torque reduction is required. The PCM retards the spark as appropriate to reduce engine torque output if torque reduction is required. The PCM also shuts off the fuel to certain injectors to reduce the engine power in the case of an abusive maneuver.

The following are instances when engine power reduction is likely to be experienced

During transmission upshifts and downshifts.
Heavy acceleration from a standing start.
The brakes are applied with moderate to heavy throttle (with the traction system active).
When the driver is performing harsh or abusive maneuvers, such as shifting into gear at high throttle angles or shifting the transmission from Reverse to Drive to create a rocking motion.

The driver is unlikely to notice the torque management actions in the first two instances. The engine power output will be moderate at full throttle in the other two cases.

The PCM calculates the amount of spark retard necessary to reduce the engine power by the desired amount. The PCM disables the fuel injectors for cylinders No. 1, 4, 6, and 7 in the case of an abusive maneuver.

INPUT DEVICES

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

Vehicles are equipped with different combinations of input devices. Not all devices are used on all models. To determine the input devices used on a specific model, see WIRING DIAGRAMS article, or see COMPONENT LOCATIONS in appropriate SYSTEM & COMPONENT TESTING article. The available input signals include the following

A/C Request Signal

The air conditioner mode selector is mounted on instrument panel. This mode selector provides a simple "on" (A/C request) signal which is monitored by the PCM. PCM uses this signal to determine control of A/C clutch relay (if equipped) and to adjust idle speed when A/C compressor clutch is engaged. On some models, PCM may also activate electric cooling fan when this signal is present. If this signal is not present on A/C-equipped vehicles, vehicle may idle rough when A/C compressor cycles.

A/C Pressure Sensor

Some models are equipped with an air conditioner pressure sensor which is used to inform PCM of A/C system pressure. PCM uses this signal to determine A/C compressor load on the engine to control idle speed with IAC valve. Failure in A/C pressure sensor circuit or with A/C pressure sensor should set a related diagnostic trouble code and A/C compressor clutch will become inoperative. A fixed high pressure value will exist if the ground circuit to sensor is faulty.

A/C Pressure Switches

A/C high and low pressure switches may be used in the PCM-monitored A/C request signal circuit. Switches are normally closed, completing the circuit between ignition and PCM. PCM will engage or disengage A/C clutch relay based upon status of this circuit. When system refrigerant pressure increases beyond a certain point, high side switch will open, causing A/C request line voltage to drop. If system refrigerant level decreases, causing refrigerant pressure to drop below normal, low side pressure switch will open, once again causing A/C request line voltage to drop. Switches may be used as normal clutch cycling devices or as safety devices which prevent compressor damage in the event of excessively high or low refrigerant pressure.

A/C Temperature Sensors

Air conditioner high side and low side temperature sensors inform PCM of A/C system temperature levels. Low temperature signal will cause A/C compressor to disengage. High temperature levels help PCM determine control of A/C compressor relative to cooling fans and idle speed.

Accelerator Pedal Position Sensor (3.0L)

The accelerator pedal position sensors 1 and 2 are located on the accelerator pedal assembly. The 3.0L engine does not use a traditional throttle cable between the accelerator pedal and throttle body. Instead, these two sensors are used to relay accelerator pedal angle input to the Engine Control Module (ECM). These two sensors are two potentiometers that vary resistance depending on pedal movement. As the accelerator pedal moves from rest to the full travel position, the APP sensor 1 signal voltage increases from 0.40 to 3.82 volts (+/- 0.15 volts), while the APP sensor 2 signal voltage increases from 0.22 to 1.92 volts. APP sensor 2 signal voltage is always one-half the value of APP sensor 1 throughout the entire pedal range.

The ECM continuously monitors each APP sensor circuit for low and high voltage faults as well as performing a comparison check between each signal. If an error in one of the sensor signals were detected, the ECM would default to one of the two limp-home modes (limited pedal range (about 10 percent) with slow acceleration). The vehicle would still be able to travel at highway speeds, as the ECM would now use the opposite sensor for the driver's pedal input.

Accelerator Pedal Position Sensor (5.7L)

The Accelerator Pedal Position (APP) sensor is mounted on the accelerator pedal assembly. The APP is actually 3 individual accelerator pedal position sensors within 1 housing. Three separate signal, low reference, and 5-volt reference circuits are used in order to connect the APP and the TAC module. The APP sensor 1 voltage should increase at the same time that the accelerator pedal is depressed, from below 1 volt at 0 percent pedal travel to above 2 volts at 100 percent pedal travel. APP sensor 2 voltage should decrease from above 4 volts at 0 pedal travel to below 2.9 volts at 100 percent pedal travel. APP sensor 3 voltage should decrease from above 3.8 volts at 0 pedal travel to below 3.1 volts at 100 percent pedal travel.

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 enrich the air/fuel mixture. If voltage swings excessively high or low, PCM may set a charging system DTC and illuminate the MIL.

Brake Switch Feedback

Models equipped with cruise control systems may monitor the brake switch circuit to determine when to engage and disengage cruise control. On vehicles equipped with a Torque Converter Clutch (TCC), the brake switch provides an input to the PCM to control the TCC solenoid located in the transmission/transaxle.

Camshaft Position Sensor (2.2L & 2.4L)

The Camshaft Position (CMP) sensor signal is a digital ON/OFF pulse, output once per revolution of the camshaft. The CMP sensor does not directly affect the operation of the ignition system. The CMP sensor information is used by the Powertrain Control Module (PCM) to determine the position of the valve train relative to the crankshaft. By monitoring the CMP and CKP signals the PCM can accurately time the operation of the fuel injectors. The CMP sensor is connected to the PCM by a 12-volt, low reference, and signal circuit.

Camshaft Position Sensor (3.0L)

The Camshaft Position (CMP) sensor is located to the left of the cylinder 2, 4 and 6 timing belt cover/exhaust camshaft sprocket. The CMP sensor is a hall effect switching device used to determine the position of the bank 2 exhaust camshaft. The CMP sensor detects a single tooth on the reluctor wheel of the camshaft, which denotes 90 degrees before top dead center cylinder No. 1 compression stroke. The sensor is used by the ECM to determine when cylinder No. 1 is approaching top dead center necessary to sycronize the correct firing order. The CMP sensor is also used to enable sequential or independent fuel injection necessary for lowering tailpipe emissions, improving driveability and to enable spark knock control.

As the reluctor wheel tooth rotates past the sensor, the sensors internal hall effect device pulls the signal circuit to ground. Therefore, the ECM expects to see one high (5 volts) to low (0 volts) voltage transition once every two crankshaft rotations as the reluctor tooth passes the sensor. The signal circuit should be at 5 volts at all times except when the transition occurs. If an error occurs in the CMP sensor circuit, the ECM will take no default actions. However, will attempt to make a good guess on which mating cylinder to provide spark and fuel to based on where the engine had stopped on the last ignition cycle. Therefore, if the ECM choose incorrectly, the engine may or may not start on the next ignition cycle. If the CMP sensor signal is lost during the ignition cycle, the ECM will still provide the correct spark and fuel in the appropriate firing order.

Camshaft Position Sensor (3.1L & 3.4L)

A 3-wire Camshaft Position (CMP) sensor is located at top of timing cover, behind water pump. As camshaft sprocket turns, sensor magnet activates a Hall Effect switch in CMP sensor. This signal is generated whenever cylinder No. 1 is at TDC of its intake stroke.

Camshaft Position (CMP) sensor The CMP sensor signal is a digital ON/OFF pulse, output once per revolution of the camshaft. The CMP sensor does not directly affect the operation of the ignition system. The CMP sensor information is used by the PCM to determine the position of the valve train relative to the crankshaft position. By monitoring the CMP and CKP signals the PCM can accurately time the operation of the fuel injectors. The PCM supplies the sensor with a 12-volt reference, low reference, and signal circuit.

Camshaft Position Sensor (3.5L, 4.0L, 4.6L & 5.7L)

The Camshaft Position (CMP) sensor is also a magneto resistive sensor, with the same type of circuits as the Crankshaft Position (CKP) sensor. The CMP sensor signal is a digital ON/OFF pulse, output once per revolution of the camshaft. The CMP sensor information is used by the PCM to determine the position of the valve train relative to the crankshaft position.

The camshaft reluctor wheel is either pressed onto the camshaft or part of the timing gear depending on the application. The feature (or target) is read in a radial or axial fashion respectively. The wheel is a smooth track, half of which is of a lower profile than the other half. This feature allows the CMP sensor to supply a signal as soon as the key is turned ON, since the CMP sensor reads the track profile, instead of a notch.

Camshaft Position Sensor (3.8L)

The Camshaft Position (CMP) sensor signal is a digital ON/OFF pulse, output once per revolution of the camshaft. The CMP sensor does not directly affect the operation of the ignition system. The CMP sensor information is used by the Powertrain Control Module (PCM) to determine the position of the valve train relative to the CKP. By monitoring the CMP and CKP signals the PCM can accurately time the operation of the fuel injectors. The CMP sensor shares 12-volt and low reference circuits with the CKP sensor. The CMP signal circuit is input to the ICM.

Crankshaft (7X) Sensor (1.9L, 2.2L & 2.4L)

The CKP sensor is a permanent magnet generator, known as a variable reluctance sensor. The magnetic field of the sensor is altered by a crankshaft mounted reluctor wheel that has seven machined slots, six of which are equally spaced 60 degrees apart. The seventh slot is spaced 10 degrees after one of the 60 degree slots. The CKP sensor produces seven pulses for each revolution of the crankshaft. The pulse from the 10 degree slot is known as the sync pulse. The sync pulse is used to synchronize the coil firing sequence with the CKP.

Crankshaft Sensor (3.0L)

The Crankshaft Position (CKP) sensor is located underneath and slightly to the left of the oil filter housing. The CKP sensor produces an AC voltage of different amplitude and frequency depending on the velocity of the crankshaft reluctor wheel. The crankshaft reluctor wheel contains 58 teeth that are 6 degrees apart with a 12 degree span that is uncut. This 12 degree span is used to locate the cylinder No. 1 top dead center piston position used for engine syncronization. The CKP sensor is conjunction with the Camshaft Position (CMP) sensor can properly syncronize spark timing, fuel timing and spark knock control.

The large numbers of teeth on the reluctor wheel are used to correctly detect engine misfires. The ECM automatically learns the variation between all of the 58 teeth under 24 different engine speed/load ranges. To correctly detect misfires, the ECM will monitor the time it takes to pass 20 of the teeth (120 degrees of crankshaft rotation) after a cylinder has fired. If the time (based on engine speed and load) to pass 20 teeth is too long, a cylinder misfire has occurred.

Crankshaft (3X/7X & 24X) Sensor (3.1L & 3.4L)

Crankshaft Position (CKP) sensors The CKP sensor B is a variable reluctance sensor. The magnetic field of the sensor is altered by a crankshaft mounted reluctor wheel that has seven machined slots, six of which are equally spaced 60 degrees apart. The seventh slot is spaced 10 degrees after one of the 60 degree slots. This sensor provides the Ignition Control Module (ICM) with 7X signals, or seven pulses for each revolution of the crankshaft. The pulse from the 10 degree slot is known as the sync pulse. Both of the sensor circuits are connected to the Ignition Control Module (ICM). A signal converter within the ICM produces digital 3X output pulse to the Powertrain Control Module (PCM), the 3X reference is known as the low resolution engine speed signal. The CKP sensor A contains a Hall Effect switch. A Hall Effect switch is a solid state switching device that produces a digital ON/OFF pulse when a rotating element passes between the sensor tip and a magnet. This rotating element is called an interrupter ring or blade. In this case the interrupter ring has 24 evenly spaced blades and windows and is part of the crankshaft damper assembly. This sensor provides the PCM with 24X signals, or 24 identical pulses per crankshaft revolution. The 24X signal is used for enhanced smoothness and idle stability at a lower calibrated RPM. The PCM supplies the sensor with a 12-volt reference, low reference, and signal circuit.

Crankshaft Sensor (3.5L, 4.0L & 4.6L)

The Crankshaft Position (CKP) sensor is a three wire sensor based on the magneto resistive principle. A magneto resistive sensor uses two magnetic pickups between a permanent magnet. As an element such as a reluctor wheel passes the magnets the resulting change in the magnetic field is used by the sensor electronics to produce a digital output pulse. This system uses two sensors within the same housing for the V6 engine, and two separate sensors for the V8 engine. The PCM supplies each sensor a 12-volt reference, low reference, and a signal circuit. The signal circuit returns a digital ON/OFF pulse 24 times per crankshaft revolution.

The crankshaft reluctor wheel is part of the crankshaft. The notches on the reluctor wheel provide a unique pattern for each pair of cylinders that are at Top Dead Center (TDC) at the same time. This is known as pulse width encoding. This pulse width encoded pattern allows the PCM to quickly recognize which pair of cylinders are at TDC.

Crankshaft (3X/18X) Sensor (3.8L)

The Crankshaft Position (CKP) sensor contains 2 Hall Effect switches in 1 housing. A Hall Effect switch is a solid state switching device that produces a digital ON/OFF pulse when a rotating element passes the sensor pick-up and interrupts the sensors magnetic field. The rotating element is called an interrupter ring or blade. In this case there are two interrupter rings built into the crankshaft balancer. The outer ring and outer switch provides the Ignition Control Module (ICM) with 18X signals, or 18 identical pulses per crankshaft revolution. The inner ring and inner switch provides the ICM with 3 pulses per revolution, each 1 of different duration. This is called the sync pulse, each pulse represents a pair of companion cylinders. The ICM supplies a 12-volt and low reference circuit to the CKP sensor, and uses the 18X and sync pulses to determine the crankshaft position, by counting how many ON-OFF 18X pulses occur during a given sync pulse. With this dual interrupter ring arrangement the ICM can identify the correct pair of cylinders to fire within as little as 120 degrees of crankshaft rotation.

Crankshaft (4X/24X) Sensor (5.7L)

The Crankshaft Position (CKP) sensor is a three wire sensor based on the magneto resistive principle. A magneto resistive sensor uses two magnetic pickups between a permanent magnet. As an element such as a reluctor wheel passes the magnets the resulting change in the magnetic field is used by the sensor electronics to produce a digital output pulse. The PCM supplies a 12-volt, low reference, and signal circuit to the CKP sensor. The sensor returns a digital ON/OFF pulse 24 times per crankshaft revolution.

The crankshaft reluctor wheel is mounted on the rear of the crankshaft. The wheel is comprised of four 90 degree segments. Each segment represents a pair of cylinders at TDC , and is further divided into six 15 degree segments. Within each 15 degree segment is a notch of 1 of 2 different sizes. Each 90 degree segment has a unique pattern of notches. This is known as pulse width encoding. This pulse width encoded pattern allows the PCM to quickly recognize which pair of cylinders are at TDC. The reluctor wheel is also a dual track-or mirror image-design. This means there is an additional wheel pressed against the first, with a gap of equal size to each notch of the mating wheel. When one sensing element of the CKP sensor is reading a notch, the other is reading a set of teeth. The resulting signals are then converted into a digital square wave output by the circuitry within the CKP sensor.

Engine Coolant Temperature Sensor

The Engine Coolant Temperature (ECT) sensor is a thermistor (temperature sensitive resistor) located in an engine coolant passage. The PCM supplies and monitors a 5-volt signal to ECT sensor through a resistor in PCM. This monitored 5-volt signal is then reduced by resistance of the engine coolant temperature. When coolant temperatures are low, ECT sensor resistance is high, and a high monitored voltage signal is seen by the PCM. When coolant temperatures are high, ECT sensor resistance is low, and a low monitored voltage is seen by the PCM. After engine start-up, temperature should rise steadily to about 194°F (90°C), then stabilize when thermostat opens.

Engine coolant temperature signal is used in the control of most systems the PCM controls (i.e., fuel delivery, ignition timing, idle speed, emission control devices). After a vehicle has been parked overnight, ECT and IAT sensor signals (resistance and temperature) should be close to same reading. An ECT sensor which is out of calibration will not set a diagnostic trouble code but will cause fuel delivery and driveability problems. Failure in ECT sensor circuit (open or short to ground) will cause monitored voltage to swing high or low and should set a related diagnostic trouble code.

Fuel Level Sensor

PCM uses fuel level sensor input to determine expected amount of fuel vapor pressure or vacuum within the fuel tank. Scan tool can display fuel level in percent for diagnostic purposes. A problem in this circuit will set a related diagnostic trouble code.

Fuel Tank Pressure Sensor

Fuel Tank Pressure (FTP) sensor is similar to MAP sensor. It is used to measure the difference between the air pressure or vacuum in the fuel tank and outside air pressure. PCM supplies a 5-volt reference and ground to the sensor, and sensor sends a voltage signal back to the PCM.

Gear Switches

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

Generator "L" Light Circuit

The control module controls generator charging by supplying a voltage (5 volts or 12 volts) on the generator "L" terminal circuit whenever the ignition is turned on or the engine is running. This control voltage is necessary for the generator to charge once the generator begins to spin. With the ignition on, engine off, the generator will ground the generator "L" terminal circuit through a resistor. When the engine is started and the generator begins to charge, the generator will open the circuit (still using the control voltage) signaling to the ECM that the generator is charging. If the voltage at the control module generator "L" terminal is low when the engine is running, the control module will send a message to the instrument panel cluster to turn on the charge telltale.

Generator "F" Field Circuit

PCM monitors the duty cycle of the generator through the "F" circuit. As generator load increases, PCM can adjust idle speed accordingly.

Ignition/Crank Signal

The PCM monitors initial cranking (RPM) signal to determine when the engine is being started. This information is used for starting enrichment. If this signal is intermittent or not available, hard starting or a no-start condition will result.

Intake Air Temperature Sensor

The Intake Air Temperature (IAT) sensor is a thermistor (temperature sensitive resistor) mounted in the intake manifold. The PCM supplies and monitors a 5-volt signal to IAT sensor through a resistor in PCM. This monitored 5-volt signal is then reduced by resistance of the intake air temperature. Low intake air temperature produces high resistance, while high intake air temperature produces low resistance. By monitoring this voltage, PCM determines manifold air temperature. IAT sensor signal is used to make fuel control calculations according to incoming air density.

Intake air temperature should read close to ambient temperature with engine cold, and rise as underhood temperature increases. After a vehicle has been parked overnight, IAT and ECT sensor signals (resistance and temperature) should be close to the same reading. Failure in IAT sensor circuit (open or short to ground) will cause monitored voltage to swing high or low and should set a related DTC.

Knock Sensor

The Knock Sensor (KS) system enables the Powertrain Control Module (PCM) to control the ignition timing advance for the best possible performance while protecting the engine from potentially damaging levels of detonation. The sensors in the KS system are used by the PCM as microphones to listen for abnormal engine noise that may indicate pre-ignition/detonation.

There are 2 types of KS currently being used

The broadband single wire sensor.
The flat response 2-wire sensor.

Both sensors use piezo-electric crystal technology to produce and send signals to the PCM. The amplitude and frequency of this signal will vary constantly depending on the vibration level within the engine. Flat response and broadband KS signals are processed differently by the PCM. The major differences are outlined below

All broadband sensors use a single wire circuit. Some types of controllers will output a bias voltage on the KS signal wire. The bias voltage creates a voltage drop the PCM monitors and uses to help diagnose KS faults. The KS noise signal rides along this bias voltage, and due to the constantly fluctuating frequency and amplitude of the signal, will always be outside the bias voltage parameters. Another way to use the KS signals is for the PCM to learn the average normal noise output from the KS. The PCM uses this noise channel, and KS signal that rides along the noise channel, in much the same way as the bias voltage type does. Both systems will constantly monitor the KS system for a signal that is not present or falls within the noise channel.
The flat response KS uses a 2-wire circuit. The KS signal rides within a noise channel which is learned and output by the PCM. This noise channel is based upon the normal noise input from the KS and is known as background noise. As engine speed and load change, the noise channel upper and lower parameters will change to accommodate the KS signal, keeping the signal within the channel. If there is knock, the signal will range outside the noise channel and the PCM will reduce spark advance until the knock is reduced. These sensors are monitored in much the same way as the broadband sensors, except that an abnormal signal will stay outside of the noise channel or will not be present.

KS diagnostics can be calibrated to detect faults with the KS diagnostic inside the PCM, the KS wiring, the sensor output, or constant knocking from an outside influence such as a loose or damaged component. In order to determine which cylinders are knocking, the PCM uses KS signal information when the cylinders are near Top Dead Center (TDC) of the firing stroke.

Manifold Absolute Pressure Sensor

The Manifold Absolute Pressure (MAP) sensor measures changes in manifold pressure. 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 (less than 2.0 volts at idle to about 4.0 volts key on engine not running at WOT). The PCM can monitor these signals and adjust air/fuel ratio and ignition timing under various operating conditions.

The Mass Airflow (MAF) sensor measures flow of air entering the engine in grams per second. This measurement of airflow is a reflection of engine load (throttle opening and air volume), similar to the relationship of engine load to MAP or vacuum sensor signal. MAF signal should remain relatively constant at cruise, gradually changing with throttle angle and rapidly changing on sudden acceleration. PCM uses MAF information to control fuel delivery. Sensor produces a frequency signal which is proportional to airflow. Failure in MAF sensor circuit should set a related diagnostic trouble code.

Oxygen Sensor (Sensor 1 - 1.9L)

CAUTION

DO NOT attempt to measure oxygen sensor output voltage using a conventional voltmeter. Current drain of voltmeter could damage sensor. Oxygen sensor voltage signal can be measured using a 10-megohm (minimum input impedance) digital voltmeter.

The 02S-1 is located in the exhaust manifold and is used by the PCM to make fuel control corrections toward a 14.7 to 1 air/fuel ratio. The 02S-1 is an electrical source that responds to oxygen content in the exhaust manifold. When the sensor reaches approximately 316°C (600°F), it produces a voltage based on the difference in oxygen between the atmosphere and exhaust gas. The PCM sends a bias voltage (391-491 mV) on the signal line which can be read on the Scan tool when the sensor is cold. When the 02S-1 is cold, it produces no voltage and has extremely high internal resistance. However, when the sensor heats up, it produces voltage that overrides the bias voltage. This voltage is read by the PCM to determine a rich/lean 02S-1 signal used to adjust injector pulse width. Under normal conditions, low sensor voltage means high oxygen content/lean air/fuel mixture and vice versa. Normal sensor readings will fluctuate between 10 mV and 999 mV.

Oxygen Sensors (Except Sensor 1 - 1.9L & Sensor 1 - 3.0L)

The main function of the fuel control Heated Oxygen Sensor (HO2S) is to provide the control module with exhaust stream information in order to allow proper fueling and maintain emissions within the mandated levels. After the sensor reaches the operating temperature, the sensor generates a voltage inversely proportional to the amount of oxygen present in the exhaust gases.

The control module uses the signal voltage from the fuel control heated oxygen sensors in a Closed Loop in order to adjust the fuel injector pulse width. While in a Closed Loop, the control module can adjust fuel delivery in order to maintain an air/fuel ratio which allows the best combination of emission control and driveability.

If the oxygen sensor pigtail wiring, connector, or terminal are damaged, replace the entire oxygen sensor assembly. Do not attempt to repair the wiring, connector, or terminals. In order for the sensor to function properly, the sensor must have a clean air reference. This clean air reference is obtained by the oxygen sensor wires. Any attempt to repair the wires, the connectors, or the terminals could result in the obstruction of the air reference. Any attempt to repair the wires, the connectors, or the terminals could degrade oxygen sensor performance.

In order to control emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), the system uses a 3-way catalytic converter. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting the HC and the CO into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting the NOx into nitrogen.

See THROTTLE ACTUATOR CONTROL (TAC) SYSTEM .

Torque Converter Clutch

See MISCELLANEOUS CONTROLS .

Fuel Pump

The fuel pump is mounted in the fuel sender assembly reservoir. The fuel pump is an electric high pressure pump. Fuel is pumped to the fuel rail at a specified flow and pressure. Excess fuel from the fuel rail assembly returns to the fuel tank through the fuel return pipe. The fuel pump delivers a constant flow of fuel to the engine even during low fuel conditions and aggressive vehicle maneuvers. The PCM controls the electric fuel pump operation through a fuel pump relay. The fuel pump flex pipe acts to dampen the fuel pulses and noise generated by the fuel pump.

Fuel Pump Relay

When ignition switch is turned to the ON position, PCM will turn on electric fuel pump by energizing the fuel pump relay. PCM will keep relay energized if engine is running or cranking (PCM is receiving reference pulses from ignition control module). If no reference pulses exist, PCM turns pump off within 2 seconds after key on.

For additional information on fuel pump activation, see appropriate BASIC DIAGNOSTIC PROCEDURES and SYSTEM & COMPONENT TESTING articles.

Fuel Pressure Regulator

The fuel pressure regulator is a diaphragm relief valve. The diaphragm has fuel pressure on one side and regulator spring pressure and intake manifold vacuum on the other side. The fuel pressure regulator maintains a constant pressure differential across the fuel injectors at all times. The pressure regulator compensates for engine load by increasing fuel pressure as the engine vacuum drops.

FUEL CONTROL (EXCEPT 3.0L)

The Powertrain Control Module (PCM) monitors voltages from several sensors in order to determine how much fuel to give the engine. The fuel is delivered under one of several conditions called modes. The PCM controls all modes.

Starting Mode

With the 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 Mass Air Flow (MAF) sensor. The PCM also uses the Engine Coolant Temperature (ECT), the Throttle Position (TP), and the Manifold Absolute Pressure (MAP) sensors to determine the proper air/fuel ratio for starting. The PCM controls the 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.

Clear Flood Mode

If the engine floods, clear the engine by pressing the accelerator pedal down to the floor and then crank the engine. When the Throttle Position (TP) sensor is at wide open throttle, the PCM reduces the injector pulse width in order to increase the air to fuel ratio. The PCM holds this injector rate as long as the throttle stays wide open and the engine speed is below a predetermined RPM. If the throttle is not held wide open, the PCM returns to the starting mode.

Run Mode

The run mode has 2 conditions called Open Loop and Closed Loop. When the engine is first started and the engine speed is above a predetermined RPM, the system begins Open Loop operation. The PCM ignores the signal from the Heated Oxygen Sensor (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 meeting the following conditions

Both HO2S have varying voltage output, showing that they are hot enough to operate properly. This depends upon the engine temperature.
The ECT sensor is above a specified temperature.
A specific amount of time has elapsed after starting the engine.

Specific values for the above 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.

Acceleration Mode

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, the PCM increases the pulse width to the injectors to provide extra fuel during acceleration. The PCM determines the amount of fuel required based upon the TP, the coolant temperature, the MAP, the MAF, and the engine speed.

Deceleration Mode

When the driver releases the accelerator pedal, air flow into the engine is reduced. The PCM reads the corresponding changes in TP, MAP, and MAF. 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.

Battery Voltage Correction Mode

When the battery voltage is low, the PCM compensates for the weak spark delivered by the ignition system in the following ways

Increasing the amount of fuel delivered.
Increasing the idle RPM.
Increasing the ignition dwell time.

Fuel Cut-Off Mode

The PCM cuts OFF fuel from the fuel injectors when the following conditions are met in order to protect the powertrain from damage and improve driveability

The ignition is OFF. This prevents engine run-on.
The ignition is ON but there is no ignition reference signal. This prevents flooding or backfiring.
The engine speed is too high, above redline.
The vehicle speed is too high, above rated tire speed.
During an extended, high speed, closed throttle coast down. This reduces emissions and increases engine braking.
During extended deceleration, in order to protect the catalytic converters.

Short Term Fuel Trim

The short term fuel trim values change rapidly in response to the HO2S signal voltages. These changes "fine tune" the engine fueling. The ideal fuel trim values are around 0 percent. A positive fuel trim value indicates that the PCM is adding fuel in order to compensate for a lean condition. A negative fuel trim value indicates that the PCM is reducing the amount of fuel in order to compensate for a rich condition. When the PCM determines that the short term fuel trim is out of the operating range, the following DTCs will set

DTC P0171: Bank 1 Too Lean
DTC P0172: Bank 1 Too Rich

Long Term Fuel Trim

The long term fuel trim is a matrix of cells arranged by RPM and Manifold Absolute Pressure (MAP). As the engine operating conditions change, the PCM will switch from cell to cell to determine what long term fuel trim factor to use in the base pulse width equation.

While in any given cell, the PCM also monitors the short term fuel trim. If the short term fuel trim is far enough from 0 percent, the PCM will change the long term fuel trim value. Once the long term fuel trim value is changed, the long term fuel trim should force the short term fuel trim back toward 0 percent. If the mixture is still not correct, the short term fuel trim will continue to have a large deviation from the ideal 0 percent. In this case, the long term fuel trim value will continue to change until the short term fuel trim becomes balanced. Both the short term fuel trim and long term fuel trim have limits which vary by calibration. If the mixture is off enough so that long term fuel trim reaches the limit of long term fuel trim control and still cannot correct the condition, the short term fuel trim would also go to the short term fuel trim limit of control in the same direction. If the mixture is still not corrected by both short term fuel trim and long term fuel trim at short term fuel trim and long term fuel trim extreme values, a fuel trim Diagnostic Trouble Code (DTC) will likely result. When the PCM determines that the long term fuel trim is out of the operating range, the following DTCs will set

DTC P0171: Bank 1 Too Lean
DTC P0172: Bank 1 Too Rich

Under the conditions of power enrichment, the PCM sets the short term fuel trim to 0 percent until power enrichment is no longer in effect. This is done so the Closed Loop factor and the long term fuel trim will not try to correct for the power enrichment condition.

FUEL CONTROL (3.0L)

The 3.0L Dual Overhead Cam (DOHC) engine utilizes Sequential Fuel Injection (SFI). SFI allows the ECM to individually control each fuel injector, which optimizes fuel economy, lowers tailpipe emissions and increases performance. The ECM pulse width modulates (PWM) each fuel injector by individually grounding each fuel injector circuit.

The ECM bases its fuel injector pulse width (the amount of fuel the engine needs) on three main parameters

Temperature of the air/fuel mixture at the intake valve. Calculation is based on the Engine Coolant Temperature (ECT) sensor and Intake Air Temperature (IAT) sensor, which is integral to the Mass Airflow (MAF) sensor.
Engine speed from the Crankshaft Position (CKP) sensor.
Engine load. Calculation is based on the Mass Airflow (MAF) sensor. The MAP sensor will be used as a default if the MAF sensor signal is not valid.

These parameters allow the ECM to calculate a base fuel injector pulse width when the system is in open loop. Open loop (for a specific bank) is when the ECM is not using the heated oxygen sensor 1 (H02S-1, pre catalyst oxygen sensor) to modify fuel.

The 3.0L H02S-1 (bank 1 or 2) are air/fuel ratio sensors that are not of the traditional switching type. These sensors allow a wider range of fuel control (8:1 up to 18:1) accuracy, which allow the ECM to remain in closed loop under all engine running conditions (except extended decelerations). When the H02S-1 is above 275°F, it will begin to allow 02 ions to pass across its diffusion plates, which will allow the supplied output pump current to start flowing through the sensor. Some of the output pump current is returned to the ECM through the input pump current circuit to limit the current through the sensor. As the air/fuel ratio changes, the sensor current draw (resistance) will vary. The ECM will attempt to maintain a fixed voltage on the signal line at a certain air/fuel ratio by varying the output pump current. When the output current and signal voltage reach a certain level for a commanded air/fuel ratio, the ECM will have reached its A/F ratio target.

The Scan tool displays a lambda value for the H02S bank 1 sensor 1 and H02S bank 2 sensor 1 used to denote the actual air/fuel ratio of each bank. Lambda signifies the actual air/fuel ratio measured by the sensor divided by 14.7. A lambda value of 1.00 means that a specific bank of cylinders is running at 14.7 to 1. When the exhaust gas has high oxygen content, the air/fuel mixture is lean and the H02S-1 lambda value will be high (around 1.2). To compensate, the ECM will command rich or increase the fuel injector pulse width. When the exhaust gas has low oxygen content, the air/fuel mixture is rich and the H02S-1 lambda value will be low (around 0.8). To compensate, the ECM will decrease the amount of fuel by reducing the injector pulse width. The ECM will normally fluctuate rich to lean around 14.7 to 1 for improved catalytic converter efficiency. This is not as drastic as the traditional switching type sensors, however.

The ECM has the ability to adapt fuel control based on previous H02S-1 signals. The Short Term Fuel Trim (STFT) value is used to adapt fuel control over a short period of time. A value of 0% is the nominal STFT value the engine should be running at. If the engine is running at 0% in closed loop, the ECM does not have to modify fuel to obtain the desired air/fuel ratio. The 0% value is based off of the calculation from the three main parameters. If for instance the vehicle is running rich, the STFT value will decrease causing the ECM to decrease the injector pulse width. The ECM will continue to do this until the H02S-1 indicates a lean condition. The same is true for the lean running condition.

The Long Term Fuel Trim (LTFT) values are based on the STFT values. There are three different engine load ranges: idle, medium load and high load that the ECM uses for fuel adaptation. When the vehicle is in one of these conditions, it will use the LTFT adaptive fuel correction value that it has stored. For instance, the vehicle could be running lean at idle, but be rich while cruising under medium load. So if the vehicle is cruising then comes down to idle, the ECM will automatically increase the injector pulse width according to the idle LTFT value. When in fuel adaptation, the ECM will take into account that a richer mixture will result when the EVAP purge solenoid is commanded ON to purge fuel vapors from the EVAP canister to the intake manifold.

To obtain a reading of how the vehicle is running overall, the LTFT values should be used while in one of the three driving states: idle, cruise and accel. To obtain a reading of how the vehicle is running at a particular instant, the STFT value should be used. The STFT and LTFT values can significantly aid in diagnosing a driveability concern if used properly.

Note. The ECM will use the H02S-2 (post catalyst oxygen sensor) from each bank to add or subtract the time it is holding the fuel control system rich or lean (this does NOT increase or decrease the amount of fuel). This technique is called fuel trim biasing which is used to improve tailpipe emissions.

IDLE SPEED

PCM controls engine idle speed based upon engine operating conditions. The PCM senses engine operating conditions and determines the best idle speed.

The Idle Air Control (IAC) valve controls engine idle speed during engine load changes to prevent stalling. The IAC valve is mounted on throttle body or on upper manifold assembly, and controls the amount of air by-passed around the throttle plate.

When engine is idling, PCM determines proper positioning of IAC valve based on battery voltage, engine coolant temperature, engine load and engine RPM. If engine RPM is too low, pintle is retracted and more air is by-passed around the throttle plate to increase engine RPM. If engine RPM is too high, pintle is extended and less air is by-passed around the throttle plate to decrease engine RPM.

If IAC valve is disconnected or connected with engine running, IAC loses its reference point and has to be reset. Resetting of IAC is accomplished on some models by turning ignition on and off. Problems in IAC circuit should set a related diagnostic trouble code.

The IAC valve affects only the idle system. If valve is stuck fully open, excessive airflow into the manifold creates a high idle speed. Valve stuck closed allows insufficient airflow, resulting in low idle speed. For calibration purposes, several different design IAC valves are used. Ensure proper design valve is used during replacement.

DIS (3.0L, 3.5L, 4.0L & 4.6L)

Camshaft Position (CMP) sensor.
Camshaft reluctor wheel.
Crankshaft Position (CKP) sensor.
Crankshaft reluctor wheel.
Ignition Coil/ICM assembly.
Powertrain Control Module (PCM).

Modes of Operation

Both CKP sensors provide identical pulses, although one signal is shifted several degrees of crankshaft rotation to the other. This amount depends on whether the sensors are separate or integrated with each other. The two CKP signals allow the PCM to perform an angle-based decode operation. This is considered a self-clocked system, where one sensor acts as a clock and the other is a data signal. The advantage of angle-based decoding is the increased accuracy and consistency of signals, even during engine acceleration and deceleration. If one sensor is not operating correctly, the PCM uses a time-based decode operation. This mode will read the pulse width of the remaining signal, and thereby provide a means of back-up with the minimum of performance loss. Diagnostic trouble codes are available to accurately diagnose the ignition system with a scan tool.

COIL-NEAR-PLUG IGNITION (5.7L)

Crankshaft Position (CKP) sensor.
Crankshaft reluctor wheel.
Camshaft Position (CMP) sensor.
Camshaft reluctor wheel.
Ignition coils.
Powertrain Control Module (PCM).

There is one normal mode of operation, with the spark under PCM control. If the CKP pulses are lost the engine will not run. The loss of a CMP signal may result in a longer crank time since the PCM cannot determine which stroke the pistons are on. Diagnostic trouble codes are available to accurately diagnose the ignition system with a scan tool.

COMPUTER CONTROLLED COIL IGNITION (3.8L)

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 cylinder 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 following components

Crankshaft Position (CKP) sensors.
Camshaft Position (CMP) sensor.
Ignition Control Module (ICM) and ignition coils.
Powertrain Control Module (PCM).

Anytime the PCM does not apply 5 volts to the IC timing signal circuit, the ICM controls ignition by triggering each coil in the proper sequence at a pre-calibrated timing advance. This is called Bypass Mode ignition used during cranking or running below a certain RPM, or during a default mode due to a system failure.

When the PCM begins receiving 18X reference and 3X reference pulses, the PCM applies 5 volts to the IC timing signal circuit. This signals the ICM to allow the PCM to control the spark timing. This is IC mode ignition. During IC mode, the PCM compensates for all driving conditions. If the IC mode changes due to a system fault, the system will stay in default until the ignition is cycled OFF to ON, or the fault is no longer present. Diagnostic trouble codes are available to accurately diagnose the ignition system with a scan tool.

ELECTRONIC IGNITION (1.9L)

The electronic ignition system on both Single Overhead Cam (SOHC) and Dual Overhead Cam (DOHC) engines provides spark energy to ignite the air/fuel mixture necessary for combustion. The Powertrain Control Module (PCM) controls spark under all engine running conditions. The system components include: the PCM, Electronic Ignition (EI) module/coil pack, spark plugs, spark plug wires and knock sensor. The spark dwell (On-time) and degrees of spark advance are dependent upon engine speed, Manifold Absolute Pressure (MAP), and Engine Coolant Temperature (ECT). The PCM can vary spark advance from 39 degrees BTDC to 3 degrees ATDC under all engine running conditions when no spark knock is present. The electronic ignition system on both SOHC and DOHC engines provides spark energy to ignite the air/fuel mixture necessary for combustion. The PCM controls spark under all engine running conditions. The system components include: the PCM, Electronic Ignition (EI) module/coil pack, spark plugs, spark plug wires and knock sensor. The spark dwell (On-time) and degrees of spark advance are dependent upon engine speed, MAP, and ECT. The PCM can vary spark advance from 39 degrees BTDC to 3 degrees ATDC under all engine running conditions when no spark knock is present.

IGNITION CONTROL

The primary function of the El module is to charge and discharge the coil packs based on PCM control. The PCM has two control circuits, one for the 2/3 coil and the other for the 1/4 coil. The PCM uses a high control signal of near 5 volts to charge up the coil and a low control signal of near 0 volts to discharge the coil. If the coil is charged and the control signal is low, the coil will fire through its secondary towers. The secondary voltage can reach a maximum of 40,000 volts.

The secondary current always travels in the same direction and in a series type circuit. For example, when the PCM fires the 1/4 coil, the current will flow out of the #1 coil tower, to the #1 spark plug wire, to the #1 spark plug, through the block, up through the #4 spark plug, through the #4 spark plug wire and back to the #4 coil tower. If one of the wires/plugs were to open, the other mating cylinder would still fire out of its coil tower because the circuit would be completed through the El module bolts.

In order to determine when to fire a cylinder, the PCM uses the Crankshaft Position (CKP) sensor. The crankshaft has 7 machined notches, 2 of which are close together representing a double pulse. The PCM uses this double pulse to identify cylinder #4 Top Dead Center (TDC). However, the PCM still has to identify whether cylinder #4 is on TDC compression or TDC exhaust. This is accomplished by the use of Compression Sense Ignition.

COMPRESSION SENSE IGNITION

Both the SOHC and DOHC engines utilize Compression Sense Ignition, which eliminates the need for a camshaft position sensor. The El module has sensing circuitry that detects when cylinder #4 has fired on its compression stroke and relays this information to the PCM. The PCM can then correctly synchronize the fuel injectors for sequential fuel injection.

The EI module uses capacitive pickup plates located under the 1/4 coil to determine when cylinder #4 has fired on compression. These plates are used to differentiate the polarity and voltage amplitude difference between the 1/4 secondary ignition circuits. Since each coil tower is of opposite polarity and the waste spark (2-4 kV) generally fires before the compression spark (10-25 kV), the module can determine cylinder #4 compression. When the EI module detects a positive to negative polarity sequence and a high negative voltage spike, it will pull the PCM 5-volt cam signal circuit to ground. The PCM knows that cylinder #4 had just fired on its compression stroke when this transition occurs.

The EI module, however, cannot always detect when cylinder #4 has fired on compression. These occurrences include

During deceleration.
Very low engine load conditions when engine is running.
If a secondary ignition problem occurs on cylinder #1 or #4.

Too few cam pulses (cam signal circuit not being pulled to ground) are a result of decreased cylinder #4 secondary resistance or increased cylinder #1 secondary resistance. Too many cam pulses (cam signal circuit being pulled to ground too often) are a result of decreased cylinder #1 secondary resistance or increased cylinder #4 secondary resistance.

SPARK KNOCK CONTROL

The PCM uses the knock sensor to determine when spark knock exists and can retard timing up to a maximum of 19 degrees. The knock sensor is a piezoelectric flat response (wide resonant band) device that produces an AC voltage of different amplitude and frequency based on engine mechanical vibration. The amplitude and frequency are dependent upon the level of knock the sensor detects.

The PCM learns a minimum noise level at idle from the knock sensor and uses stored normal noise level calibration values for the rest of the RPM band. The knock sensor signal is only used during the TDC combustion event of the firing cylinder. When in a combustion event, the PCM filters the knock signal and compares it to the normal calibration noise level for that RPM. If the PCM has determined that knock is present during the combustion event, it will retard timing on the next firing cylinders until the knock is eliminated. The PCM will always try to work back to a zero compensation level or no spark retard.

If knock is present, the PCM will increment three counters that can be read on the Scan tool. The LOW, MID and HIGH SPARK MODIFIERS represent three different RPM bands the PCM uses to store knock retard degrees. The Scan tool also displays the actual amount of spark retard degrees as SPARK RETARD CYL #1-4. If excessive spark knock is detected, the retarding of timing will cause a reduced power condition.

ELECTRONIC IGNITION (2.2L - VIN 4 & 2.4L)

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 following components

Crankshaft Position (CKP) sensor.
Camshaft Position (CMP) sensor.
Ignition Control Module (ICM) and ignition coils.
Powertrain Control Module (PCM).

There is one normal mode of operation, with the spark under PCM control. If the CKP pulses are lost, the engine will not run. The loss of a CMP signal may result in a longer crank time since the PCM cannot determine which stroke the pistons are on. DTCs are available to accurately diagnose the ignition system with a scan tool.

ELECTRONIC IGNITION (2.2L - VIN F)

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 its compression stroke is called the event cylinder. The cylinder that is at TDC of its 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 ignition coils and Ignition Control Module (ICM) are contained within one assembly. The ignition coil/ICM assembly is mounted in the center of the engine camshaft cover, with short boots connecting the coils to the spark plugs. The coil driver modules within the ICM are commanded ON/OFF by the Powertrain Control Module (PCM). The EI system consists of the following components

Crankshaft Position (CKP) sensor.
Ignition Control Module (ICM) and ignition coils.
Powertrain Control Module (PCM).

There is one normal mode of operation, with the spark under PCM control. If the CKP pulses are lost, the engine will not run. The loss of a CMP signal may result in a longer crank time since the PCM cannot determine which stroke the pistons are on. Diagnostic trouble codes are available to accurately diagnose the ignition system with a scan tool.

EMISSION SYSTEMS

Note. To determine emission systems usage, see appropriate EMISSION APPLICATIONS article.

SECONDARY AIR INJECTION SYSTEM

The secondary Air Injection (AIR) system helps reduce exhaust emissions. The system forces fresh filtered air into the exhaust stream in order to accelerate the catalyst operation. The system includes the following components

AIR Pump (Electric)

The AIR pump supplies filtered air through the secondary air injection system into the exhaust stream. The control module provides ground for the pump relay, then the battery voltage is applied to the pump. The filter is the only serviceable part of the pump.

AIR Shut Off Valve

The AIR shut off valve is vacuum operated. When the secondary air injection system is enabled, vacuum is applied to the valve. The vacuum opens the valve, and allows air from the AIR Pump to flow to the check valves.

AIR Vacuum Control Solenoid

The AIR vacuum control solenoid controls the AIR Shut Off Valve. When the secondary air injection system is enabled, the control module provides a ground to the solenoid. Enabling the solenoid allows the engine vacuum to be applied to the AIR shut off valve.

Check Valves

The check valves prevent back flow of the exhaust gases into the secondary air injection system. An inoperative AIR pump that had shown indications of exhaust gases in the outlet port would indicate a check valve failure.

Plumbing

The plumbing carries the air from the pump to the exhaust stream. The plumbing includes the hoses, the pipes, and the clamps. You can test the plumbing for leaks using a soapy water solution. With the AIR pump running, the bubbles with form if a leak exists.

Three-Way Catalytic Converter

A Three-Way Catalytic Converter (TWC) is used on all vehicles to reduce exhaust emissions. This type of converter reduces hydrocarbon (HC), carbon monoxide (CO) and oxides of nitrogen (NOx) levels.

Converter contains a reducing agent (Rhodium and Platinum) to reduce NOx and an oxidizing agent (Palladium and Platinum) to oxidize HC and CO. This causes HC and CO to oxidize (combine with oxygen) into the harmless base elements: water (H 2 O) and carbon dioxide (CO 2 ). Oxygen is removed from NOx, causing it to reduce to the harmless base elements nitrogen (N) and oxygen (O 2 ).

EXHAUST GAS RECIRCULATION

The Exhaust Gas Recirculation (EGR) system is designed to reduce oxides of nitrogen (NOx) emissions by lowering combustion temperatures. This is accomplished when a metered amount of exhaust gas is recirculated into the intake manifold and mixed with the air/fuel mixture.

Linear EGR System

The main element of the system is the linear EGR valve. The EGR valve feeds small amounts of exhaust gas back into the combustion chamber. With the fuel/air mixture diluted by the exhaust gases, combustion temperatures are reduced.

The linear EGR valve is designed to accurately supply EGR to an engine independent of intake manifold vacuum. The valve controls EGR flow from the exhaust to the intake manifold through an orifice with a Powertrain Control Module (PCM) controlled pintle. During operation, the PCM controls pintle position by monitoring the pintle position feedback signal. The feedback signal can be monitored with a scan tool as EGR position sensor. The EGR position sensor should always be near the commanded EGR position, called the Desired EGR Pos. The PCM uses information from the following sensors to control the pintle position

The Engine Coolant Temperature (ECT) sensor.
The Throttle Position (TP) sensor.
The Mass Air Flow (MAF) sensor.

The linear EGR valve is usually activated under the following conditions

Warm engine operation.
Above idle speed.

EVAPORATIVE EMISSION CONTROL

The Evaporative (EVAP) emission control system limits fuel vapors from escaping into the atmosphere. 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. The EVAP system consists of the following components

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.

EVAP Purge Solenoid

The EVAP purge solenoid controls the flow of vapors from the EVAP system to the intake manifold. 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.

EVAP Vent Solenoid

The EVAP vent solenoid controls fresh airflow 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 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 - low voltage. As FTP decreases, FTP voltage increases, low pressure or vacuum - high voltage.

EVAP Service Port

The EVAP service port is located in the EVAP purge pipe between the EVAP purge solenoid and the EVAP canister. The service port is identified by a Green colored cap.

POSITIVE CRANKCASE VENTILATION

The Positive Crankcase Ventilation (PCV) system is used to provide more effective elimination of crankcase vapors. Fresh air from the air filter housing or throttle body is supplied to the crankcase where it is mixed with blow-by gases and passed through a PCV valve into the intake manifold or supercharger inlet (3.8L VIN 1). This mixture is then passed into the combustion chamber and burned.

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

Under conditions in which abnormal amounts of blow-by gases are produced (such as worn cylinders or rings), the system is designed to allow the excess gases to flow back through crankcase vent hose into the intake or throttle body to be consumed during normal combustion.

DESCRIPTION

The PCM is equipped with a self-diagnostic system which detects system failures or abnormalities. When a malfunction occurs, PCM will illuminate the Malfunction Indicator Light (MIL) located on instrument cluster. When a malfunction is detected and MIL is illuminated, a corresponding Diagnostic Trouble Code (DTC) will be stored in PCM memory. Malfunctions are designated as either "emission related" or "non-emission related", and are divided into 4 code types to identify type of fault. The 4 code types are defined as follows

Type "A" Emission related faults that illuminate MIL at first occurrence of a fail condition.
Type "B" Emission related faults that illuminate MIL if a fault occurs in 2 consecutive ignition cycles.
Type "C" Non-emission related faults that do not illuminate MIL. Driver Information Center (DIC) may display a message.
Type "D" Non-emmission faults and diagnostic aids that do not turn on any telltale lights or messages.

Emission related DTCs (type "A" or "B") cause MIL to illuminate and remain on until the malfunction is repaired. If MIL illuminates and remains on during vehicle operation, cause of malfunction must be determined using affected Diagnostic Trouble Code (DTC) located in appropriate SELF-DIAGNOSTICS article. PCM records and updates failure records/operating conditions to Freeze Frame. If a sensor fails, PCM will use a substitute value in its calculations to continue engine operation. In this condition, vehicle is functional but impaired driveability is likely. Use a scan tool to clear codes and turn off MIL.

Non-emission related DTCs (type "C") do not illuminate the MIL, but may display a message on the Driver Information Center (DIC). The corresponding diagnostic trouble code, however, will be retained in PCM memory. PCM records operating conditions at time of failure into memory. If related fault does not reoccur within 40 warm-up cycles, related diagnostic trouble code will be erased from PCM memory. Intermittent failures may be caused by a sensor, connector or wiring related problem. See appropriate TROUBLE SHOOTING - NO CODES article.

As a bulb and system check, the Malfunction Indicator Light (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. To access codes, see appropriate SELF-DIAGNOSTICS article.

Several states require that a vehicle pass On-Board Diagnostic (OBD) system tests and the I/M emission inspection in order to renew license plates. This is accomplished by viewing the I/M System Status display on a scan tool. Using a scan tool, the technician can observe the I/M System Status in order to verify that the vehicle meets the criteria that complies with the local area requirements.

Conditions For Updating I/M System Status

Each system requires at least one, and sometimes several, diagnostic tests. The results of these tests are reported by a Diagnostic Trouble Code (DTC). A system monitor is complete when either all of the DTCs comprising the monitor have Run and Passed, or any one of the DTCs comprising the monitor have illuminated the Malfunction Indicator Lamp (MIL). Once all of the tests are completed, the I/M System Status display will indicate YES in the Completed column. For example, when the Heated Oxygen Sensor (HO2S) Heater Test indicates YES, all of the oxygen sensor heaters have been diagnosed. If the vehicle has four heated oxygen sensors, all four heater circuits have been diagnosed. The I/M System Status will indicate NO under the Completed column when any of the required tests for that system have not run. The following is a list of conditions that would set the I/M System Status indicator to NO

The vehicle is new from the factory and has not yet been driven through the necessary drive conditions to complete the tests.
The battery has been disconnected or discharged below operating voltage.
The control module power or ground has been interrupted.
The control module has been reprogrammed.
The control module DTCs have been cleared as part of a service procedure.

Monitored Emission Control Systems

The OBD II System monitors all emission control systems that are on-board. Not all vehicles have a full complement of emission control systems. For example, a vehicle may not be equipped with secondary Air Injection (AIR) or Exhaust Gas Recirculation (EGR). The OBD II regulations require monitoring of the following

Air conditioning system.
Catalytic converter efficiency.
Comprehensive component monitoring. Emission related inputs and outputs.
Evaporative (EVAP) emissions system.
Exhaust Gas Recirculation (EGR) system.
Fuel delivery system.
Heated catalyst monitoring.
Misfire monitoring.
Oxygen sensor (O2S or HO2S) system.
Oxygen sensor heater (HO2S heater) system.
Secondary Air Injection (AIR) system.

For specific monitor information, see appropriate SELF-DIAGNOSTICS article.

PCM is equipped with a serial data line. Serial data is a stream of electrical impulses which can be interpreted by special testers or other control modules. When the ignition is turned on, each module sends a state of health message to the other modules using the serial data line. Ensuring the modules are working properly. If a module stops communicating, other control modules expecting information from that module will set a DTC. Serial data and codes must be accessed using a scan tool connected to the Data Link Connector (DLC). Update intervals and information contained within the data stream vary with model application.

3.0L

The 3.0L engine does not use a traditional throttle cable found on most other vehicles. The Throttle Actuator Control (TAC) module, which is part of the throttle body assembly, contains a direct current actuator motor controlled by the ECM used to move the throttle plate (in either direction) and two throttle position sensors to determine throttle plate position. The throttle plate is held at a 7 percent rest position to a mechanical stop by a constant force return spring. This spring will hold the throttle plate to the stop when there is no current flowing to the actuator motor. There is another return spring, which creates constant force on the throttle plate only when the throttle plate is moved towards the full closed position.

Each time the ignition is turned on, the ECM will perform a closed throttle test. During this test, it will close the throttle from the 7 percent rest position to the full closed position and allow it to return. This is performed to ensure the springs have enough force to close the throttle in case of an actuator motor failure.

The ECM will also go through a throttle body relearn procedure 29 seconds after the ignition is turned on with the engine off as long as certain conditions have been met. See DTC P1526 in appropriate SELF-DIAGNOSTICS article for these conditions. During this learn procedure, the ECM will move the throttle from the rest to full closed position, then to around 14 percent. During this period, the TP sensors lower limits, rest position of the TP sensors, the actuator motor force to overcome both spring pressures as well as the return rates of both springs are learned. If any of these parameters are out of range, a DTC will be set. The scan tool contains the parameter TAC LEARN COUNTER, which can be monitored during the learn procedure. The counter should start at 0 then count up to 9 counts during the 3 second relearn procedure. If the counter starts at 0 and goes to 9, the sensors, springs and motor are okay. A relearn procedure is performed only once per ignition cycle. Anytime a throttle body or ECM is replaced, a relearn procedure MUST be performed. See DTC P1526 in appropriate SELF-DIAGNOSTICS article.

5.7L

The Throttle Actuator Control (TAC) system uses the vehicle electronics and components in order to calculate and control the position of the throttle blade. This system eliminates the need for a mechanical cable attachment from the accelerator pedal to the throttle body. This system also performs the cruise control functions. The TAC system components include but is not limited to the following

The Accelerator Pedal Position (APP) sensor. This sensor is mounted on the accelerator pedal assembly. The APP is actually 3 individual accelerator pedal position sensors within 1 housing. Three separate signal, low reference, and 5-volt reference circuits are used in order to connect the APP and the TAC module. The APP sensor 1 voltage should increase at the same time that the accelerator pedal is depressed, from below 1 volt at 0 percent pedal travel to above 2 volts at 100 percent pedal travel. APP sensor 2 voltage should decrease from above 4 volts at 0 pedal travel to below 2.9 volts at 100 percent pedal travel. APP sensor 3 voltage should decrease from above 3.8 volts at 0 pedal travel to below 3.1 volts at 100 percent pedal travel.
The throttle body. The throttle body for the TAC system is similar to a conventional throttle body with some exceptions. One exception is the use of a motor to control the Throttle Position (TP) instead of a mechanical cable. The other exception is the new design TP sensor. The TP sensor mounts on the side of the throttle body opposite the throttle actuator motor. The TP sensor is actually 2 individual TP sensors within 1 housing. Separate low reference and 5-volt reference circuits are used in order to connect the TP sensors and the TAC module. The TP sensor 1 signal voltage increases at the same time that the throttle opens. The voltage increases, from approximately 1 volt at 0 throttle to above 3.5 volts at 100 percent throttle. TP sensor 2 signal voltage decreases at the same time that the throttle is opened. The voltage increases from approximately 3.8 volts at 0 throttle to below 1 volt at 100 percent throttle.
The TAC module. The TAC module is the control center for the electronic throttle system. The TAC module and the PCM communicate via a dedicated redundant serial data circuit. The TAC module and the PCM monitor the commanded throttle position and compare the commanded position to the actual throttle position. This is accomplished by monitoring the APP and the TP sensor. These 2 values must be within a calibrated value of each other. The TAC module also monitors each individual circuit of the TP sensor, and of the APP to verify proper operation.
The Powertrain Control Module (PCM).

Each of these components interface together in order to ensure accurate calculations, and in order to control the throttle position.

MISCELLANEOUS CONTROLS

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

On many models, PCM regulates operation of A/C compressor clutch through a PCM-controlled A/C compressor clutch relay. This allows the PCM to disengage 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 or rises beyond normal operating levels.

Refrigerant pressure sensing may be accomplished by monitoring high and low pressure switches or a pressure sensor which will register either high or low pressure levels. Power steering load is monitored through a power steering pressure switch. Hot restart is monitored through the coolant temperature sensor. For component application and related wiring, see appropriate A/C COMPRESSOR CLUTCH CONTROLS article in AIR CONDITIONING & HEATING.

Some models are equipped with an A/C pressure sensor which is used to inform PCM of A/C system pressure levels. Low pressure signal will cause A/C compressor to disengage to prevent system damage. High pressure levels cause PCM to engage high speed cooling fans while A/C compressor clutch is engaged. Extremely high pressure levels will cause PCM to disengage A/C compressor clutch to prevent system damage.

ELECTRIC COOLING FAN

On many models, PCM regulates operation of electric cooling fan through a PCM-controlled cooling fan relay which controls the ground circuit or power circuit for the cooling fan. This allows PCM to operate cooling fan based upon engine coolant temperature. Most systems will engage electric cooling fan whenever A/C clutch is engaged, regardless of engine temperature. A malfunction of the cooling fan will cause engine overheating and possible detonation.

Some models use more than one cooling fan. The second fan may function as an auxiliary cooling device when A/C is engaged or (on models using A/C high and low pressure switches) during periods of engine overheating, or high A/C refrigerant pressures.

For component application and related wiring, see appropriate ELECTRIC COOLING FANS article in ENGINE COOLING.

The Electronic Variable Orifice (EVO) actuator is a linear solenoid mounted in power steering pump. PCM controls both power supply and ground path for the solenoid using a Pulse Width Modulated (PWM) signal. During periods of low speed turns (as determined by VSS input), EVO actuator is commanded to open more, allowing pump to provide an increased fluid flow for increased steering assist. During high-speed, straight-line steering, EVO actuator is commanded to restrict flow of steering fluid to steering gear. Unused steering fluid is returned to reservoir by way of a by-pass. EVO actuator is part of the Variable Effort Steering (VES) system.

HOT LIGHT OR COOLANT TEMPERATURE LIGHT

When engine coolant temperature sensor input indicates temperature exceeds specified range, PCM will turn on the TEMP or HOT light by providing a ground for the light circuit. As a bulb check, PCM also supplies a ground to turn on light when ignition is first turned on.

Torque Converter Clutch (Electronic Transaxle/Transmission)

The torque converter clutch functions similarly to the non-electronic type. A TCC Pulse Width Modulated (PWM) solenoid valve is used that regulates hydraulic pressure to make locking and unlocking of the TCC smoother.

Electronic Transmission

The transaxle/transmission is controlled by the Powertrain Control Module (PCM). PCM controls other vehicle functions as well as the transmission. The PCM monitors a number of engine/vehicle functions and uses the data to control 1-2/3-4 and 2-3 shift solenoids, TCC and, on some models, pressure control solenoid to regulate TCC engagement, upshift pattern, downshift pattern and line pressure (shift quality).

1-2/3-4 Shift Solenoid 1-2/3-4 shift solenoid is attached to valve body and is a normally open exhaust valve. PCM activates solenoid by grounding it through an internal quad-driver. The 1-2/3-4 solenoid is on in 1st and 4th gears but off in 2nd and 3rd gears. When on, solenoid redirects fluid to act on the shift valves.
2-3 Shift Solenoid 2-3 shift solenoid is attached to valve body and is a normally open exhaust valve. PCM activates solenoid by grounding it through an internal quad-driver. The 2-3 solenoid is off in 3rd and 4th gears but on in 1st and 2nd gears. When on, solenoid redirects fluid to act on the shift valves.
Pressure Control Solenoid Pressure Control (PC) solenoid is attached to the valve body and controls line pressure by moving a pressure regulator valve against spring pressure. PCM varies line pressure based upon engine load. Engine load is calculated from several inputs including Manifold Absolute Pressure (MAP) sensor, Transmission Fluid Temperature (TFT) sensor, TP sensor and gear position. Line pressure is actually varied by changing the amperage applied to pressure control solenoid from .02 (maximum pressure) to 1.1 amps (minimum pressure). The PCM continuously varies the PC solenoid valve amps to maintain the correct average current flowing through the valve.

Shift Light (Except Corvette)

The shift light is used on M/T models. Light indicates the best transmission shift point for maximum fuel economy based on engine speed and load. Power for light is supplied through the GAUGES fuse. Light illuminates when PCM supplies a ground circuit for bulb. For wiring reference, see WIRING DIAGRAMS article.

1-To-4 Light (Corvette)

The 1-to-4 light is used on M/T models. Condition is referred to as "Skip Shift". The 1-to-4 light indicates when driver should shift transmission from 1st gear to 4th gear for maximum fuel economy. Power for light is supplied through the CLUSTER fuse. Light illuminates when PCM supplies a ground circuit for bulb. For wiring reference, see WIRING DIAGRAMS article.

2nd & 3rd Gear Block-Out Relay (Camaro, Corvette & Firebird)

Note. 2nd and 3rd gear blockout relay may also be referred to as a computer aided gear select solenoid or skip shift solenoid.

Power for the 2nd and 3rd gear block-out relay winding is supplied by the ENGIGN 1 or ENG SEN fuse. When PCM determines driver should shift transmission from 1st gear to 4th gear for maximum fuel economy, PCM will provide a ground for the 2nd and 3rd gear block-out relay. When relay is energized, voltage supplied by the ENGIGN1 or ENG SEN fuse will pass through relay and energize the 2nd and 3rd gear block-out solenoid mounted in the transmission. When Skip Shift system is activated, skip shift solenoid mechanically blocks second and third gear positions. For wiring reference, see WIRING DIAGRAMS article.

Reverse Lockout

Reverse lockout system (consisting of a lockout solenoid which operates a reverse lockout mechanism) is controlled by the PCM. System is used to prevent driver from shifting into reverse at speeds greater than 3 MPH. When ignition is on, power is supplied to the reverse lockout solenoid. At speeds greater than 3 MPH, PCM sends a ground signal to energize the reverse lockout solenoid. Reverse lockout system mechanically prevents shift lever from going into reverse position.

Engine Control System - Theory & Operation Buick LeSabre VIII

INTRODUCTION

Mass Airflow Sensor

Speed Density

SUPERCHARGER (3.8L VIN 1)

COMPUTERIZED ENGINE CONTROLS

POWERTRAIN CONTROL MODULE

Torque Management

INPUT DEVICES

A/C Request Signal

A/C Pressure Sensor

A/C Pressure Switches

A/C Temperature Sensors

Accelerator Pedal Position Sensor (3.0L)

Accelerator Pedal Position Sensor (5.7L)

Battery Voltage

Brake Switch Feedback

Camshaft Position Sensor (2.2L & 2.4L)

Camshaft Position Sensor (3.0L)

Camshaft Position Sensor (3.1L & 3.4L)

Camshaft Position Sensor (3.5L, 4.0L, 4.6L & 5.7L)

Camshaft Position Sensor (3.8L)

Crankshaft (7X) Sensor (1.9L, 2.2L & 2.4L)

Crankshaft Sensor (3.0L)

Crankshaft (3X/7X & 24X) Sensor (3.1L & 3.4L)

Crankshaft Sensor (3.5L, 4.0L & 4.6L)

Crankshaft (3X/18X) Sensor (3.8L)

Crankshaft (4X/24X) Sensor (5.7L)

Engine Coolant Temperature Sensor

Fuel Level Sensor

Fuel Tank Pressure Sensor

Gear Switches

Generator "L" Light Circuit

Generator "F" Field Circuit

Ignition/Crank Signal

Intake Air Temperature Sensor

Knock Sensor

Manifold Absolute Pressure Sensor

Oxygen Sensor (Sensor 1 - 1.9L)

Oxygen Sensors (Except Sensor 1 - 1.9L & Sensor 1 - 3.0L)

Oxygen Sensors (Sensor 1 - 3.0L)

RPM Reference Signal

Throttle Position Sensor

Vehicle Speed Sensor

OUTPUT SIGNALS

A/C Compressor Clutch

Air Injection System

Boost Control Solenoid (Supercharger)

Canister Purge Control Solenoid

Coil-Near-Plug Ignition

Computer Controlled Coil Ignition (C 3 I)

Cooling Fan Relay

Digital EGR Valve

Direct Ignition System

EGR Control Solenoid

Electronic Variable Orifice Actuator

Fuel Injectors

Fuel Pump & Fuel Pump Relay

HOT Light Or Coolant Temperature (TEMP) Light

Idle Air Control Valve

Integrated Direct Ignition

Linear EGR Valve

Knock Sensor Operation

Malfunction Indicator Light

Self-Diagnostics

Serial Data

Shift Light

Shift Solenoids (Electronically-Controlled Transmission)

Throttle Actuator