Engine Controls - Theory & Operation Chevrolet Chevy Van G20

INTRODUCTION

This article covers the basic description and operation of engine performance related systems and components. Before diagnosing vehicles or systems with which you are not completely familiar, read through this article.

SPEED DENSITY

All gasoline vehicles are equipped with a MAP sensor, using the speed density method to compute the airflow rate. Manifold pressure is used to calculate the airflow rate to the ECM. 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 output voltage, the ECM determines manifold pressure. If MAP sensor fails, the ECM will supply a fixed MAP value and use the TPS to control fuel.

On 2.5L models, a Manifold Air Temperature (MAT) is used. Sensor allows ECM to determine intake air temperature. Signal is used by ECM to delay EGR until intake air temperature reaches about 40°F (5°C). If intake air temperature becomes excessively high, ECM will retard timing slightly to compensate.

COMPUTERIZED ENGINE CONTROLS (GASOLINE) DESCRIPTION

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 following sub-systems: Electronic Control Module (ECM), Input Devices (sensor and switch input signals) and Output Signals.

ELECTRONIC CONTROL MODULE (ECM) DESCRIPTION

ECM is located in passenger compartment. For exact location of ECM, see ECM LOCATION in appropriate SELF-DIAGNOSTICS article. 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. This will correct itself and normal performance will return if vehicle is allowed to "relearn" optimum control conditions. This is accomplished by driving vehicle 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 by ECM from various engine sensors, into digital signals for use by the CPU.

CENTRAL PROCESSING UNIT (CPU)

Digital signals received by CPU are used 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 commands 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

The 3 types of memories used in ECMs are: Read Only Memory (ROM), Random Access Memory (RAM) and Programmable Read Only Memory (PROM).

1. Read Only Memory (ROM) - ROM is programmed information that can only be read by ECM. The ROM program cannot be changed. If battery voltage is removed, ROM information will be 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 will be retained.

Note. INPUT DEVICES are components which modify or produce voltage signals monitored by the ECM. OUTPUT SIGNALS are components controlled by the ECM.

INPUT DEVICES

Each sensor or switch furnishes electronic (voltage) signals to ECM. The ECM uses these input signals to compute ignition timing, air/fuel ratio and idle speed for proper driveability and emission control. Various models are equipped with different combinations of input devices. Not all devices are used on all models. To determine the input usage, see appropriate wiring diagram in WIRING DIAGRAMS article. The available input signals include the following

A/C ON (A/C REQUEST) SIGNAL (3.1L)

The air conditioner "on" switch is mounted in instrument panel. This switch provides a simple "on" or "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 SYSTEM/COMP TESTS article.

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 will open, causing compressor clutch to disengage.

If system freon level decreases, causing freon pressure to drop below normal, low side pressure switch will open, causing compressor clutch to disengage, preventing compressor damage.

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 enrichen 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 will shut down for as long as condition exists. If condition is short-term, vehicle may stumble and SERVICE ENGINE SOON light will flicker. If condition lasts long enough, vehicle will stall.

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), one circuit of brake switch is in series with the power supply for the TCC solenoid located in 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 reduced by resistance of the CTS. When coolant temperatures are low, CTS resistance is high and a high monitored voltage signal is seen by the ECM. When coolant temperatures are high, CTS resistance is low and a low monitored voltage is seen by the ECM. 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.

CRANK SIGNAL

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

FUEL PUMP FEEDBACK

Fuel pump circuit between fuel pump relay/oil pressure switch and fuel pump is monitored by the ECM. 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 will result 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. High gear switch information is used by ECM in controlling emission components and engagement of Torque Converter Clutch (TCC).

KNOCK SENSOR (EXCEPT 2.5L)

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. Controller normally applies a 12-volt signal to terminal B7 at the ECM. ECM interprets this as a "no knock" condition. When controller receives a knock signal from the knock sensor, controller will remove the 12-volt signal from the ECM monitored knock signal line. ECM will then retard ignition timing until the engine knock ceases (12-volt signal is returned by the controller). 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 suspected as the cause of a driveability problem, perform functional check of ESC system. See SYSTEM/COMP TESTS article.

MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR

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 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 will substitute a fixed MAP value and will use 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 SYSTEM/COMP TESTS article.

MANIFOLD AIR TEMPERATURE (MAT) SENSOR (2.5L)

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 allows ECM to determine intake air temperature. Signal is used by ECM to delay EGR until intake air temperature reaches about 40°F (5°C). If intake air temperature becomes excessively high, ECM will retard timing slightly to compensate. 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.

OXYGEN (O2) SENSOR

The O2 sensor is mounted in the exhaust system where it 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 signal line. This is referred to as "cross counts".

The O2 sensor will 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 will function in "open loop" mode and ECM will not make air/fuel adjustments based upon O2 sensor signals but will use 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 will begin 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 it to "open loop". A problem in the O2 sensor circuit should set a related trouble code.

PARK/NEUTRAL SWITCH (P/N)

This switch is connected to transmission gear selector. The switch signals ECM when transmission is in Park or Neutral. Information from P/N switch is used by ECM for determining control of ignition timing, converter clutch and idle speed. To check function of P/N switch, perform functional check of switch. See SYSTEM/COMP TESTS article.

POWER STEERING (P/S) SWITCH ("S" & "T" SERIES WITH 2.5L)

This switch informs ECM of engine load conditions which exist when steering wheel is turned from center to full lock position. Information is used by ECM to help control idle speed. To check function of P/S switch, perform functional check of switch. See SYSTEM/COMP TESTS article.

RPM REFERENCE SIGNAL

The RPM is monitored by ECM through ignition module tach/pulse signals (on circuit No. 430) produced by the HEI module (RPM reference line of 4-wire EST connector) or camshaft position sensor signal (Hall Effect signal on 2.5L). Signal is used by ECM for determining control of timing, fuel delivery, EGR function and idle speed. Signal is used by ECM to trigger fuel injectors.

THROTTLE POSITION SENSOR (TPS)

The TPS is a variable mechanical resistor connected either 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. The second 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). This signal is used by ECM for determining control of fuel, idle speed, spark timing and converter clutch. A problem in the TPS circuit may set a related trouble code.

VEHICLE SPEED SENSOR (VSS)

Depending upon vehicle application, VSS is either a Permanent Magnet (PM) generator mounted in transmission or a Light Emitting Diode (LED) mounted in instrument panel cluster, behind speedometer. The VSS sends a pulsing signal to ECM, which ECM converts into Miles Per Hour (MPH). This sensor input is used by ECM in controlling converter clutch engagement.

OUTPUT SIGNALS

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

1. Air Injection Control Solenoid (Emission Systems)
2. Canister Purge Solenoid Valve (Emission Systems)
3. SERVICE ENGINE SOON Light (Self-Diagnostic System)
4. EGR Control Solenoid Valve (Emission Systems)
5. ESC Timing Retard (Ignition System)
6. Fuel Injectors (Fuel Control)
7. Fuel Module (Fuel Delivery)
8. Fuel Pump & Fuel Pump Relay (Fuel Delivery)
9. HEI-EST Ignition (Ignition System)
10. Idle Air Control (IAC) Valve (Fuel Injected - Idle Speed)
11. Self-Diagnostics (Self-Diagnostic System)
12. Serial Data (Self-Diagnostic System)
13. Converter Clutch (Miscellaneous ECM Controls)
14. Transmission Downshift Relay (THM-400 - Miscellaneous ECM Controls)

COMPUTERIZED ENGINE CONTROLS (DIESEL)

The Diesel Electronic Control (DEC) system is used on the 6.2L Diesel engine with light duty emissions. The DEC system consists of the Electronic Control Module (ECM), Input Devices and Output Signals. The DEC system electronically controls EGR system operation, Torque Converter Clutch (TCC) engagement, cold advance, and glow plug system.

ELECTRONIC CONTROL MODULE (ECM)

The Electronic Control Module (ECM) is located in the passenger compartment, behind glove box. It constantly monitors the information from various sensors to control the EGR, TCC, cold advance, and glow plug systems. The ECM processes input signals from sensors and then sends the 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 code(s) which identify problem areas to aid technicians in making system repairs.

The 3 types of memories used in ECMs are: Read Only Memory (ROM), Random Access Memory (RAM) and Programmable Read Only Memory (PROM).

1. Read Only Memory (ROM) - ROM is programmed information that can only be read by ECM. The ROM program cannot be changed. If battery voltage is removed, ROM information will be 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 will be 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. The available input signals include the following

CTS is a thermistor (a resistor that changes its resistance value based on temperature). 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).

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

MANIFOLD ABSOLUTE PRESSURE SENSOR (MAP)

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 back to the ECM.

The signal is compared to the EGR duty cycle calculated by the ECM. If there is a difference in the vacuum value sensed and the ECM command, the ECM makes minor correction adjustments. 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.

VSS sends a pulsing signal to ECM so can calculate vehicle speed. It is mounted on the transmission. This calculation is used to control TCC engagement.

Note. Output signals are regulated by the ECM to maintain correct driveability and exhaust emissions.

ELECTRONIC CONTROLLER/GLOW PLUG RELAY

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

COLD ADVANCE CONTROL

The cold advance control circuit is designed to advance injection pump timing about 4 degrees during cold engine operation. This circuit is activated by the ECM, through the cold advance relay, to energize the cold advance solenoid. The ECM opens the circuit when coolant temperature is above 95°F (35°C).

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

TORQUE CONVERTER CLUTCH

The Torque Converter Clutch (TCC) system uses a solenoid operated valve, in the automatic transmission, to couple the engine flexplate to the output shaft of the transmission through the torque converter.

In order for the TCC to apply, the following conditions must be met: transmission fluid pressure must be correct. The ECM must complete the ground circuit to energize the TCC apply solenoid in the transmission. The solenoid moves a check ball and allows hydraulic pressure to apply the torque converter clutch.

After the torque converter clutch applies, the ECM uses the information from the throttle position sensor to release the clutch when vehicle is accelerating or decelerating at a certain rate. The normally closed brake switch opens when brake pedal is depressed, de-energizing the TCC solenoid.

EXHAUST GAS RECIRCULATION SYSTEM

See EXHAUST GAS RECIRCULATION under EMISSION SYSTEMS (DIESEL) in this article.

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 of about 18 psi (1.3 kg/cm 2 ).

A pressure regulator mounted on throttle body unit keeps fuel available to injector(s) at a constant pressure of 9-13 psi (.6-.9 kg/cm 2 ) at idle. Excess fuel is returned to fuel tank through pressure regulator return line.

When the ignition switch is turned to the ON position, ECM will turn on the electric fuel pump by energizing the fuel pump relay. The ECM will keep the pump on if the engine is running or cranking (ECM is receiving reference pulses from the ignition module). If there are no reference pulses, ECM turns pump off within 2 seconds after key is turned on. For additional information, see FUEL PUMP RELAY under FUEL DELIVERY in this article.

FUEL PRESSURE REGULATOR

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 the throttle body. Spring tension maintains a constant fuel pressure to injector regardless of engine load.

FUEL PUMP RELAY

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

As a back-up system to fuel pump relay, fuel pump is also activated by the oil pressure switch. 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, operating 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 sender or sensor.

Fuel pump circuit between fuel pump relay/oil pressure switch and fuel pump is monitored by the ECM. 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 will result in the setting of a related trouble code in ECM memory.

For additional information on fuel pump activation, see BASIC TESTING and SYSTEM/COMP TESTS articles.

FUEL MODULE (5.7L & 7.4L)

A fuel module (mounted near ECM) will override ECM 2-second fuel pump relay timer and fuel pump will run for 20 seconds when ignition is turned on. This will occur even if vehicle is not started. This function helps to restart vehicle during high ambient temperatures (possible vapor lock conditions).

FUEL CONTROL

The ECM, using input signals, determines adjustments to the air/fuel mixture in order to provide the optimum ratio for proper combustion under all operating conditions. Throttle Body Injection (TBI) systems can operate in the "open loop" or "closed loop" mode. Description of these modes is as follows: 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 Manifold Absolute Pressure (MAP) sensor readings. Engine will remain 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 oxygen sensor has reached operating temperature, coolant temperature has reached 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 mixture as possible. If oxygen sensor cools off (due to excessive idling) or a fault occurs in the oxygen sensor circuit, vehicle will once again enter "open loop" mode.

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.

FUEL CUT-OFF

Injectors are de-energized when ignition is turned off so that dieseling is prevented. Injectors will not be energized if RPM reference pulses are not received by the ECM, even with ignition on. This prevents flooding before starting. Fuel cut-off will also occur at high engine RPM to prevent internal damage to engine. Some fuel injected models may also cut off fuel injector signals during periods of high speed, closed throttle deceleration (when fuel is not needed).

THROTTLE BODY INJECTION

Injector is located in throttle body unit. Dual injector 220 series throttle body is used on all models except 2.5L. The 2.5L uses the 700 series single injector throttle body. Battery voltage is supplied to the injector when the ignition is on. ECM energizes solenoid by providing a ground path through its internal circuitry. By regulating the injector ground circuit, ECM controls injector "on" time (pulse width) to provide proper amount of fuel to engine.

Pressure to injector is maintained at 9-13 psi (.6-.9 kg/cm 2 ) by the pressure regulator. Excess fuel passes through pressure regulator and is returned 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 providing 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.

During starting, clear flood mode, deceleration and heavy acceleration, fuel delivery is controlled by internal ECM calibration.

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. Air/fuel ratio is determined by ECM 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 longer (richer air/fuel mixture ratio). When coolant temperature is high, injector pulse width becomes shorter (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 will be 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 - Fuel enrichment during heavy acceleration is provided by ECM. 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 non-synchronized (not in proportion to distributor reference signals). 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.

IDLE SPEED

Engine idle speed is controlled by the ECM depending upon engine operating conditions. The ECM senses engine operating conditions and determines the best idle speed.

IDLE AIR CONTROL VALVE

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 and controls the amount of air by-passed around the throttle plate. The IAC valve moves its pintle in and out in steps referred to as "counts" (0 counts-fully seated, 255 counts-fully retracted) to control engine idle speed. Counts can be measured using a Scan tester plugged 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 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. On other models it may be necessary to drive 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 design IAC valves are used. Ensure proper design valve is used during replacement.

FUEL DELIVERY

An electric fuel pump is mounted on the left side of frame and 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) 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. Engine RPM is controlled by a rotary fuel metering valve. 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

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.

GLOW PLUGS

Glow plugs are small 6-volt heaters powered by 12 volts to give rapid heating. The glow plugs are operated by the electronic controller and 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 turns off.

The glow system for the LH6 (light duty emissions version) is somewhat different from 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, glow plug temperature inhibit is controlled by the ECM which receives temperature information from the coolant temperature sensor, located in the water crossover of the engine.

The computer then 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 for diesel, see appropriate TESTS W/CODES article.

Note. Using a jumper wire on by-pass relay will cause glow plug failure.

GLOW PLUG INHIBIT SWITCH

The LL 4 (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 the color of their cap. A switch with a Black cap is a temperature-controlled switch. A switch with a Natural cap color is an optional switch which is always closed, allowing for more frequent cycling of glow plugs.

GLOW PLUG AFTER-START

The glow plug controller provides glow plug operation after starting a cold engine. This after-start operation is initiated when the ignition switch is returned to RUN from the 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 is used to activate the HPCA circuit. On heavy duty emissions (LL4 models), the HPCA circuit is controlled by a coolant temperature switch located on rear of right-hand cylinder head. The circuit advances injection timing about 4 degrees when engine is cold.

When engine temperature is below 80°F (27° C), the circuit decreases housing pressure from 10 psi to zero (.7 to zero kg/cm 2 ). At the same time, 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 will close again when engine temperature falls below 85°F (30°C).

IGNITION SYSTEM (GASOLINE)

HEI-EST DISTRIBUTOR

The Delco-Remy High Energy Ignition Electronic Spark Timing (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 means of a 4-wire connector, leading to Electronic Control Module (ECM).

No vacuum or centrifugal advance mechanisms are used. All spark timing changes are controlled by the Electronic Control Module (ECM) based upon monitored input signals. Some models use an additional Electronic Spark Control (ESC) ignition retard system in the event 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 has started, 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 it to the appropriate spark plug terminal. The distributor module may have either a 7-terminal ignition module or an 8-terminal ignition module (sealed connector module) depending on application.

The 2.5L HEI-EST system is also equipped with a Hall Effect switch inside of the distributor. The Hall Effect switch produces a camshaft signal which is used by the ECM to trigger injectors. Loss of the camshaft signal will result in a no-start condition.

IGNITION TIMING ADVANCE

At engine speeds less than 400 RPM, the 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), the ECM takes over control of the ignition timing.

Ignition timing is controlled by the ECM based upon input signals from the 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 the MAP sensor.

The PROM portion of the ECM has a programmed spark advance curve based on engine speed. Spark timing is calculated by ECM whenever an ignition pulse is present. Spark advance is controlled only when engine is running (not during cranking). Input signal values are used by ECM 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 SYSTEM/COMP TESTS article.

1. Reference (RPM) - Alternating current signals from the pick- up coil are converted by the ignition module to digital signals which are used to trigger the ignition coil. On models without Hall Effect sensor, signal is passed on by the ignition module for use by the ECM. On models equipped with Hall Effect sensor (2.5L), a separate signal is produced by the Hall Effect switch for use by the ECM. This ignition module or Hall Effect signal supplies RPM data and crankshaft position reference to the ECM. Since the signal on this circuit is used as an injector trigger reference on fuel injected vehicles, if circuit is open or grounded, engine will not run.
2. By-Pass - When an engine speed signal of approximately 400 RPM is received by the ECM, ECM 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 will set a related trouble code in ECM memory. The engine will run at base timing plus a small amount of advance built into the HEI module.
3. EST - When 5 volts is present on the by-pass circuit and ignition module has turned control of engine timing over to ECM, the ECM advances or retards spark on this circuit based on calculations involving the 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

In conjunction with the HEI-EST system, an Electronic Spark Control (ESC) retard system is used on all except 2.5L engine. System consists of the following: a detonation (knock) sensor, a high energy ignition system, an ESC controller, and the ECM.

When detonation (engine knock) occurs, detonation sensor produces a low voltage AC signal. This signal goes to the 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 zero volts. The ECM interprets this as a need to retard timing. The ECM then retards spark timing until the ESC controller returns the 12-volt signal.

If ESC signal wire were to become open or grounded, ECM would continuously provide full ignition timing retard. A malfunction in the ESC circuit should set a related trouble code. If a code is not present and ESC system is suspected as the cause of driveability problems, perform functional check of ESC system. See SYSTEM/COMP TESTS article.

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, 2.8L engine) or an electric air control with Relief Tube valve (ECT, 4.3L and V8 engines), solenoid, check valve(s) and plumbing.

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

ELECTRIC AIR CONTROL (EAC) VALVES WITH RELIEF TUBE (ECT)

VALVES

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. Under rich mixture condition or SERVICE ENGINE SOON light is on, solenoid is de-energized.

Scheme 2

Scheme 3

AIR PUMP

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

Note. Always cover centrifugal filter fan before cleaning engine to prevent liquid from entering air pump. DO NOT oil air pump.

CHECK VALVE

The check valve prevents the backflow of exhaust gases into the air injection system. The check valve closes when exhaust gas pressure in exhaust manifold exceeds pressure delivered by pump. This occurs when air pump by-passes at high speeds, air delivery is switched to catalytic converter, air is diverted to atmosphere or air cleaner, or when air pump malfunctions.

AIR MANAGEMENT SYSTEM

When ECM energizes the electronic air control solenoid on a cold vehicle, air is allowed to flow through the control valve to the exhaust manifold. As coolant temperature increases, or system goes into closed loop, the ECM opens the solenoid ground circuit, de-energizing the control solenoid. When this occurs, air is routed to the 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 at the same time oxidizing HC and CO. An air supply pipe from the AIR system injects air between the beds of the converter. This is so the second converter bed can oxidize 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

The port EGR valve is controlled manifold vacuum regulated by an ECM controlled solenoid.

NEGATIVE BACKPRESSURE EGR

Vacuum is applied to upper EGR diaphragm via a hose connected 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 will be closed, 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 (EEC)

Carbon canister storage is used for evaporative fuel control on all vehicles. The function of evaporative emission control system is to store gasoline fumes from fuel tank in a carbon canister until fumes can be drawn into engine for burning during combustion process.

There are 4 basic components which may be used in evaporative emission system.

1. Activated carbon canister (All models - open at top or bottom for fresh air intake).
2. Vacuum operated canister control valve (4.3L and V8 high altitude - mounted remotely).
3. Thermostatic vacuum switch (2.8L and 3.1L - mounted in coolant passage in intake manifold).
4. 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.

CARBON CANISTER

Evaporative fumes from the fuel tank are vented through hose(s) into a canister containing activated carbon. The activated carbon absorbs and holds fuel vapors when the engine is not operating. When the 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 the canister into the engine. Regulation of vapors through this purge line may be controlled by a vacuum canister purge valve or thermostatic vacuum switch.

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

Canister Control Valve (CCV)

Canister control valve 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 will draw in the internal vacuum diaphragm, opening the port between the canister and purge vacuum. When engine is off, valve diaphragm is closed by internal spring pressure, 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 above idle speed, control vacuum pulls upper diaphragm upward. This allows purging of canister through PCV system.

Thermostatic Vacuum Switch

On 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 will be closed, preventing purging of canister. When coolant temperature increases to greater than 115°F (46°C), switch will open, allowing purging of canister.

Fuel Tank Pressure Control Valve

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 enter canister or go directly to the engine when purge is enabled.

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 is used to provide for 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 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 where 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 air inlet and be consumed during normal combustion.

The PCV valve is held closed by spring pressure 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 will close to prevent ignition of fumes in crankcase.

THERMOSTATIC AIR CLEANER (TAC)

Many models are equipped with a system for preheating the air entering the fuel injection unit during cold engine operation.

This system maintains incoming air temperature to a point where the carburetor or fuel injection system can maintain lean air/fuel ratios to reduce hydrocarbon (HC) and carbon monoxide (CO) emissions, and reduces carburetor icing.

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.

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 precalibrated value, this valve opens, allowing the intake of cooler outside air.

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 carburetor.

VACUUM MOTOR

When engine vacuum is applied to the vacuum motor, the air control door closes off the intake of outside air. Air is then drawn into the air cleaner 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 closes off the intake of heated air.

THROTTLE RETURN CONTROL SYSTEM

Throttle Return Control (TRC) system is used on all Heavy Duty emission models. System consists of throttle lever actuator, solenoid vacuum control valve and engine speed switch.

Manifold vacuum is routed through solenoid vacuum valve, which is normally closed, to throttle lever actuator. Upon vehicle deceleration, electronic speed switch signals solenoid vacuum valve to open when engine speed is greater than preset RPM.

When valve opens, manifold vacuum is directed to throttle lever actuator, which extends to open throttle slightly. When engine speed drops to less than preset RPM, solenoid valve closes, retracting throttle lever actuator and returning throttle to curb idle position.

EXHAUST GAS RECIRCULATION

Note. For additional information on 6.2L light duty emission EGR system, see appropriate TESTS W/CODES for diesel article.

Purpose of the Exhaust Gas Recirculation (EGR) system is to limit formation of oxides of nitrogen (NOx) emissions. This is done by reducing peak combustion chamber temperatures during which NOx is formed. EGR system consists of EGR valve, Exhaust Pressure Regulator (EPR) valve, EGR, 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 mount of exhaust gas into the combustion chamber, diluting the air/fuel mixture and reducing combustion chamber peak temperatures, and thereby reducing NOx formation.

EPR VALVE

The EPR valve is mounted between the exhaust manifold and the exhaust pipe. Valve is used to increase exhaust backpressure during idle, which increases the exhaust flow through the EGR system. EPR valve resembles the EFE or heat riser type valves which were used on earlier carbureted vehicles. Valve is opened and closed by a vacuum diaphragm type actuator. Actuator is regulated by an ECM controlled EGR/EPR solenoid.

EGR/EPR SOLENOIDS

The EGR/EPR solenoids are mounted at the rear of the engine as a single assembly. ECM controls EGR by controlling amount of "on" and "off" time of EGR solenoid using input from engine speed sensor and TPS. When EGR is not needed, the EGR vent solenoid is energized by the ECM to vent vacuum. The same vacuum that controls the EGR valve controls the 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 will make a correction. If variation is too great for ECM to correct, an error is detected. The ECM will go then into default mode and set 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 auto. trans.), cruise control and heater and A/C servos. The vacuum pump is either belt driven or gear driven.

The belt-driven vacuum pump used on "G" and "P" Series is bracket mounted to the right front of the engine. With the exception of the pulley, the vacuum pump is replaced as an assembly.

The gear-driven pump, used on "R" and "V" Series, is mounted at the top, rear of the engine. It is driven by a cam inside the drive assembly to which it mounts. The drive housing assembly has a drive gear on the lower end 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. This is accomplished when valve regulates (meters) 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 gasses being 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 will cause oil leaks, while too much vacuum will pull 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 will illuminate the SERVICE ENGINE SOON light located on instrument panel. When malfunction is detected and light is turned on, a corresponding trouble code will be stored in ECM memory. Malfunctions are designated as either "hard failures" or as "intermittent failures". For procedures on retrieving stored codes, see appropriate SELF-DIAGNOSTICS article.

"HARD FAILURES"

Hard failures cause SERVICE ENGINE SOON light to glow and remain on until the 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. If a sensor fails, ECM will use a substitute value in its calculations to continue engine operation. In this condition, vehicle is functional, but loss of good driveability will most likely be encountered.

"INTERMITTENT FAILURES"

Intermittent failures cause SERVICE ENGINE SOON light to flicker or illuminate and go out about 10 seconds after the intermittent fault goes away. The corresponding trouble code, however, will be retained in ECM memory. If related fault does not reoccur within 50 engine restarts, related trouble code will be erased from ECM memory. Intermittent failures may be caused by sensor, connector or wiring related problems. See H - TESTING W/O CODES article.

SERVICE ENGINE SOON LIGHT

As a bulb and system check, SERVICE ENGINE SOON light will glow 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 the 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. To verify proper operation of SERVICE ENGINE SOON light and retrieve trouble codes on diesel vehicles, see DIAGNOSTIC CIRCUIT CHECK chart in TESTS W/CODES for diesel article.

SERIAL DATA

ECM is equipped with a serial data line. Serial date is a stream of electrical impulses which can be interpreted by special testers of other control modules. Serial data must be accessed using special Scan testers connected 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. This allows the ECM 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 freon pressure drops below or rises above normal operating levels. Freon pressure sensing may be accomplished through the monitoring of 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 wiring schematics in MISCELLANEOUS ECM CONTROLS in SYSTEM/COMP TESTS article.

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 will open, causing compressor clutch to disengage.

If system freon level decreases, causing freon pressure to drop, low side pressure switch will open, causing compressor clutch to disengage and preventing compressor damage.

COOLING FAN (3.1L ONLY)

ECM regulates operation of the electric cooling fan through an ECM controlled relay which controls the ground circuit or power circuit for the cooling fan. This allows the ECM to operate the cooling fan based upon engine temperature. Most systems will engage the electric cooling fan whenever the A/C clutch is engaged, regardless of engine temperature. As a back-up system, many models utilize a coolant override switch which will also engage the cooling fan in the event that the ECM fails to energize the cooling fan relay, or the cooling fan relay malfunctions. A malfunction of the cooling fan will cause engine overheating and possible detonation.

For component application and related wiring, see wiring schematics in MISCELLANEOUS ECM CONTROLS in SYSTEM/COMP TESTS article.

CONVERTER CLUTCH

The purpose of the transmission/transaxle converter clutch feature is to eliminate power loss of torque converter stage when vehicle is in a cruise condition. This allows 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 the transmission) may also be in series with solenoid power or ground circuit. On other models, switch status may only be monitored by the ECM, without sharing power or ground with the converter solenoid. For wiring reference, see MISCELLANEOUS ECM CONTROLS in SYSTEM/COMP TESTS article.

Converter clutch will engage when vehicle is moving greater than a precalibrated speed, engine is at normal operating temperature, throttle position sensor output is not changing (indicating a steady road speed), transmission 3rd gear or high gear switch is closed (if equipped), and brake switch is 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. This allows torque converter to directly connect engine to the transmission. When operating conditions indicate that transmission should operate as normal, converter clutch solenoid is de-energized. This allows transmission to return to normal automatic operation. Since power for the converter solenoid is delivered through the brake switch, transmission will also return 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 SYSTEM/COMP TESTS article.

SHIFT LIGHT

The shift light is used on vehicles equipped with manual transmission. Light indicates the best transmission shift point for maximum fuel economy. Power for light is supplied through the GAUGES fuse. Light is illuminated when the ECM supplies a ground circuit for the bulb. For wiring reference, see MISCELLANEOUS ECM CONTROLS in SYSTEM/COMP TESTS article.