Engine Controls - Theory & Operation GMC Typhoon I

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.

TURBOCHARGER (4.3L)

The turbocharger is basically an air compressor or air pump. Its major parts include a turbine wheel, shaft, compressor wheel, turbine housing, compressor housing and center housing. The center housing contains a turbine seal, compressor seal and bearings.

The internal combustion engine is an air-breathing machine. The amount of power produced by the engine is determined not by the amount of fuel it uses, but by the amount of air it breathes in a certain period of time. Air must mix with fuel to complete the combustion cycle. When the air/fuel ratio reaches a certain point, additional fuel produces only black smoke, not more power; the denser the smoke, the more the engine is being overfueled.

The turbocharger increases the quantity and density of air in the engine combustion chambers. The increased volume of air allows more fuel to be used while maintaining the proper air/fuel ratio. The increased air and fuel allows the engine to produce more horsepower than a non-turbocharged engine.

The turbocharger uses the normally wasted energy in the engine exhaust gas. As load on the engine is increased and the throttle is opened wider, more air/fuel mixture flows into the combustion chambers. The increased flow is burned and produces a larger volume of exhaust gas. The gas enters the exhaust manifolds, flows through the turbocharger turbine housing and turns the turbine wheel and shaft. The shaft is coupled to the compressor wheel. The compressor wheel compresses the air it receives and sends it to the intake manifold. The higher pressure in the intake manifold allows a denser charge to enter the combustion chambers.

Intake manifold pressure, or "boost", is controlled by an exhaust by-pass valve, or wastegate. The wastegate is operated by a spring-loaded diaphragm-type actuator which responds to boost pressure. The actuator, which is controlled by the wastegate solenoid, opens the wastegate to allow exhaust gases to by-pass the turbine wheel, thereby maintaining the correct boost level. The wastegate solenoid is controlled by the ECM through a turbo boost relay.

The rotating assembly in the turbocharger can reach speeds of 130,000-140,000 RPM. An adequate supply of clean engine oil is essential for cooling and lubrication. Whenever a basic engine bearing has been damaged or the turbocharger is replaced, the oil and oil filter should be changed and the turbocharger flushed with clean engine oil.

Charge Air Cooler

Before entering the combustion chambers, the air from the turbocharger is routed through an air-to-water charge air cooler. As the turbocharger compresses air, the air's temperature rises. The heated, compressed air then flows through the charge air cooler core, where it is cooled by coolant passing through the charge air cooler. The cooler, denser air allows a denser air/fuel charge to enter the combustion chambers, producing significantly more power.

The hot coolant is then routed through the charge air cooler radiator, where it releases the heat it absorbed at the charge air cooler. An electric pump mounted to the charge air cooler radiator circulates coolant through the system. Pump is controlled by the ECM through a charge air cooler air pump relay.

Mass Airflow (3.8L)

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.

The ECM uses MAF information to control fuel delivery. Sensor produces a frequency signal which cannot be easily measured in testing (32-150 Hertz). This varying signal is proportional to airflow.

Speed Density (Except 3.8L)

All gasoline vehicles except 3.8L are equipped with a MAP sensor, and use the speed density method to compute the airflow rate. ECM uses manifold pressure to calculate the airflow rate. The MAP sensor responds to manifold vacuum changes due to engine load and speed changes. The ECM sends a voltage signal to the MAP sensor. Manifold pressure changes result in resistance changes in the MAP sensor.

By monitoring MAP sensor signal voltage, the ECM determines manifold pressure. If MAP sensor fails, the ECM supplies a fixed MAP value, and uses the TPS to control fuel.

2.5L and 4.3L turbo models also use a Manifold Air Temperature (MAT) sensor. Sensor allows ECM to determine intake air temperature. ECM uses signal to delay EGR until intake air temperature reaches about 40°F (5°C). If intake air temperature becomes excessively high, ECM compensates by slightly retarding timing.

COMPUTERIZED ENGINE CONTROLS (GASOLINE)

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 3-way catalytic converter can control oxides of nitrogen (NOx), hydrocarbon (HC) and carbon monoxide (CO) emissions.

The computerized engine control system consists of Electronic Control Module (ECM), input devices (sensor and switch input signals) and output signals.

ELECTRONIC CONTROL MODULE (ECM)

Note. Some models use a Powertrain Control Module (PCM). The difference between an ECM and PCM is the PCM also controls electronic transmission internals and cruise control system. Unless stated otherwise, references to ECM also apply to PCM-equipped vehicles.

ECM is located in passenger compartment. For exact location of ECM, see ECM LOCATION in appropriate TESTS W/CODES article in the ENGINE PERFORMANCE Section or under COMPONENT LOCATIONS in I - SYS/COMP TESTS article in the ENGINE PERFORMANCE Section. The ECM consists of the Arithmetic Logic Unit (ALU), Central Processing Unit (CPU), power supply and system memories.

The ECM has a "learning" ability which allows it to make minor corrections for fuel system variations. If battery power is interrupted, a vehicle performance change may be noticed. ECM corrects itself, and normal performance returns if vehicle is allowed to "relearn" optimum control conditions. "Relearning" occurs when vehicle is driven at normal operating temperature under part throttle, moderate acceleration and idle conditions.

Arithmetic Logic Unit (ALU)

This internal component of the ECM converts electrical signals received from various engine sensors into digital signals for use by the CPU.

Central Processing Unit (CPU)

CPU uses digital signals to perform all mathematical computations and logic functions necessary to deliver proper air/fuel mixture. CPU also calculates spark timing and idle speed. The CPU controls operation of emission control, "closed loop" fuel control and diagnostic system.

Power Supply

Power for ECM reference output signals (5 volts) and control devices (12 volts) is received from the battery through ignition circuit when ignition switch is in ON position. Keep-alive memory power is received directly from the battery.

Memories

ECM uses 5 types of memory

1. Read Only Memory (ROM) ROM is programmed information which only ECM can read. The ROM program cannot be changed. If battery voltage is removed, ROM information is retained.
2. Random Access Memory (RAM) RAM is the scratch pad for the CPU. Data input, diagnostic codes and results of calculations are constantly updated and temporarily stored in RAM. If battery voltage is removed from ECM, all information stored in RAM is lost.
3. Programmable Read Only Memory (PROM) PROM is factory programmed engine calibration data which "tailors" ECM for specific transmission, engine, emission, vehicle weight and rear axle ratio application. The PROM can be removed from ECM. If battery voltage is removed, PROM information is retained.
4. Calibration Package (CALPAC) Some models use a PROM and a CALPAC. CALPAC provides fuel delivery back-up so engine runs in case of PROM or ECM failure. Any time ECM is replaced, PROM and CALPAC must both be installed into replacement ECM. If battery voltage is removed, CALPAC information is retained.
5. Memory Calibration (MEM-CAL) Some vehicles may use a ECM containing a MEM-CAL unit. This assembly contains functions of PROM and CALPAC. If power to ECM is removed, MEM-CAL information is retained. MEM-CAL also contains an internal ESC module on models equipped with ESC.

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 control unit.

INPUT DEVICES

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

A/C On (A/C Request) Signal

The air conditioner "on" switch is mounted in instrument panel. This switch provides a simple "on"("A/C request") signal, which is monitored by the ECM. The ECM uses this signal to determine control of the A/C clutch relay (if equipped) and to adjust idle speed when air conditioner compressor clutch is engaged. On some models, ECM may also activate radiator 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. To check function of the A/C switch, perform functional check of switch. See I - SYS/COMP TESTS article in the ENGINE PERFORMANCE Section.

Battery Voltage

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

Brake Switch Feedback

On models equipped with cruise control systems, ECM may monitor the brake switch circuit to determine when to engage and disengage cruise control. On vehicles equipped with a Torque Converter Clutch (TCC), one circuit of brake switch is in series with the power supply for the TCC solenoid located in automatic transmission.

Coolant Temperature Sensor (CTS)

The CTS is a thermistor (temperature sensitive resistor) located in an engine coolant passage. The ECM supplies and monitors a 5-volt signal to CTS. This monitored 5-volt signal is then modified by resistance of the CTS. When coolant temperatures are low, CTS resistance is high and the ECM sees a high monitored voltage signal. When coolant temperatures are high, CTS resistance is low and the ECM sees a low monitored voltage. When fully warmed, CTS 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 converter clutch application. A CTS which is out of calibration will not set a trouble code, but can cause fuel delivery and driveability problems. A coolant sensor circuit problem should set a related trouble code.

Crankshaft Position Sensor

Crankshaft position sensor, used on 3.8L engine, utilizes a Hall Effect switch mounted near vibration damper. The sensor monitors vibration damper position (crankshaft position) and sends signals to ignition module. These signals provide ECM with a TDC position reference for each piston, as well as supplying an engine speed (RPM) signal. This allows ECM to compute crankshaft position and RPM and fire appropriate ignition coil at the proper time. Crankshaft position sensor signal is also used in conjunction with the camshaft position sensor signal to determine triggering of the sequentially-fired fuel injectors. For additional information, see COMPUTER CONTROLLED COIL IGNITION (C(3)I) under IGNITION SYSTEM (GASOLINE).

Cranking Signal

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

Digital Ratio Adapter Controller (DRAC)

DRAC compensates for various axle and tire ratios by monitoring the Vehicle Speed Sensor (VSS) signal and modifying it before passing it on to the ECM and speedometer unit. On models equipped with a DRAC, VSS buffer is an internal part of DRAC.

Fuel Pump Feedback

ECM monitors fuel pump circuit between fuel pump relay/oil pressure switch and fuel pump. This enables the ECM 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 trouble code in ECM memory.

Gear Switches

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

Knock Sensor

The knock sensor is a piezoelectric device which detects abnormal engine vibrations (spark knock) in the engine. This vibration results in the production of a very low AC signal, which is sent from the knock sensor to the ESC controller or the MEM-CAL portion of the ECM (on models equipped with 4L80-E transmission). The ECM then retards ignition timing until the engine knock ceases. Two knock sensors are used on 4.3L CPI engines.

For additional information on knock sensor operation, see ESC DETONATION RETARD OPERATION under IGNITION TIMING CONTROL SYSTEMS (GASOLINE).

A fault in the ESC circuit may set a related trouble code. When a related trouble code is not present and the ESC system is the suspected cause of a driveability problem, perform functional check of ESC system. See I - SYS/COMP TESTS article in the ENGINE PERFORMANCE Section.

Manifold Absolute Pressure (MAP) Sensor (Except 3.8L)

The 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 ECM (1.5 volts at idle to about 4.5 volts at WOT). The ECM can monitor these signals and adjust air/fuel ratio and ignition timing under various operating conditions.

If MAP sensor fails, the ECM substitutes a fixed MAP value, and uses the TPS to control fuel delivery. A fault in the MAP circuit should set a related trouble code. If a related trouble code is not present and MAP sensor is suspected of causing a driveability problem, perform functional check of MAP sensor. See I - SYS/COMP TESTS article in the ENGINE PERFORMANCE Section.

Manifold Air Temperature (MAT) Sensor (2.5L & 4.3L Turbo)

MAT sensor is a thermistor (temperature sensitive resistor) mounted in the intake manifold. Low intake air temperature produces high internal sensor resistance, while high temperature causes low internal sensor resistance. The ECM supplies and monitors a 5-volt signal to sensor through a pull-down resistor in ECM.

MAT sensor, also known as an intake air temperature sensor, allows ECM to determine intake air temperature. ECM uses signal to delay EGR until intake air temperature reaches about 40°F (5°C). If intake air temperature becomes excessively high, ECM compensates by slightly retarding ignition timing. After a vehicle has sat overnight, MAT and CTS signals (resistance and temperature) should be close to same reading. Failure in MAT sensor circuit should set a related trouble code.

Mass Airflow (MAF) Sensor (3.8L)

The 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 a MAP or vacuum sensor signal. MAF signal should remain relatively constant at cruise, gradually changing with throttle angle and rapidly changing on sudden acceleration. The ECM uses this information to control fuel delivery.

This frequency generator type MAF sensor produces a frequency signal that cannot be easily measured in testing (32-150 Hertz). This varying signal is proportional to airflow. A fault in the MAF sensor circuit should set a related trouble code.

Oxygen (O2) Sensor

The O2 sensor is mounted in the exhaust system and monitors oxygen content of exhaust gases. The oxygen content causes the Zirconia/Platinum-tipped O2 sensor to produce a voltage signal which is proportional to exhaust gas oxygen concentration (0-3%) compared to outside oxygen (20-21%). This voltage signal is low (about .1 volt) when a lean mixture is present and high (about 1.0 volt) when a rich mixture is present. As ECM compensates for a lean or rich condition, this voltage signal constantly fluctuates between high and low, crossing a .45-volt reference voltage supplied by ECM on the O2 sensor signal line. This is referred to as "cross counts".

The O2 sensor does not function properly (produce voltage) until its temperature reaches 600°F (316°C). At temperatures less than the normal operating range of the sensor, vehicle functions in "open loop" mode, and ECM does not make air/fuel adjustments based upon O2 sensor signals, but uses TPS and MAP or MAF values to determine air/fuel ratio from a table built into memory. When ECM reads a voltage signal greater than .45 volt from the O2 sensor, ECM begins to alter commands to injector to produce either a leaner or richer mixture.

Once vehicle has entered "closed loop", a fault in the O2 circuit (cooled-down sensor or open or shorted O2 sensor circuit) is the only thing which can return vehicle to open loop. A problem in the O2 sensor circuit should set a related trouble code.

On 4.3L engines, O2 sensor uses an internal heating element. Heating element allows O2 sensor to warm more quickly, allowing fuel system to enter closed loop operation sooner. Heating element also prevents fuel system from re-entering open loop operation, which would be a normal response to prolonged idling.

Park/Neutral (P/N) Switch

This switch is connected to transmission gear selector and signals ECM when transmission is in Park or Neutral. ECM uses this information for determining control of ignition timing, converter clutch and idle speed. To check function of P/N switch, perform functional check of switch. See I - SYS/COMP TESTS article in the ENGINE PERFORMANCE Section.

Power Steering (P/S) Pressure Switch (2.5L)

This switch informs ECM of engine load conditions which exist when steering wheel is turned from center to full lock position. ECM uses this information to help control idle speed. To check function of P/S switch, perform functional check of switch. See I - SYS/COMP TESTS article in the ENGINE PERFORMANCE Section.

Pressure Switch Manifold (PSM) (4L80-E Transmission)

The PSM is actually 5 pressure switches combined into a single unit mounted on the transmission valve body. The PCM supplies battery voltage on 3 separate wires to PSM. By grounding one or more of the switches in various combinations, PCM detects what gear range the vehicle operator has selected.

RPM Reference Signal

ECM monitors RPM through ignition module tach/pulse signals (on circuit No. 430) produced by the HEI module (RPM reference line of 4-wire EST connector) or Hall Effect camshaft and crankshaft sensors (3.8L). ECM uses signal to determine control of timing, fuel delivery (fuel pump relay energizing), EGR function and idle speed. ECM also uses signal to trigger fuel injectors.

Throttle Position Sensor (TPS)

The TPS is a variable mechanical resistor connected directly to the throttle shaft linkage. The TPS has 3 wires connected to it. One is connected to a 5-volt reference voltage supply from ECM, another is connected to ECM ground and the third is the signal return which is monitored by ECM. The voltage signal from the TPS varies from closed throttle (.5-1.0 volt) to wide open throttle (4.5-5 volts). ECM uses this signal to determine control of fuel, idle speed, spark timing and converter clutch. A problem in the TPS circuit may set a related trouble code.

Transmission Temperature Sensor (TTS) (4L80-E Transmission)

TTS is a thermistor (temperature sensitive resistor) mounted to the transmission valve body. The PCM supplies and monitors a 5-volt signal to TTS. This monitored 5-volt signal is then modified by resistance of TTS. When transmission fluid temperatures are low, TTS resistance is high and PCM sees a high monitored voltage signal. When transmission fluid temperatures are high, TTS resistance is low and PCM sees a low monitored voltage.

PCM uses transmission fluid temperature input in control of converter clutch application and shift quality. Sensor circuit problem should set a related trouble code.

Vehicle Speed Sensor (VSS)

VSS is a Permanent Magnet (PM) generator mounted in transmission or transfer case. The VSS sends a pulsing signal to either the ECM or Digital Ratio Adapter Controller (DRAC), which passes the signal on to the ECM. ECM then converts this signal into Miles Per Hour (MPH) by monitoring the time interval between pulses. ECM uses this sensor input in controlling converter clutch engagement.

On all models except 3.1L, 3.8L and 4WD equipped with 4L80-E transmission, a VSS buffer is built into the DRAC. On 3.1L, a VSS buffer is located between VSS and ECM. On 3.8L and 4WD equipped with 4L80-E transmission, VSS buffer is built into ECM.

OUTPUT SIGNALS

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

A/C Clutch Relay

MISCELLANEOUS ECM CONTROLS.

Air Injection Control Solenoid

EMISSION SYSTEMS.

Charge Air Cooler Pump Relay (4.3L Turbo)

TURBOCHARGER.

Computer Controlled Coil Ignition (C(3)I)

IGNITION SYSTEM.

Cooling Fan Relay (3.1L)

MISCELLANEOUS ECM CONTROLS.

SERVICE ENGINE SOON Light

SELF-DIAGNOSTIC SYSTEM.

EGR Control Solenoid Valve

EMISSION SYSTEMS.

ESC Timing Retard

IGNITION SYSTEM.

EVRV Solenoid

EMISSION SYSTEMS.

Fuel Injectors

FUEL CONTROL.

Fuel Module

FUEL DELIVERY.

Fuel Pump & Fuel Pump Relay

FUEL DELIVERY.

HEI-EST Ignition

IGNITION SYSTEM.

Idle Air Control (IAC) Valve

IDLE SPEED.

Self-Diagnostics

SELF-DIAGNOSTIC SYSTEM.

Serial Data

SELF-DIAGNOSTIC SYSTEM.

Shift Solenoids (4L80-E Transmission)

MISCELLANEOUS ECM CONTROLS.

Torque Converter Clutch

MISCELLANEOUS ECM CONTROLS.

Transmission Shift Light (Manual Transmission)

MISCELLANEOUS ECM CONTROLS.

Turbo Boost Relay (4.3L Turbo)

TURBOCHARGER.

Wastegate Solenoid (4.3L Turbo)

TURBOCHARGER.

COMPUTERIZED ENGINE CONTROLS

(DIESEL)

6.2L Diesel engine with light duty emissions uses Diesel Electronic Control (DEC) system. DEC system consists of Electronic Control Module (ECM), input devices and output signals. DEC system electronically controls EGR system operation, Torque Converter Clutch (TCC) engagement, cold advance and glow plug system.

Electronic Control Module (ECM) is located in passenger compartment, behind glove box. It constantly monitors information from various sensors to control EGR, TCC, cold advance and glow plug systems. ECM processes input signals from sensors and then sends necessary electrical responses to control these systems.

The ECM performs the diagnostic function of the DEC system. It can recognize operational problems, alert driver through the SERVICE ENGINE SOON light and store codes which identify problem areas to technicians making system repairs.

ECM uses 3 types of memory

1. Read Only Memory (ROM) ROM is programmed information which only ECM can read. ROM program cannot be changed. If battery voltage is removed, ROM information is retained.
2. Random Access Memory (RAM) RAM is the scratch pad for the CPU. Data input, diagnostic codes and results of calculations are constantly updated and temporarily stored in RAM. If battery voltage is removed from ECM, all information stored in RAM is lost.
3. Programmable Read Only Memory (PROM) PROM is factory programmed engine calibration data which "tailors" ECM for specific transmission, engine, emission, vehicle weight and rear axle ratio application. PROM can be removed from ECM. If battery voltage is removed, PROM information is retained.

Each sensor or switch furnishes electronic (voltage) signals to ECM. The ECM uses these input signals to control EGR, TCC, cold advance and glow plug systems. Various models are equipped with different combinations of input devices. Not all devices are used on all models. To determine the input usage on a specific model, see appropriate wiring diagram in WIRING DIAGRAMS article in the ENGINE PERFORMANCE Section. The available input signals include

CTS is a thermistor (temperature sensitive resistor). A coolant temperature of -40°F (-40°C) produces a high resistance (100,000 ohms), while a coolant temperature of 266°F (130°C) produces a low resistance (70 ohms).

ECM supplies a 5-volt reference signal through an internal resistor to CTS and measures return voltage. Voltage is high when coolant temperature is low, and low when coolant temperature is hot. By measuring voltage, ECM knows engine coolant temperature. Engine coolant temperature affects cold advance and glow plug systems.

Manifold Absolute Pressure (MAP) Sensor

MAP sensor, mounted on left side of cowl, monitors vacuum in the EGR system. It senses the actual vacuum in the EGR vacuum line and sends a signal to the ECM.

The signal is compared to the EGR duty cycle calculated by the ECM. If there is a minor difference in the vacuum value sensed and the ECM command, the ECM corrects. When a major difference is sensed, the ECM recognizes a fault and sends a full EGR signal.

TPS, mounted on the injection pump, is a variable resistor monitoring throttle opening angle for the ECM. The sensor is connected to a 5-volt reference signal and has a high resistance value when throttle is closed. At wide open throttle, the TPS resistance value is low, and output to the ECM will be near 5 volts.

Engine Speed Sensor

The engine speed sensor is a camshaft driven pick-up and is mounted at center rear of engine. The sensor receives a 5-volt reference signal and allows the ECM to measure engine RPM by the number of times reference voltage is pulsed. The engine speed sensor pulses 4 times per revolution.

Mounted on the transmission, VSS sends a pulsing signal to ECM for vehicle speed calculation. This calculation is used to control TCC engagement.

Note. Output signals are regulated by the ECM to maintain correct driveability and exhaust emissions. For theory and operation of components, refer to indicated system.

Electronic Controller/Glow Plug Relay

FUEL CONTROL.

Cold Advance Control

FUEL CONTROL.

MISCELLANEOUS ECM CONTROLS.

Exhaust Gas Recirculation System

EMISSION SYSTEMS (DIESEL).

Fuel Module (7.4L, 5.7L G Van & 5.7L Over 8500 GVWR)

Fuel module overrides the ECM 2-second timer and fuel pump runs 20 seconds before shutting off when vehicle is not started. This added circuit corrects hot restart problems which could cause vapor lock during high ambient temperatures.

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 in the fuel pump controls maximum fuel pump pressure.

On TBI fuel systems, pressure regulator is mounted on throttle body. On Port Fuel Injection (PFI) systems, pressure regulator is mounted on the fuel rail. Regulator keeps fuel available to injector(s) at a constant pressure. Excess fuel is returned to fuel tank through pressure regulator return line.

When ignition switch is turned to ON position, ECM turns on electric fuel pump by energizing fuel pump relay. ECM keeps pump on if engine is running or cranking (ECM is receiving reference pulses from ignition module). If there are no reference pulses and vehicle is not equipped with a fuel module, ECM turns pump off within 2 seconds after key is turned on. For additional information, see FUEL PUMP RELAY and FUEL MODULE under FUEL DELIVERY.

Fuel Pressure Regulator (CPI)

A constant fuel pressure of 54-64 psi (3.8-4.5 kg/cm 2 ) is maintained by a factory preset, nonadjustable, spring loaded diaphragm contained within CPI assembly. Spring tension maintains a constant fuel pressure to injector regardless of engine load.

Fuel Pressure Regulator (TBI)

A constant fuel pressure of 9-13 psi (.6-9 kg/cm 2 ) is maintained by a factory preset, nonadjustable, spring loaded diaphragm contained within throttle body. Spring tension maintains a constant fuel pressure to injector regardless of engine load.

Fuel Pressure Regulator (4.3L Turbo PFI)

Fuel pressure regulator is a diaphragm-operated relief valve with injector pressure on one side and manifold pressure (vacuum) on the other. Pressure regulator compensates for engine load by increasing fuel pressure when low manifold vacuum is experienced.

During periods of high manifold vacuum, regulator-to-fuel tank return orifice is fully open, keeping fuel pressure on the low side of its regulated range. As throttle valve opens, vacuum to regulator diaphragm decreases, allowing spring tension to gradually close off return passage. At wide open throttle (when vacuum is at its lowest), return orifice is restricted, providing maximum fuel volume and maintaining constant fuel pressure to injectors.

Fuel Pump Relay

When ignition switch is turned to ON position, ECM turns electric fuel pump on by energizing fuel pump relay. ECM keeps relay energized if engine is running or cranking (ECM is receiving reference pulses from ignition module). If there are no reference pulses, ECM turns pump off within 2 seconds after key on. See FUEL MODULE under FUEL DELIVERY.

As a back-up system to fuel pump relay, the oil pressure switch also activates fuel pump. The oil pressure switch is normally open until oil pressure reaches approximately 4 psi (.28 kg/cm 2 ). If fuel pump relay fails, the oil pressure switch closes when oil pressure is obtained, and operates the fuel pump. An inoperative fuel pump relay may result in extended cranking times due to the time required to build up oil pressure. Oil pressure switch may be combined into a single unit with an oil pressure gauge sending unit or sensor.

ECM monitors fuel pump circuit between fuel pump relay/oil pressure switch and fuel pump, enabling ECM to determine if fuel pump is being energized by fuel pump relay or oil pressure switch. A failure in this monitored circuit results in the setting of a related trouble code in ECM memory.

For additional information on fuel pump activation, see BASIC TESTING and I - SYS/COMP TESTS articles in the ENGINE PERFORMANCE Section.

FUEL CONTROL

The ECM, 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.

Open Loop

When engine is cold and engine speed is greater than 400 RPM, ECM operates in open loop mode. In open loop, ECM calculates air/fuel ratio based upon coolant temperature and MAP or MAF sensor readings. Engine remains in open loop operation until O2 sensor reaches operating temperature, coolant temperature reaches preset temperature and a specific period of time has elapsed after engine starts.

Closed Loop

When O2 sensor reaches operating temperature, coolant temperature reaches a preset temperature and a specific period of time has passed since engine start-up, ECM operates in closed loop. In closed loop, ECM controls air/fuel ratio based upon O2 sensor signals (in addition to other input parameters) to maintain as close to a 14.7:1 air/fuel ratio as possible. If O2 sensor cools off (due to excessive idling) or a fault occurs in the O2 sensor circuit, vehicle once again enters open loop mode.

On 4.3L engines, oxygen sensor is equipped with an internal heating element. This element enables system to reach "closed loop" sooner and maintain closed loop even during periods of extended idle.

Central Port Injection (CPI)

CPI is one of 3 fuel systems used on 4.3L engines. The non-repairable injector assembly consists of a fuel meter body, fuel pressure regulator, fuel injector and 6 poppet nozzles with fuel tubes. CPI assembly is housed in the lower manifold assembly. Fuel pump and pressure regulator maintain fuel pressure at 54-64 psi (3.8-4.5 kg/cm 2 ) under all operating modes.

When injector is energized, pressurized fuel passes down fuel distribution tubes to poppet nozzles located at rear of intake valves. Fuel pressure forces poppet valves open, spraying fuel into cylinders when intake valves are open. As fuel pressure drops (due to all poppets opening or injector de-energizing), poppet nozzle spring pressure closes poppet nozzle until pressure again builds high enough to overcome poppet nozzle spring pressure. Excess fuel is returned to the fuel tank via the fuel return line.

Throttle Body Injection

Injectors are located in throttle body unit. All models except 2.5L use 220 Series dual injector throttle body. 2.5L models use 700 Series single injector throttle body. Battery voltage is supplied to injector when ignition is on. ECM energizes injector solenoid by providing a ground path through its internal circuitry. By regulating injector ground circuit, ECM controls injector "on" time (pulse width) to provide proper amount of fuel to engine.

Pressure regulator maintains pressure to injector at 9-13 psi (.6-.9 kg/cm 2 ). Excess fuel passes through pressure regulator and returns to fuel tank.

In the "run" mode, ECM uses tach (RPM) signal to determine when to pulse injector. Fuel injectors are pulsed once for each engine revolution; each spray provides 1/2 the fuel required for the combustion process. Thus, 2 injections of fuel (2 rotations of crankshaft) are mixed with incoming air to produce the fuel charge for each combustion cycle. On models equipped with dual injectors in the throttle body, injectors are pulsed alternately.

Port Fuel Injection (PFI)

Individual, electrically pulsed injectors (one per cylinder) are located in intake manifold fuel rails. These injectors are next to intake valves in cylinder head.

The 4.3L Turbo PFI system features simultaneous double-fire injection. Fuel injectors are pulsed once for each engine revolution; each spray provides 1/2 the fuel required for the combustion process. Thus, 2 injections of fuel (2 rotations of crankshaft) are mixed with incoming air to produce the fuel charge for each combustion cycle.

The 3.8L engine uses Sequential Fuel Injection (SFI). Injectors on these models are pulsed sequentially in spark plug firing order. Main differences between sequential and simultaneous systems are injectors, wiring and the ECM.

On both systems, 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 ECM to control pulse width. The ECM controls pulse width using information provided by various sensors.

Fuel System Operating Modes

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

1. Starting During engine starts, ECM delivers one injector pulse for each distributor reference pulse received (synchronized mode). Injector pulse width is based upon coolant temperature and throttle position. ECM determines air/fuel ratio when throttle position is less than 80 percent open. Engine starting air/fuel ratio ranges from 1.5:1 at -33°F (-36°C) to 14.7:1 at 201°F (94°C). At lower coolant temperatures, injector pulse width is wider (richer air/fuel mixture ratio). When coolant temperature is high, injector pulse width becomes narrower (leaner air/fuel ratio).
2. Clear Flood If engine is flooded, driver must depress accelerator pedal to Wide Open Throttle (WOT) position. At this position, ECM adjusts injector pulse width equal to an air/fuel ratio of 20:1. This air/fuel ratio is maintained as long as throttle remains in wide open position and engine speed is less than 600 RPM. If throttle position becomes less than 80 percent open and/or engine speed exceeds 600 RPM, ECM changes injector pulse width to that used during engine starting (based upon coolant temperature and manifold vacuum).
3. Heavy Acceleration ECM provides fuel enrichment during heavy acceleration. Sudden opening of throttle valve causes rapid increase in MAP signal. Pulse width is directly related to MAP, throttle position and coolant temperature. Higher MAP and wider throttle angles give wider injector pulse width (richer mixture). During enrichment, injector pulses are not in proportion to distributor reference signals (non- synchronized). Any reduction in throttle angle cancels fuel enrichment.
4. Deceleration During normal deceleration, fuel output is reduced. This reduction in available fuel serves to remove residual fuel from intake manifold. During sudden deceleration, when MAP, throttle position and engine speed are reduced to preset levels, fuel flow is cut off completely. This deceleration fuel cut-off overrides normal deceleration mode. During either deceleration mode, injector pulses are not in proportion to distributor reference signals.
5. Battery Voltage Correction ECM compensates for low battery voltage by increasing injector pulse width and increasing idle RPM. ECM is able to perform these commands because of a built-in memory/learning function.
6. 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 ECM, even with ignition on. This prevents flooding before starting. Fuel cut-off also occurs at high engine RPM 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).

IDLE SPEED (GASOLINE)

ECM controls engine idle speed depending upon engine operating conditions. The ECM senses engine operating conditions and determines the best idle speed.

The IAC valve controls engine idle speed to prevent stalling during engine load changes. The IAC valve is mounted on throttle body and controls the amount of air by-passed around the throttle plate. The IAC valve controls engine idle speed by moving its pintle in and out in steps referred to as "counts" (0 counts, fully seated; 255 counts, fully retracted). Counts can be measured by plugging a Scan tester into the Assembly Line Data Link (ALDL).

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. Normal counts on an idling engine should be 4-60. When engine is idling, ECM determines proper positioning of IAC valve based on battery voltage, coolant temperature, engine load and engine RPM.

If IAC valve is disconnected or reconnected with engine running, IAC loses its reference point and must be reset. On some models, IAC is reset by turning ignition on and off. Other models require driving vehicle at normal operating temperature over 35 MPH with circuit properly connected. Problems in IAC circuit should set a related 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 IAC valves are used. Ensure replacement valve is proper design.

FUEL DELIVERY

A mechanical pump is mounted on the right side of engine block. Camshaft eccentric drives pump. Pump pulls fuel from the fuel tank through a primary filter. The fuel is then pumped through a secondary filter, mounted on firewall (pickups) or rear of air cleaner (vans), and to the injection pump.

The 6.2L diesel engine uses a mechanical, high pressure rotary diesel injection pump, which is gear-driven by camshaft at camshaft speed. Pump injects a precisely metered amount of fuel to each cylinder at the proper time.

High pressure fuel lines carry the fuel to an injection nozzle in each cylinder. All fuel lines are the same length to ensure no variance in timing. A rotary fuel metering valve controls engine RPM. As the accelerator pedal is depressed, throttle linkage opens fuel metering valve to allow increased fuel delivery.

Diesel Injection Pump

The high pressure diesel injection pump is mounted at top of engine, below intake manifold. The pump is gear-driven by camshaft. Pump precisely governs time and amount of fuel injection.

A built-in fuel pressure regulator and transfer pump picks up fuel at pump inlet, pushing it through a passage to the pump head. The pump head distributes fuel at transfer pump pressure 8-12 psi (.5-.8 kg/cm 2 ) to metering valve, governor and automatic advance mechanisms. Fuel then passes to rotary fuel metering valve and into a charging passage. As pump shaft rotates, fuel is directed at high pressure through each delivery pipe to an injector. (Scheme 1)

Scheme 1

Fuel Injection Lines

Eight high pressure fuel injection lines are routed from the injection pump to an injector in each cylinder. The lines are of equal length to prevent a difference in timing between cylinders. Lines are not interchangeable and are pre-bent by the manufacturer.

The electronic controller/glow plug relay is mounted at rear of left cylinder head. It monitors and controls glow plug operation. Controller uses four pins to determine glow plug operating requirements. Pin "B" senses voltage at starter motor solenoid. Pin "C" senses glow plug voltage. Pin "D" supplies 12 volts, through cold advance relay, to operate controller when coolant temperature is less than 80°F (27°C). Pin "E" is controller ground.

A normally operating system works as follows: at room temperature and with ignition on and engine off, the glow plugs come on for 4-6 seconds and then go off for about 4.5 seconds. The glow plugs then cycle on for about 1.5 seconds and off for about 4.5 seconds, for a total start sequence of about 20 seconds. If the engine is cranked during or after start sequence, the glow plugs will cycle on and off for a total of 25 seconds after the ignition switch is returned from the crank position, whether engine starts or not.

Glow Plugs

Glow plugs are small 6-volt heaters, powered by 12 volts to give rapid heating. Electronic controller operates glow plugs, which cycle on when ignition switch is turned to the RUN position (prior to starting the engine). The glow plugs remain pulsing a short time after engine starting, then automatically turn off.

The glow system for the LH6 (light duty emissions version) is different than the system for the LL4 (heavy duty emissions version). The LH6 system has the same glow plugs, glow plug controller and WAIT light. However, there is no temperature inhibit switch. Instead, the ECM, which receives temperature information from the coolant temperature sensor located in the water crossover of the engine, controls glow plug temperature inhibit.

The computer transposes this temperature information into a voltage signal, which it sends to the cold advance relay, ignition circuit and glow plug controller. The cold advance relay is located at the junction block in the engine compartment on the right side of the cowl. For diagnostic information on computer controlled system, see appropriate TESTS W/CODES article in the ENGINE PERFORMANCE Section.

Glow Plug Inhibit Switch

The LL4 (heavy duty emissions version of 6.2L engine) is equipped with a glow plug inhibit switch. The inhibit switch is calibrated to open above 125°F (51.5°C) to prevent glow plug operation above this temperature.

Two types of inhibit switches are used. Switches can be identified by their cap color. A switch with a Black cap is a temperature-controlled switch. The other switch (with a non-Black cap) is an optional switch which is always closed to allow for more frequent cycling of glow plugs.

Glow Plug After-Start

Glow plug controller provides glow plug operation after starting a cold engine. This after-start operation is initiated when ignition switch is returned to RUN from START position.

Injection Nozzles

Each of the 8 combustion chambers is equipped with an injection nozzle. The injection nozzle has a single fuel inlet fitting and 2 fuel return fittings (one on each side of fuel inlet fitting). The nozzle is threaded into the cylinder head. Injection nozzles are spring loaded and calibrated to open at a specified fuel line pressure. The combustion chamber end of the nozzle has a replaceable compression seal and carbon stop seal.

Housing Pressure Cold Advance (HPCA)

The HPCA circuit is used to improve cold starting and aid in emission control. On light duty emissions (LH6 models), an ECM signal activates the HPCA circuit. On heavy duty emissions (LL4 models), a coolant temperature switch located on rear of right cylinder head controls the HPCA circuit. The circuit advances injection timing about 4 degrees when engine is cold.

When engine temperature is less than 80°F (27° C), the circuit decreases housing pressure from 10 psi (.7 kg/cm 2 ) to zero. Meanwhile, the fast idle solenoid is activated. When the temperature switch opens, the HPCA circuit is de-energized and housing pressure rises, retarding pump timing. The temperature switch closes again when engine temperature falls to less than 85°F (30°C).

The cold advance control circuit is designed to advance injection pump timing about 4 degrees during cold engine operation. ECM activates this circuit through the cold advance relay to energize the cold advance solenoid. The ECM opens the circuit when coolant temperature is greater than 95°F (35°C).

When coolant temperature is less than the switching point and with ignition on, the cold advance solenoid is continuously energized without the engine running. When coolant temperature is less than the switching point and with the engine running, injection pump housing pressure is decreased from 10 psi (.7 kg/cm 2 ) to zero, which advances injection pump timing by about 4 degrees. As engine warms, the cold advance solenoid is de-energized and the injection pump housing pressure is returned to 10 psi (.70 kg/cm 2 ).

Curb Idle Speed

Curb idle is controlled by mechanical adjustment of the low idle speed screw. For idle speed adjustment procedure, see CURB IDLE under IDLE SPEED (DIESEL) in ADJUSTMENTS article in the ENGINE PERFORMANCE Section.

Fast Idle Speed

Fast idle solenoid controls fast idle. For adjustment of fast idle speed, see FAST IDLE under IDLE SPEED (DIESEL) in ADJUSTMENTS article in the ENGINE PERFORMANCE Section.

IGNITION SYSTEM (GASOLINE)

HIGH ENERGY IGNITION ELECTRONIC SPARK TIMING (HEI-EST) DISTRIBUTOR

The Delco-Remy HEI-EST system consists of distributor housing, rotor, cap, 7 or 8-terminal ignition module, magnetic pick-up, pole piece, pick-up coil, connecting harness and the EST portion of the ECM. The distributor is connected to the EST system by a 4-wire connector leading to Electronic Control Module (ECM).

No vacuum or centrifugal advance mechanisms are used. Based upon monitored input signals, ECM controls all spark timing changes. Some models use an additional Electronic Spark Control (ESC) system, which retards timing in case of engine detonation (knock). Most models are equipped with sealed ignition coil and ignition module connectors.

When the external teeth on the timing core approach, align with and pass the pick-up coil windings, an alternating current is produced in the pick-up coil windings. In the cranking mode, this alternating current signals switching transistors in the HEI module to make or break the ignition coil primary ground circuit. Once the engine starts, ECM takes control of primary ground circuit (EST mode).

When the primary ground circuit is removed, the magnetic field created by the flow of current in the primary windings collapses across the primary and secondary windings of the coil. This induces a high-voltage surge in the secondary windings of the coil. Secondary voltage is then discharged to the rotor, which distributes voltage to the appropriate spark plug terminal. Depending on application, the distributor module may have either a 7-terminal or an 8-terminal (sealed connector) ignition module.

The Computer Controlled Coil Ignition (C(3)I) system, used on the 3.8L engine, eliminates the need for a mechanical distributor. The C(3)I ignition system consists of a coil pack (3 coils), ignition module, camshaft and crankshaft sensors, wiring harness and the Electronic Spark Timing (EST) portion of the Electronic Control Module (ECM).

In the C(3)I system, each cylinder is paired with the cylinder that is opposite it in the firing order. Cylinder No. 1 is paired with 4, 2 with 5, and 3 with 6. Spark occurs simultaneously in the cylinder approaching the compression stroke and in the cylinder approaching the exhaust stroke. The cylinder on the exhaust stroke requires less voltage for the spark plug to fire. This leaves the bulk of the available voltage to fire the spark plug for the cylinder on the compression stroke. The process is repeated when the cylinders reverse roles. Each cylinder pair is fired by its own ignition coil.

Input from the Hall Effect cam and crank sensors is used by the ignition module to determine when to trigger the appropriate coil pack. Module passes on camshaft sync-pulse signal to the ECM to initialize sequential fuel injector timing.

1. Type II Ignition Coil Pack On Type II ignition coil pack, 3 separate twin tower coils are independently mounted over the C(3)I ignition module. Each coil provides the spark for 2 simultaneously paired spark plugs. Each coil can be replaced separately.
2. Camshaft Position Sensor The 3.8L Hall Effect camshaft sensor is located on the timing cover, behind and below water pump. The ECM uses camshaft "sync-pulse" signals (passed to ECM by the ignition module) to determine the exact position of the No. 1 piston. Signal is used by ECM to properly initialize fuel injector firing. If camshaft sensor signal is lost, Code 41 will be set. Engine can be restarted and will run in sequential mode; however, odds are 1 in 6 that injectors will spray correctly without camshaft signal. This provides "walk home" protection against cam sensor failure.
3. Combination 3X & 18X Crankshaft Sensor In addition to the camshaft sensor, the 3.8L uses a Hall Effect crankshaft sensor containing 2 interrupter rings. The outside ring contains 18 evenly spaced interrupters, producing 18 pulses per crankshaft revolution. The inner ring has 3 interrupters spaced at irregular intervals (10, 20 and 30 degrees apart). The ignition module monitors signals generated by the 2 interrupter rings. The 18X ring will change state once during the 10-degree gap of the 3X ring, twice during the 20-degree gap and 3 times during the 30-degree gap. The changing relationship between the 2 rings allows the ignition module to identify the correct ignition coil to fire within the first 120 degrees of crankshaft rotation. This system provides for a faster start and a more accurate measurement of crankshaft sensor signals. If the 3X signal to ignition module is lost while the engine is running, the fuel injection system will continue to run in sequential mode; however, loss of 3X or 18X signal will prevent vehicle from restarting.
4. Fuel Control Signal In addition to the RPM reference (18X) signal and fuel sync (camshaft) signals generated by the ignition module on 3.8L, a fuel control reference signal must also be passed on to the ECM in order to inform ECM proper signals are being generated to the ignition module. The fuel control signal is generated by the C(3)I module from calculations involving signals from the crankshaft sensor 18X and the 3X pulse rings.

Ignition Timing Advance

At engine speeds less than 400 RPM, ignition module controls spark advance by triggering coil(s) at a predetermined interval based on engine speed only. At engine speeds greater than 400 RPM (EST mode), ECM controls ignition timing.

ECM controls ignition timing based upon input signals from engine RPM reference line (ignition module), coolant temperature sensor, manifold air temperature sensor, throttle position sensor, knock sensor, vehicle speed sensor, gear position switch and MAP or MAF sensor.

The PROM portion of the ECM has a programmed spark advance curve based on engine speed. ECM calculates spark timing whenever an ignition pulse is present. Spark advance is controlled only when engine is running (not during cranking). ECM uses input signal values to modify PROM information, increasing or decreasing spark advance to achieve maximum performance with minimum emissions. To check ignition system operation, see BASIC TESTING or I - SYS/COMP TESTS article in the ENGINE PERFORMANCE Section.

1. Reference (RPM) On all engines, ignition module converts signals from pick-up coil or Hall Effect sensors to digital signals which are used to trigger ignition coil. Since the signal on this circuit is also used as an injector trigger reference on fuel injected vehicles, engine will not run if circuit is open or grounded.
2. By-Pass When the ECM receives an engine speed signal of approximately 400 RPM, it considers engine to be running and applies 5 volts to the ignition module on the by-pass wire. This causes ignition module to switch timing control over to the variable timing control circuit in the ECM. On some models, this by- pass wire contains a connector located between the 4-wire connector and the ECM. This is disconnected when adjusting base timing. On all models, an open or grounded by-pass circuit sets a related trouble code in ECM memory. The engine runs at base timing plus a small amount of advance built into the HEI module.
3. EST When 5 volts is present on by-pass circuit and ignition module has turned control of engine timing over to ECM, ECM advances or retards spark on this circuit based on calculations involving reference signal and other sensor input signals. If base timing is incorrectly set, entire advance curve will be incorrect.
4. Ground This is the reference ground circuit. It is grounded at distributor and ECM, ensuring there is no voltage drop in the EST circuit which could affect ignition operation.

ESC Detonation Retard Operation

Some models use an Electronic Spark Control (ESC) retard system along with the HEI-EST system. System consists of a knock (detonation) sensor, a high energy ignition system, an ESC controller (some models) and the ECM. On some models, the function of the ESC controller is built into the Memory Calibration (MEM-CAL) unit of the ECM.

When engine knock (detonation) occurs, knock sensor produces a low voltage AC signal. This signal goes to the ESC controller or directly to the MEM-CAL unit inside the ECM, depending upon application.

On models using an ESC controller, controller supplies the ECM with a 12-volt signal. When detonation occurs, controller grounds the 12-volt signal to the ECM, pulling the signal down to near zero volts. The ECM interprets this as a signal to retard timing. The ECM then retards spark timing until the ESC controller returns the 12-volt signal. If signal wire becomes open or grounded on models using ESC controller, ECM continuously provides full ignition timing retard.

On vehicles using ECMs containing MEM-CAL units, the ECM supplies a 5-volt DC reference signal on the knock sensor signal line. Internal circuitry of the knock sensor pulls this voltage down to about 2.5 volts. When knock occurs, the knock sensor produces an AC voltage signal which rides on the 2.5-volt DC signal to the ECM. The voltage and frequency of this signal depend upon knock signals received by the sensor. The ECM retards spark timing until signals from detonation sensor cease. Two knock sensors are used on 4.3L CPI engines.

A malfunction in the ESC circuit should set a related trouble code. If a code is not present and ESC system is suspected cause of driveability problems, perform functional check of ESC system. See I - SYS/COMP TESTS article in the ENGINE PERFORMANCE Section.

AIR INJECTION SYSTEM

Air Injection Reaction (AIR) system is used to reduce carbon monoxide (CO) and hydrocarbon (HC) emissions. The AIR system provides additional oxygen to continue combustion process after exhaust gases leave the combustion chamber. This added air also brings catalytic converter up to operating temperature more quickly when engine is cold. The AIR system diverts air either to the exhaust manifold ports or to the air cleaner.

The system consists of an air pump, an Electric Air Control (EAC) valve (2.8L engine) or an electric air control valve with relief tube (4.3L and V8 engines), solenoid, check valve(s) and plumbing. (Scheme 2)or (Scheme 3)

Note. On EAC valve, divert and signal tube locations are reversed from previous model year.

Electric Air Control (EAC) Valves With Relief Tube

When engine is cold or at wide open throttle, ECM energizes solenoid on valve, and air is directed to exhaust manifold ports. When coolant temperature increases, solenoid is de-energized and air goes into air cleaner.

At higher engine speeds, air is directed to air cleaner through pressure relief valve (if equipped), even though solenoid may be energized. Air should not be entering exhaust manifold during closed loop mode.

During deceleration, the increased manifold vacuum signal directs air to air cleaner. Check valve on air injection pipe prevents exhaust gases from entering air pump. Solenoid is de-energized under rich mixture condition or if SERVICE ENGINE SOON light is on.

Scheme 2

Scheme 3

Air Pump

The air pump is a belt-driven, positive displacement vane-type pump. A centrifugal filter mounted behind the pulley purges air drawn into pump of dirt and contaminants. The air pump is permanently lubricated and requires no periodic service.

Check Valves

Check valves prevent the backflow of exhaust gases into the air injection system. Check valves close when exhaust gas pressure in exhaust manifold exceeds pressure delivered by pump. This occurs when air pump by-passes at high speeds, when air delivery is switched to catalytic converter, when air is diverted to either atmosphere or air cleaner or when air pump malfunctions.

Air Management System

When ECM energizes electronic air control solenoid on a cold vehicle, air is allowed to flow through control valve to exhaust manifold. As coolant temperature increases or system enters closed loop, ECM opens solenoid ground circuit, de-energizing control solenoid. Air is then routed to air cleaner.

CATALYTIC CONVERTER

A 3-way catalytic converter with dual bed is used to reduce exhaust emissions. This type of converter can reduce hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx).

The upstream section of the converter contains a reducing/oxidizing bed to reduce NOx while oxidizing HC and CO. An air supply pipe from the AIR system injects air between the beds of the converter. Thus, the second converter bed oxidizes any remaining HC and CO to efficiently reduce exhaust emissions.

EXHAUST GAS RECIRCULATION (EGR)

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.

There are 2 types of EGR systems used. Port EGR is used on 2.8L, 3.1L, 4.3L ("S" and "T" Series), 7.4L and 5.7L (over 8500 GVWR). Negative backpressure EGR is used on 2.5L, 4.3L (except "S" and "T" Series), 5.0L and 5.7L (under 8500 GVWR).

Port EGR

Port EGR valve is controlled by manifold vacuum regulated by an ECM-controlled solenoid. Vehicles equipped with 4L80-E transmissions use a pulse width-modulated EGR vacuum control solenoid, referred to as an Electronic Vacuum Regulator Valve (EVRV).

Negative Backpressure EGR

Vacuum is applied to upper EGR diaphragm via a hose connected to intake manifold vacuum. Manifold vacuum is also applied to lower EGR diaphragm (through intake port at base of EGR valve).

When manifold vacuum in lower chamber is insufficient to overcome spring tension on lower diaphragm, bleed valve closes, allowing vacuum in upper chamber to open EGR valve. With engine at idle or under light load, high manifold vacuum applied to lower chamber opens air bleed valve in lower diaphragm. This bleeds off vacuum in upper chamber, keeping the EGR valve closed.

EVAPORATIVE EMISSION CONTROL

All vehicles use carbon canister storage for evaporative fuel control. Evaporative emission control system stores gasoline fumes from fuel tank in a carbon canister until fumes can be drawn into engine for burning during combustion process.

The 4 basic components which may be used in evaporative emission system are activated carbon canister (all models, open at top or bottom for fresh air intake), vacuum operated canister control valve (4.3L and V8 high altitude, mounted remotely), thermostatic vacuum switch (2.8L and 3.1L, mounted in coolant passage in intake manifold) and tank pressure control valve (4.3L and V8 high altitude, mounted in hose between canister and fuel tank). For specific component application and vacuum hose routing, see VACUUM DIAGRAMS article in the ENGINE PERFORMANCE Section.

Carbon Canister

Evaporative fumes from fuel tank are vented through hose(s) into a canister containing activated carbon. Activated carbon absorbs and holds fuel vapors when engine is not operating. When engine is started and engine speed is greater than idle (purge at idle would cause too rich a mixture), engine vacuum draws fuel vapors from canister into engine. A vacuum canister purge valve or thermostatic vacuum switch regulates vapors through this purge line.

Carbon canisters are open in design. When the engine is started, engine vacuum draws outside air into canister either through the top or bottom, and then through a filter in bottom of canister. This helps to purge vapors from the activated carbon.

Canister Control Valve (CCV) (High Altitude 4.3L Except "S" & "T" Series & V8)

CCV is vacuum operated. When the engine is not running, vapor from the fuel tank is stored in the carbon canister. When the vehicle is started, vacuum to the upper port draws in the internal vacuum diaphragm, opening the port between the canister and purge valve. When engine is off, internal spring pressure closes valve diaphragm, preventing vapor from venting to atmosphere.

The canister control valve acts as both vapor vent valve and purge valve. When engine is running, manifold vacuum from PCV system pulls lower diaphragm upward. When engine is operating at greater than idle speed, control vacuum pulls upper diaphragm upward. This allows purging of canister through PCV system.

Thermostatic Vacuum Switch (2.8L & 3.1L)

A wax pellet-type thermostatic vacuum switch is installed in the engine coolant passage in the intake manifold. Two vacuum fittings on switch connect to charcoal canister and the TBI unit. When coolant temperature is less than 115°F (46°C), switch closes, preventing purging of canister. When coolant temperature increases to greater than 115°F (46°C), switch opens, allowing purging of canister.

Fuel Tank Pressure Control Valve (High Altitude 4.3L Except "S" & "T" Series & V8)

Fuel tank pressure control valve allows vapors to flow from the fuel tank into the EEC system. When fuel tank pressure exceeds the spring pressure on the valve diaphragm, the valve opens and allows vapors to either enter canister or, when purge is enabled, go directly to the engine.

The tank pressure control valve is located inside the gas cap on the 3.1L, in the engine compartment on "C" and "K" Series and near the fuel tank on other models.

POSITIVE CRANKCASE VENTILATION (PCV)

The PCV system provides more effective elimination 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 a PCV valve 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 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), 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 open against spring pressure, permitting crankcase fumes to enter the intake manifold. Should the engine backfire, the PCV valve closes to prevent ignition of fumes in crankcase.

THERMOSTATIC AIR CLEANER (TAC)

Many models are equipped with a system for preheating the air entering the throttle body during cold engine operation.

This system maintains incoming air temperature to a point where fuel injection system can maintain lean air/fuel ratios to reduce hydrocarbon (HC) and carbon monoxide (CO) emissions. TAC systems are either vacuum motor controlled or wax pellet controlled.

Vacuum Motor Controlled (2.8L & 3.1L)

This system consists of an air cleaner assembly with integral air control door, vacuum control temperature sensor, vacuum motor, heat shroud (on exhaust manifold), heated air tube and vacuum hoses.

1. Vacuum Control Temperature Sensor The vacuum control temperature sensor controls the operation of the air control door. During initial start-up situations, this valve directs engine vacuum to the air control vacuum motor. The motor closes the air intake door, allowing the intake of heated manifold air. When the intake air temperature reaches a pre-calibrated value, this valve opens, allowing the intake of cooler outside air.
2. Air Control Door The air control door temperature sensor closes when the temperature of air entering the air cleaner is less than the calibrated temperature of the temperature sensor. This allows engine vacuum to operate the air control door vacuum motor, and warm manifold air to be routed to the throttle body.
3. Vacuum Motor When engine vacuum is applied to the vacuum motor, the air control door stops the intake of outside air. The air cleaner then draws in air from around the exhaust manifold. As air inside the air cleaner warms, the temperature sensor begins to open, bleeding off vacuum to the vacuum motor. As vacuum to vacuum motor decreases, the air control door begins to open. As air control door opens, outside air is allowed to enter air cleaner assembly. When air entering air cleaner reaches a predetermined temperature, the air control door opens completely and stops the intake of heated air.

Wax Pellet Controlled (Except 2.8L & 3.1L)

A self-contained, wax pellet actuated assembly mounted in the air cleaner controls the air regulator damper (hot/cold air delivery door). When incoming air is cold, wax material sealed in the actuator is in a solid contracted state. As incoming air warms, wax material expands by changing to a liquid state. This forces piston outward, repositioning air regulator damper and allowing air (either a mix of hot and cold or all cold) to enter engine.

Note. For additional information on 6.2L light duty emission EGR system, see appropriate TESTS W/CODES article in the ENGINE PERFORMANCE Section.

The Exhaust Gas Recirculation (EGR) system limits formation of oxides of nitrogen (NOx) emissions by reducing peak combustion chamber temperatures in which NOx is formed. EGR system consists of EGR valve, Exhaust Pressure Regulator (EPR) valve, EPR and EGR vent solenoids and EGR fault detection. A vacuum pump is required to provide a vacuum source to operate the EGR system.

EGR Valve

EGR valve reintroduces a small amount of exhaust gas into combustion chamber, diluting air/fuel mixture and reducing combustion chamber peak temperatures, thereby reducing NOx formation.

EPR Valve

The EPR valve is mounted between the exhaust manifold and the exhaust pipe. Valve increases exhaust backpressure during idle, which increases exhaust flow through EGR system. EPR valve resembles the EFE or heat riser type valves of earlier, carbureted vehicles. A vacuum diaphragm-type actuator opens and closes valve. An ECM-controlled EGR/EPR solenoid regulates actuator.

EGR/EPR Solenoids

EGR/EPR solenoids are mounted at rear of engine as a single assembly. Using input from engine speed sensor and TPS, ECM controls EGR by controlling amount of "on" and "off" time of EGR solenoid. When EGR is not needed, ECM energizes EGR vent solenoid to vent vacuum. Vacuum which controls EGR valve controls EPR valve. ECM energizes EPR solenoid to close EPR valve at idle to increase exhaust backpressure.

EGR Fault Detection

The ECM uses input from the MAP sensor to measure amount of absolute pressure in EGR vacuum line. If a minor variation between calculated EGR and actual EGR is monitored by ECM, the ECM corrects. If variation is too great for ECM to correct, an error is detected. The ECM then enters default mode and sets a related trouble code in memory.

Vacuum Pump

A vacuum pump is mounted on the engine and provides vacuum for operating emission controls (light duty emissions), transmission modulator (heavy duty emissions with M40 automatic transmission), cruise control and heater and A/C servos. The vacuum pump is either belt- or gear-driven.

The belt-driven vacuum pump is bracket mounted to the right front of the engine. Except for the pulley, the vacuum pump is replaced as an assembly.

The gear-driven pump is mounted at the top rear of the engine and contains a permanently-mounted speed sensor. Pump is driven by a cam inside the drive assembly to which it mounts. On the lower end of the drive housing assembly is a drive gear which meshes with the camshaft gear in the engine. The drive gear causes the cam in the drive housing to rotate.

CRANKCASE DEPRESSION REGULATOR (CDR)

The CDR valve, located on the right valve cover, is used on both light and heavy duty diesel engines. Valve prevents crankcase pressure from accumulating during idle by regulating (metering) crankcase pressure back into the engine. Intake manifold vacuum acts against a spring-loaded diaphragm to control flow of crankcase gases. Higher intake manifold vacuum levels pull diaphragm closer to the top of the outlet tube, reducing amount of gases drawn from crankcase. As intake manifold vacuum drops, spring pressure pushes diaphragm away from top of outlet, allowing more gases to flow from crankcase into intake manifold.

Optimum pressure in crankcase is one inch of water (as measured with a manometer) at idle to 3-4 inches at full load. Too little vacuum causes oil leaks; too much vacuum pulls oil into the air crossover.

SELF-DIAGNOSTIC SYSTEM

The ECM is equipped with a self-diagnostic system which detects system failures or abnormalities. When a malfunction occurs, ECM illuminates the SERVICE ENGINE SOON light located on instrument panel. When malfunction is detected and light is turned on, a corresponding trouble code is stored in ECM memory. Malfunctions are designated as either "hard failures" or "intermittent failures". For procedures on retrieving stored codes, see appropriate TESTS W/CODES article in the ENGINE PERFORMANCE Section.

"Hard Failures"

Hard failures cause SERVICE ENGINE SOON light to glow and remain on until malfunction is repaired. If light comes on and remains on during vehicle operation, cause of malfunction must be determined using diagnostic charts located in TESTS W/CODES article in the ENGINE PERFORMANCE Section. If a sensor fails, ECM uses a substitute value in its calculations to continue engine operation. Although vehicle is functional in this condition, driveability will probably be adversely affected.

"Intermittent Failures"

Intermittent failures cause SERVICE ENGINE SOON light to flicker or illuminate and go out about 10 seconds after intermittent fault goes away. However, ECM retains corresponding trouble code in memory. If related fault does not reoccur within 50 engine restarts, related trouble code is erased from ECM memory. Sensor, connector or wiring related problems may cause intermittent failures. See TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

As a bulb and system check, SERVICE ENGINE SOON light glows when ignition switch is turned to ON position and engine is not running. When engine is started, light should go out. If not, a malfunction has been detected in computerized engine control system or SERVICE ENGINE SOON light circuit is faulty.

To verify proper operation of SERVICE ENGINE SOON light on gasoline vehicles, see DIAGNOSTIC CIRCUIT CHECK in BASIC TESTING article in the ENGINE PERFORMANCE Section. To verify proper operation of SERVICE ENGINE SOON light and retrieve trouble codes on diesel vehicles, see DIAGNOSTIC CIRCUIT CHECK chart in appropriate TESTS W/CODES article in the ENGINE PERFORMANCE Section.

ECM is equipped with a serial data line. Serial data is a stream of electrical impulses which can be interpreted by special testers of other control modules. Serial data must be accessed by connecting special Scan testers to the Assembly Line Data Link (ALDL) connector. Update intervals and information contained within the data stream vary with model application.

MISCELLANEOUS ECM CONTROLS

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

A/C CLUTCH

On many models, ECM regulates operation of the A/C clutch through an ECM-controlled relay. The ECM 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 freon pressure drops to less than or rises to greater than normal operating levels.

Freon pressure is sensed through the monitoring of high and low pressure switches or a pressure sensor which registers either high or low pressure levels. Power steering load is monitored through a power steering pressure switch (2.5L). Hot restart is monitored through the coolant temperature sensor. For component application and related wiring, see wiring schematics under MISCELLANEOUS ECM CONTROLS in I - SYS/COMP TESTS article in the ENGINE PERFORMANCE Section.

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 freon pressure increases beyond a certain point, high side switch opens, causing compressor clutch to disengage.

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

COOLING FAN (3.1L & 3.8L)

ECM regulates operation of the electric cooling fan through an ECM controlled relay which controls the ground or power circuit for the cooling fan. ECM operates the cooling fan based upon engine temperature. Most systems engage electric cooling fan whenever the A/C clutch is engaged, regardless of engine temperature. As a back-up system, many models use a coolant override switch, which engages the cooling fan in case the ECM fails to energize the cooling fan relay or if the cooling fan relay malfunctions. A malfunction of the cooling fan causes engine overheating and possible detonation.

For component application and related wiring, see wiring schematics under MISCELLANEOUS ECM CONTROLS in I - SYS/COMP TESTS article in the ENGINE PERFORMANCE Section.

Converter Clutch

The transmission/transaxle converter clutch eliminates power loss of torque converter stage when vehicle is in a cruise condition, allowing driver convenience of automatic transmission and fuel economy of a manual transmission. Fused battery ignition is supplied to converter solenoid through a brake switch.

On some models, 2nd, 3rd and 4th gear hydraulic apply switches (located within transmission) may also be in series with solenoid power or ground circuit. On other models, switch status may only be monitored by ECM, without sharing power or ground with converter solenoid. For wiring reference, see wiring schematics under MISCELLANEOUS ECM CONTROLS in I - SYS/COMP TESTS article in the ENGINE PERFORMANCE Section.

Converter clutch engages when vehicle is moving faster than a pre-calibrated speed, engine is at normal operating temperature, throttle position sensor output is not changing (indicating a steady road speed) and transmission 3rd gear or high gear switch (if equipped) and brake switch are closed.

When vehicle speed is great enough (about 20-45 MPH as indicated by the vehicle speed sensor), ECM energizes converter clutch solenoid mounted in transmission, allowing torque converter to directly connect engine to the transmission. When operating conditions indicate transmission should operate as normal, converter clutch solenoid is de-energized, allowing transmission to return to normal automatic operation. Since power for the converter solenoid is delivered through the brake switch, transmission also returns to normal automatic operation when brake pedal is depressed. To check function of converter clutch system, perform functional check of system. See MISCELLANEOUS ECM CONTROLS in I - SYS/COMP TESTS article in the ENGINE PERFORMANCE Section.

Electronic Transmission (4L80-E)

On gasoline vehicles equipped with 4L80-E transmission, Powertrain Control Module (PCM) controls transmission and other vehicle functions. On diesel vehicles, Transmission Control Module (TCM) controls electronic transmission, but no other components. PCM/TCM monitors a number of engine/vehicle functions and uses data to control shift solenoid "A", shift solenoid "B", TCC and the force motor and regulate TCC engagement, upshift pattern, downshift pattern and line pressure (shift quality).

1. Shift Solenoid "A" Shift solenoid "A" is attached to the valve body and is a normally open exhaust valve. PCM/TCM activates solenoid by grounding it through an internal quad-driver. Solenoid "A" is on in 1st and 4th gears, but off in 2nd and 3rd. When on, solenoid redirects fluid to act on the shift valves. Solenoid "A" is Blue. Code 82 is associated with solenoid "A".
2. Shift Solenoid "B" Shift solenoid "B" is attached to the valve body and is a normally open exhaust valve. PCM/TCM activates solenoid by grounding it through an internal quad-driver. Solenoid "B" is on in 3rd and 4th gears, but off in 1st and 2nd. When on, solenoid redirects fluid to act on the shift valves. Solenoid "B" is Red. Codes 81, 86 and 87 are associated with solenoid "B".
3. Force Motor Force motor is attached to the valve body and controls line pressure by moving a pressure regulator valve against spring pressure. Force motor replaces the throttle valve or vacuum modulator used on past transmissions. PCM/TCM varies line pressure based upon engine load. Engine load is calculated from various inputs, especially the TPS. Line pressure is actually varied by changing the amperage applied to the force motor from zero (high pressure) to 1.1 amps (low pressure). The force motor is periodically pulsed to prevent fluid contamination from causing pressure regulator valve to stick.

Shift Light

Shift light may be used on vehicles equipped with manual transmission. Light indicates best transmission shift point for maximum fuel economy. Power for light is supplied through GAUGES fuse. Light illuminates when ECM supplies a ground circuit for bulb. For wiring reference, see wiring schematics under MISCELLANEOUS ECM CONTROLS in I - SYS/COMP TESTS article in the ENGINE PERFORMANCE Section.