Engine Controls - Theory & Operation Cadillac Escalade GMT400

MODEL IDENTIFICATION

Vehicle model is identified by fifth character of Vehicle Identification Number (VIN). VIN is stamped on metal pad on top of left end of instrument panel, near windshield. See MODEL IDENTIFICATION table.

MODEL IDENTIFICATION

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 (6.5L DIESEL)

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 over-fueled.

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 PCM through a turbo boost relay.

The rotating assembly in the turbocharger can reach speeds of 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.

Speed Density

All engines are equipped with a Manifold Absolute Pressure (MAP) sensor (gasoline), or Barometric Pressure (BARO) sensor (diesel), and use the speed density method to compute airflow rate. PCM uses manifold pressure to calculate the airflow rate. The MAP/BARO sensor responds to manifold vacuum changes due to engine load and speed changes. The PCM sends a voltage signal to the MAP/BARO sensor. Manifold pressure changes result in resistance changes in the MAP/BARO sensor.

By monitoring MAP/BARO sensor signal voltage, the PCM determines manifold pressure. On gasoline engines, if MAP sensor fails, the PCM supplies a fixed MAP sensor value, and uses the TP sensor to control fuel.

Some models also use an Intake Air Temperature (IAT) sensor. The IAT sensor allows PCM to determine intake air temperature. PCM uses signal to delay EGR until intake air temperature reaches about 40°F (5°C). If intake air temperature becomes excessively high, PCM compensates by slightly retarding timing.

COMPUTERIZED ENGINE CONTROLS (GASOLINE ENGINES)

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

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

POWERTRAIN CONTROL MODULE (PCM) & VEHICLE CONTROL MODULE (VCM)

Note. Models are equipped with a Powertrain Control Module (PCM) or a Vehicle Control Module (VCM). The difference between VCM and PCM is the PCM controls electronic transmission internals, cooling fan and cruise control system. The VCM provides control of the engine systems as well as the anti-lock brake system. References to PCM also apply to VCM-equipped vehicles, unless stated otherwise.

For location of PCM/VCM, see COMPONENT LOCATIONS in appropriate SYSTEM & COMPONENT TESTING article. The PCM consists of the Arithmetic Logic Unit (ALU), Central Processing Unit (CPU), power supply and system memories.

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

Memories

PCM may use one or more of 5 types of memory

1. 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 PCM failure. Anytime PCM is replaced, PROM and CALPAC must both be installed into replacement unit. If battery voltage is removed, CALPAC information is retained.
2. Electronically Erasable Programmable Read Only Memory (EEPROM) Some models may use an EEPROM. This is the same as a PROM except it can be electronically reprogrammed by the manufacturer using special equipment.
3. Memory Calibration (MEM-CAL) Some vehicles may use a PCM containing a MEM-CAL unit. This assembly contains functions of PROM and CALPAC. If power to PCM is removed, MEM-CAL information is retained. MEM-CAL also contains an internal Knock Sensor (KS) module on models equipped with a KS system.
4. Programmable Read Only Memory (PROM) PROM is factory programmed engine calibration data which "tailors" PCM for specific transmission, engine, emission, vehicle weight and rear axle ratio application. The PROM can be removed from PCM. If battery voltage is removed, PROM information is retained.
5. 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, all information stored in RAM is lost.
6. Read Only Memory (ROM) ROM is programmed information which only PCM can read. The ROM program cannot be changed. If battery voltage is removed, ROM information is retained.

INPUT DEVICES

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

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. The available input signals include

A/C On (A/C Request) Signal

The air conditioner power switch is mounted in the instrument panel. This switch provides a simple "on" (A/C request) signal, which is monitored by PCM. The PCM uses this signal to determine control of the A/C clutch relay (if equipped) and to adjust idle speed when A/C compressor clutch is engaged. On FWD Vans, control unit also activates 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 appropriate SYSTEM & COMPONENT TESTING article.

Battery Voltage

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

Brake Switch Feedback

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

Camshaft Position (CMP) Sensor (Except 3.4L)

CMP sensor utilizes a Hall Effect sensor that serves a similar function as Crankshaft Position (CKP) sensor. CMP sensor is mounted in distributor or on engine block in close proximity to camshaft. On 2.2L, primary function of CMP sensor is to correlate crankshaft to camshaft position so PCM can determine which cylinder is ready to be fueled by fuel injector. On all others, CMP sensor creates a 1X signal and is used by PCM to identify which cylinder(s) are misfiring. The 1X signal will not affect driveability.

Camshaft Position (CMP) Sensor (3.4L)

CMP sensor is located at top of timing cover, behind water pump. As camshaft sprocket turns, a magnet activates Hall Effect switch in CMP sensor. This signal is generated whenever cylinder No. 1 is at TDC of its compression stroke.

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

Cranking Signal

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

Crankshaft Position (CKP) Sensor

CKP sensor utilizes a pick-up coil type sensor mounted on side of engine block or on bottom of timing cover. The CKP sensor monitors crankshaft position and sends signals to ignition control module. These signals provide PCM with a TDC position reference for each piston, as well as supplying an engine speed (RPM) signal. This allows PCM to fire appropriate ignition coil at the proper time, determine triggering of the fuel injectors, and to compute crankshaft position and RPM. CKP sensor signal is also used to detect a cylinder misfire by monitoring changes in crankshaft speed. For additional information, see IGNITION SYSTEM (GASOLINE ENGINES) .

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 PCM and speedometer. On models equipped with a DRAC, VSS buffer is an internal part of DRAC.

Engine Coolant Temperature (ECT) Sensor

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

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

Exhaust Gas Recirculation (EGR) Pintle Position

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

Fuel Pump Feedback

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

Gear Switches

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

Intake Air Temperature (IAT) Sensor

IAT 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 PCM supplies and monitors a 5-volt signal to sensor through a pull-down resistor in PCM.

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

Knock Sensor (KS)

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 KS module (integral to PCM). The PCM then retards ignition timing until the engine knock ceases. Two knock sensors are used on some models.

A fault in the KS circuit may set a Diagnostic Trouble Code (DTC). See appropriate SELF-DIAGNOSTICS article. When a related DTC is not present and the KS system is the suspected cause of a driveability problem, perform functional check of KS system. See appropriate SYSTEM & COMPONENT TESTING article.

Manifold Absolute Pressure (MAP) Sensor

MAP sensor measures changes in manifold pressure. Changes in manifold pressure result from engine load and speed changes. The MAP sensor converts these changes in manifold pressure into a voltage output signal to PCM (1.5 volts at idle to about 4.5 volts at WOT). The PCM can monitor these signals and adjust air/fuel ratio and ignition timing under various operating conditions.

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

Oxygen Sensor (O2S)

O2S is mounted in exhaust system and monitors oxygen content of exhaust gases. The oxygen content causes the Zirconia/Platinum-tipped O2S to produce a voltage signal which is proportional to exhaust gas oxygen concentration (0-3%) compared to outside oxygen (20-21%). This voltage signal is low (about 0.1 volt) when a lean mixture is present and high (about 1.0 volt) when a rich mixture is present. As PCM compensates for a lean or rich condition, this voltage signal constantly fluctuates between high and low, crossing a.45-volt reference voltage supplied by PCM on the O2S signal line. This is referred to as "cross counts".

The O2S 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 PCM does not make air/fuel adjustments based upon O2S signals, but uses TP and MAP or MAF values to determine air/fuel ratio from a table built into memory. When PCM reads a voltage signal greater than.45 volt from the O2S, PCM begins to alter commands to fuel injector to produce a leaner mixture.

Once vehicle has entered "closed loop" mode, a fault in the O2S circuit (cooled-down sensor or open or shorted O2S circuit) will return vehicle to "open loop" mode. A problem in the O2S circuit should set a related diagnostic trouble code.

On most engines, O2S uses an internal heating element. This type of sensor is referred to as a Heated Oxygen Sensor (HO2S). Heating element allows HO2S to warm more quickly, causing fuel system to enter "closed loop" mode sooner. Heating element also prevents fuel system from re-entering "open loop" mode, which would be a normal response to prolonged idling.

Park/Neutral Position (PNP) Switch

This switch is connected to transmission gear selector and signals PCM when transmission is in Park or Neutral. PCM uses this information for determining control of ignition timing, Torque Converter Clutch (TCC) and idle speed. To check function of PNP switch, perform functional check of switch. See appropriate SYSTEM & COMPONENT TESTING article.

Transmission Fluid Pressure (TFP) Switch

The TFP switch (if equipped) is actually 5 pressure switches combined into a single unit mounted on transmission valve body. The PCM supplies battery voltage on 3 separate wires to TFP switch. As switches are actuated in various combinations during transmission operation, PCM can detect what gear range transmission is in.

Throttle Position (TP) Sensor

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

Transmission Fluid Temperature (TFT) Sensor

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

PCM uses TFT sensor input to control Torque Converter Clutch (TCC) application and shift quality. Sensor circuit problem should set a related diagnostic 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 the PCM or Digital Ratio Adapter Controller (DRAC), which passes the signal on to the PCM. The PCM then converts this signal into miles per hour by monitoring the time interval between pulses. PCM uses this sensor input in controlling Torque Converter Clutch (TCC) engagement, shift speed, etc.

OUTPUT SIGNALS

Note. Models have different combinations of computer-controlled components. Not all listed components are used on every model. For theory and operation of components, refer to indicated system.

A/C Clutch Relay

See MISCELLANEOUS PCM/VCM CONTROLS .

Cruise Control Stepper Motor

See MISCELLANEOUS PCM/VCM CONTROLS .

Malfunction Indicator Light (MIL)

See SELF-DIAGNOSTIC SYSTEM .

EGR System

See EMISSION SYSTEMS (GASOLINE ENGINES) .

Electronic Ignition (EI)

See IGNITION SYSTEM (GASOLINE ENGINES) .

Fuel Injectors

See FUEL CONTROL under FUEL SYSTEM (GASOLINE ENGINES).

Fuel Pump & Fuel Pump Relay

See FUEL DELIVERY under FUEL SYSTEM (GASOLINE ENGINES).

Idle Air Control (IAC) Valve

See IDLE SPEED under FUEL SYSTEM (GASOLINE ENGINES).

Self-Diagnostics

See SELF-DIAGNOSTIC SYSTEM .

Serial Data

See SELF-DIAGNOSTIC SYSTEM .

Shift Solenoids (Electronic Transmission)

See MISCELLANEOUS PCM/VCM CONTROLS .

Torque Converter Clutch (TCC)

See MISCELLANEOUS PCM/VCM CONTROLS .

Transmission Shift Light (Manual Transmission)

See MISCELLANEOUS PCM/VCM CONTROLS .

COMPUTERIZED ENGINE CONTROLS (DIESEL ENGINES)

The 6.5L Diesel engine (with or without turbocharger) uses a Powertrain Control Module (PCM). The system consists of a PCM, input devices and output signals. PCM electronically controls fuel flow, fuel timing advance/retard, idle speed, EGR system operation, cruise control, Torque Converter Clutch (TCC) engagement, transmission shifting and glow plug system.

POWERTRAIN CONTROL MODULE (PCM)

The PCM is located in passenger compartment, behind instrument panel. It constantly monitors information from various sensors to control fuel flow, injector timing, cruise control, transmission shifting, throttle, EGR, TCC, cold advance and glow plug systems. PCM processes input signals from sensors and then sends necessary electrical responses to control these systems.

The PCM performs the diagnostic function of the system. It can recognize operational problems, alert driver through the Malfunction Indicator Light (MIL) and store Diagnostic Trouble Codes (DTCs) which identify problem areas to technicians to aid in making system repairs.

PCM uses 3 types of memory

1. Read Only Memory (ROM) ROM is programmed information which only PCM can read. ROM program cannot be changed. If battery voltage is removed, ROM information is retained.
2. Random Access Memory (RAM) RAM is the scratchpad 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 PCM, all information stored in RAM is lost.
3. Programmable Read Only Memory (PROM) PROM is factory programmed engine calibration data which "tailors" PCM for specific transmission, engine, emission, vehicle weight and rear axle ratio application. PROM can be removed from PCM. If battery voltage is removed, PROM information is retained.

Each sensor or switch furnishes or modifies electronic voltage signals to PCM. The PCM uses these input signals to control fuel flow, injector timing, cruise control, transmission shifting, 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

Accelerator Pedal Position (APP) Sensor

APP sensor, mounted on accelerator pedal, contains 3 separate variable resistor circuits that monitor throttle opening angle for PCM. Each APP circuit, connected to a 5-volt reference signal, has a high resistance value when throttle is closed. At wide open throttle, sensor resistance value is low and output to PCM will be about 5 volts.

Barometric Absolute Pressure (BARO) Sensor

BARO sensor is part of MAP sensor mounted on left side of cowl, and monitors atmospheric pressure during ignition key on, engine off. The signal is converted into an altitude value by the PCM. PCM uses this information to adjust fuel flow, injector timing and transmission shifting.

Boost Sensor

Boost sensor, used on turbo engines, monitors boost pressure and is used by PCM to open wastegate which limits pressure. At full load under WOT, boost sensor indicates high pressure (high voltage). At closed throttle under deceleration, boost sensor indicates low pressure (low voltage).

CKP sensor is a Hall Effect type sensor mounted at front of crankshaft. Front crankshaft hub includes a wheel with 4 slots. PCM uses CKP sensor signal to determine engine RPM. This signal is used to improve idle. If pump cam signal is lost, PCM will use crankshaft position signal data to control injection timing and fuel flow.

ECT sensor is a thermistor (temperature sensitive resistor). An engine coolant temperature of -40°F (-40°C) produces a high resistance (100 k/ohms), while an engine coolant temperature of 266°F (130°C) produces a low resistance (70 ohms).

PCM supplies a 5-volt reference signal through an internal resistor to ECT sensor and measures return voltage. Voltage is high when coolant temperature is low, and low when coolant temperature is hot. By measuring voltage, PCM can determine engine coolant temperature. Engine coolant temperature affects injector timing and glow plug system.

Fuel Temperature Sensor

Sensor is part of pump cam signal sensor and works like IAT sensor. PCM uses this signal to adjust fuel delivery.

Injection Pump Cam Signal

The injection pump cam signal is an optical sensor mounted on the fuel injection pump. The sensor receives a 5-volt reference signal and allows the PCM to measure fuel injector pulse ring RPM and position. This signal is critical to accurate fuel injection timing and start of injection.

IAT sensor is a thermistor (temperature sensitive resistor). Air temperature of -40°F (-40°C) sensor produces a high resistance (100 k/ohms), while air temperature of 266°F (130°C) sensor produces a low resistance (70 ohms).

PCM supplies and monitors a 5-volt reference signal through an internal resistor to IAT sensor and measures return voltage. Voltage is high when temperature is low, and low when temperature is hot. Engine air temperature affects fuel delivery and injector timing.

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

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

TFT sensor is a thermistor (temperature sensitive resistor). PCM supplies and monitors a 5-volt reference signal through an internal resistor to sensor. Monitored voltage is high when temperature is low (high sensor resistance), and low when temperature is hot.

Mounted on the transmission, VSS sends a pulsing signal to PCM for vehicle speed calculation. PCM uses this calculation for cruise control and fuel cut-off.

Note. PCM regulates output signals to maintain correct driveability and exhaust emissions. For theory and operation of components, refer to indicated system.

Glow Plug Relay

See FUEL CONTROL under FUEL SYSTEM (DIESEL ENGINES).

See MISCELLANEOUS PCM/VCM CONTROLS .

Exhaust Gas Recirculation System

See EMISSION SYSTEMS (DIESEL ENGINES) .

Fuel Solenoid

See FUEL DELIVERY under FUEL SYSTEM (DIESEL ENGINES).

Injector Timing Stepper Motor

See FUEL CONTROL under FUEL SYSTEM (DIESEL ENGINES).

Torque Converter Clutch

See MISCELLANEOUS PCM/VCM CONTROLS .

See MISCELLANEOUS PCM/VCM CONTROLS .

Turbo Boost Relay (6.5L Turbo)

See TURBOCHARGER (6.5L DIESEL) under AIR INDUCTION SYSTEM.

Wastegate Solenoid (6.5L Turbo)

See TURBOCHARGER . (6.5L DIESEL) under AIR INDUCTION SYSTEM.

Fuel Pump

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

On Central Sequential Port Injection (CSI) systems, pressure regulator is mounted to fuel metering body under upper intake manifold. On Sequential Multiport Fuel Injection (SFI), pressure regulator is attached to end of fuel rail. Pressure 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, PCM turns on electric fuel pump by energizing fuel pump relay. PCM keeps pump on if engine is running or cranking (PCM is receiving reference pulses from ignition module). If there are no reference pulses, PCM turns pump off within 2 seconds after ignition is turned on.

Most models also include a second control path through the oil pressure switch which will turn the fuel pump on after the switch detects oil pressure. Cranking time will be longer if fuel pump does not receive current until oil pressure switch contacts close.

Fuel Pressure Regulator (CSI)

Fuel pressure regulator is a diaphragm-operated relief valve with injector pressure on one side and manifold pressure (vacuum) on the other. Pressure regulator maintains a pressure of 60-66 psi (4.2-4.6 kg/cm 2 ) under all operating conditions. Pressure regulator is a factory preset, nonadjustable, spring-loaded diaphragm attached to CSI assembly. Spring tension maintains a constant fuel pressure to injector regardless of engine load.

Fuel Pressure Regulator (SFI)

Fuel pressure regulator is a diaphragm-operated relief valve with injector pressure on one side and manifold pressure (vacuum) on the other. Pressure regulator maintains a pressure of 56-62 psi (3.9-4.4 kg/cm 2 ) under all operating conditions. Pressure regulator compensates for engine load by increasing fuel pressure when low manifold vacuum is experienced.

Fuel Pump Relay

When ignition switch is turned to ON position, PCM turns electric fuel pump on by energizing fuel pump relay. PCM keeps relay energized if engine is running or cranking (PCM is receiving reference pulses from ignition module). If there are no reference pulses, PCM turns pump off within 2-20 seconds after key on.

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 about 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. Cranking time will be longer if fuel pump does not receive current until oil pressure switch contacts close. Oil pressure switch may be combined into a single unit with an oil pressure gauge sending unit or sensor.

PCM monitors fuel pump circuit between fuel pump relay/oil pressure switch and fuel pump, enabling PCM 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 diagnostic trouble code in PCM memory.

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

FUEL CONTROL

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

Open Loop

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

Closed Loop

When O2S reaches operating temperature, coolant temperature reaches a preset temperature and a specific period of time has passed since engine start-up, PCM operates in "closed loop" mode. In "closed loop" mode, PCM controls air/fuel ratio based upon O2S signals (in addition to other input parameters) to maintain as close to a 14.7:1 air/fuel ratio as possible. If O2S cools off (due to excessive idling) or a fault occurs in O2S circuit, vehicle will re-enter "open loop" mode.

On most engines, O2S is equipped with an internal heating element. This type of sensor is known as a Heated Oxygen Sensor (HO2S). The heating element enables system to reach and maintain "closed loop" mode sooner, even during periods of extended idle.

Central Sequential Port Injection (CSI)

CSI is a non-repairable injector assembly consisting of a fuel meter body, fuel pressure regulator, fuel injector and poppet nozzles with fuel tubes. CSI assembly is housed in the lower manifold assembly. Fuel pump and pressure regulator maintain fuel pressure at 60-66 psi (4.2-4.6 kg/cm 2 ) under all operating conditions.

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.

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 PCM.

Constant fuel pressure is maintained to the injectors. Air/fuel mixture is regulated by amount of time injector stays open (pulse width). Various sensors provide information to the PCM to control pulse width.

Fuel System Operating Modes

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

1. Starting During engine starts, PCM delivers one injector pulse for each distributor reference pulse received (synchronized mode). Injector pulse width is based upon coolant temperature and throttle position. PCM determines air/fuel ratio when throttle position is less than 80 percent open. Engine starting air/fuel ratio ranges from 0.8:1 at -40°F (-40°C) to 16.8:1 at 230°F (110°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, PCM adjusts injector pulse width equal to an air/fuel ratio of 16.5:1. 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 65 percent open and/or engine speed exceeds 600 RPM, PCM changes injector pulse width to that used during engine starting (based upon coolant temperature and manifold vacuum).
3. Heavy Acceleration PCM provides fuel enrichment during heavy acceleration. Sudden opening of throttle valve causes rapid increase in MAP or MAF signal. Pulse width is directly related to MAP or MAF, throttle position and coolant temperature. Higher MAP or MAF 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 or MAF, 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 PCM compensates for low battery voltage by increasing injector pulse width and increasing idle RPM. PCM is able to perform these commands because of a built-in memory/learning function.
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 PCM, even with ignition on. This prevents flooding before starting. Fuel cut-off also occurs at high engine RPM or excessive vehicle speed to prevent internal damage to engine. Some models may also cut off fuel injector signals during periods of sudden, closed throttle deceleration (when fuel is not needed).

IDLE SPEED

PCM controls engine idle speed depending upon engine operating conditions. PCM senses engine operating conditions and determines 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 observing scan tool display while connected to the Data Link Connector (DLC).

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 near 18. When engine is idling, PCM 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, then 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 diagnostic trouble code.

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

FUEL DELIVERY

All models use an electric, fuel-lift pump mounted on the left frame rail. Pump pulls fuel from the fuel tank through a primary filter. The fuel is then pumped through a secondary filter, mounted on firewall or rear of intake manifold, and to the fuel injection pump.

The diesel engine uses a mechanical, high-pressure, rotary fuel 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 based on inputs from the PCM.

High-pressure fuel lines carry fuel to an injection nozzle in each cylinder. All fuel lines are the same length to ensure no variance in timing. An electric fuel solenoid controls engine RPM. As the Accelerator Pedal Position (APP) sensor is pressed down, PCM operates fuel solenoid through fuel solenoid driver to allow increased fuel delivery.

Fuel Injection Pump

The high-pressure fuel 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 based on PCM-controlled fuel solenoid and injector timing stepper motor.

Electrical controls, mounted on injection pump, are used by the PCM to control fuel flow and injector timing. An engine shut-off solenoid blocks fuel from entering charging passage. The pump cam sensor, on top of injection pump, detects RPM of the pulse ring. It also includes a fuel temperature sensor. These signals help PCM control amount of fuel delivery and injector timing. An injector timing stepper motor advances or retards injector timing. PCM controls amount of fuel delivered through fuel solenoid driver which operates fuel solenoid, mounted at front of injection pump. (Scheme 1)

Fuel under regulated low pressure enters 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 Plug System

The PCM uses various inputs to determine when glow plug operation is required. PCM uses relay to operate the glow plugs. The glow plug relay is mounted at rear of left cylinder head.

A normally operating system works as follows: at room temperature with ignition on and engine off, the glow plugs come on for 4-6 seconds and then go off for about 3 seconds. The glow plugs then cycle on for about one second and off for about 4.5 seconds, for a total start sequence of about 16 seconds. If the engine is cranked during or after start sequence, the glow plugs will cycle on and off for a total of 16 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. PCM 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. For diagnostic information on computer controlled system, see appropriate SELF-DIAGNOSTICS article.

Glow Plug After-Start

PCM 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.

Electronic signals from the Acceleration Pedal Position (APP) sensor, temperature sensors, pump cam sensor and crankshaft position sensor are received by the PCM. Based on these signals, PCM controls fuel flow through fuel solenoid and fuel solenoid driver.

Advance & Retard Control

The injector timing stepper motor is designed to advance or retard injection pump timing plus or minus 4 degrees. PCM activates this circuit based on signals from the pump cam sensor, crankshaft position sensor and various temperature sensors. This control is mainly used during cold engine operation and for idle speed.

Curb Idle Speed

Curb idle speed is adjustable on 6.5L diesel (L57) only. L57 engine is equipped with mechanical fuel injection. For adjustment procedure, see IDLE SPEED (6.5L DIESEL) under IDLE SPEED & MIXTURE in appropriate ON-VEHICLE ADJUSTMENTS article. On models equipped with electronic fuel injection, curb idle speed is controlled by PCM based on signals from crankshaft RPM, pump cam sensor and various temperature sensors. No mechanical adjustment of curb idle speed is possible.

Fast Idle Speed

Fast idle solenoid is controlled by PCM. No adjustments are possible.

Enhanced Ignition System

Used on the 4.3L, 5.0L, 5.7L and 7.4L engines, the enhanced ignition system consists of the VCM, distributor, ignition coil driver module, ignition coil and Camshaft Position (CMP) sensor. Ignition control and by-pass functions are controlled by the VCM.

1. Camshaft Position (CMP) Sensor CMP sensor is similar to CKP sensor. CMP sensor provides one pulse (1X signal) per camshaft revolution. VCM uses this signal in conjunction with the crankshaft position to determine which cylinder(s) are misfiring.
2. Crankshaft Position (CKP) Sensor CKP sensor is located in the front engine cover. Air gap between sensor and target wheel is preset and is not adjustable. Target wheel has 4 slots, 60 degrees apart, and is keyed to the crankshaft. Rotation of target wheel creates a change in the magnetic field of the sensor which results in an induced voltage pulse. One crankshaft revolution will result in 4 pulses (4X signal). Based on these pulses, VCM is able to determine crankshaft position and engine speed. VCM will then activate the fuel injector and provide spark to distributor.
3. Distributor Distributor assembly contains the Camshaft Position (CMP) sensor, cap, rotor and shaft. A Diagnostic Trouble Code (DTC) will set when distributor is installed a tooth off in relation to the camshaft. Distributor is not serviceable.
4. Ignition Coil Driver Module Module is mounted next to coil. VCM signals the ignition coil driver to turn on primary current to the ignition coil by pulling the IC line high (4 volts). The ignition control driver turns the primary current on and off by applying and removing ground to primary winding. Module does not have a back-up function that would allow engine to run if IC signal is lost.

Used on the 2.2L, 3.4L, 4.8L, 5.3L and 6.0L engines, the EI ignition system eliminates the need for a mechanical distributor. It consists of coil packs, EI modules, Crankshaft Position (CKP) sensor (2 used on 3.4L), Camshaft Position (CMP) sensor, wiring harness and the Ignition Control (IC) portion of the Powertrain Control Module (PCM).

On 2.2L and 3.4L, the ignition system uses a "waste spark" method of spark distribution. Each cylinder is paired with the cylinder that is opposite it in the firing order (1-4 and 2-3), or (1-4, 2-5 and 3-6). Spark occurs simultaneously in the cylinder approaching the compression stroke and in cylinder approaching 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. On 4.8L, 5.3L and 6.0L, ignition coils are fired sequentially and do not use the "waste spark" method.

Input from the CKP sensor and EI modules are used by the PCM to control ignition timing and triggering fuel injectors.

1. Crankshaft Position (CKP) Sensor (2.2L 7X Signal) CKP sensor is mounted on side of engine block. CKP sensor detects position of 7 notches (7X signal) in internal crankshaft wheel. These signals are received by the EI module which allows PCM to determine ignition and fuel injector firing time. Six notches are equal distance apart (60 degrees). PCM recognizes TDC of cylinder No. 1 based on time between pulses and location of seventh notch (sync pulse) placed 10 degrees from one of the other notches. The IC module must receive 7X signal to fire the correct ignition coil. PCM uses every other notch to fire fuel injectors. The 2nd notch after the sync pulse provides a signal, triggering EI module to fire coils No. 2 and 3. Fifth notch signals coils No. 1 and 4 to fire.
2. Crankshaft Position (CKP) Sensor (3.4L 7X/24X Signal) The 7X CKP sensor is located on side of engine block. Sensor detects position of 7 notches (7X signal) cast into crankshaft. These signals are received by the EI module which allows PCM to determine ignition and fuel injector firing time. Six notches are equal distance apart (60 degrees). PCM recognizes TDC of cylinder No. 1 based "on" time between pulses and location of seventh notch (sync pulse) placed 10 degrees from one of the other notches. The IC module must receive 7X signal to the correct ignition coil. The 24X CKP sensor is located on bracket at front of timing cover and produces a 24X signal used to smooth engine operation at engine speeds less than 1200 RPM. The 24X signal is generated by an interrupter ring with 24 equally spaced notches attached to rear of crankshaft balancer.
3. Crankshaft Position (CKP) Sensor (4.8L, 5.3L & 6.0L 4X/24X Signal) The CKP sensor is located on lower right rear of engine block. CKP sensor detects position of 24 notches (24X signal) on reluctor wheel, mounted on rear of crankshaft. The PCM uses the 24X signal to determine when a particular cylinder is on either a firing or exhaust stroke. A 4X signal is also produced by the reluctor wheel. PCM uses the 4X signal to monitor misfires, tachometer output, spark control, fuel control and certain diagnostics.
4. Ignition Coils (4.8L, 5.3L & 6.0L) Eight ignition coils are independently mounted above each cylinder on the rocker covers. Ignition coils are fired sequentially and are connected to the PCM by eight individual Ignition Control (IC) circuits. Each coil can be replaced separately.
5. Ignition Coil Pack (2.2L & 3.4L) On ignition coil pack, 2 or 3 separate twin tower coils are independently mounted over the EI module. Each coil provides the spark for 2 simultaneously paired spark plugs. Each coil can be replaced separately.
6. Ignition Control Module (ICM) ICM receives ignition control signals from the PCM which in turn triggers the corresponding ignition coils. Since the PCM controls spark timing and ignition control during crank and run cycles, there is no by-pass mode. ICM is not serviceable.

Ignition Timing Control

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

AIR INJECTION SYSTEM (7.4L)

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

System consists of an air pump, air pump clutch, AIR relay, check valve(s) and plumbing.

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. VCM controls air pump by grounding air pump relay control unit, energizing the relay and supplying voltage to air pump clutch, engaging air pump.

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.

CATALYTIC CONVERTER

A Three-Way-Catalytic (TWC) converter is used to reduce exhaust emissions. This type of converter can reduce hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx).

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 injection 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. A linear EGR valve is used on all engines.

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

EVAPORATIVE EMISSION SYSTEM

All vehicles use carbon canister storage for evaporative fuel control. Evaporative emission control system stores gasoline fumes from fuel tank in a carbon canister. After engine is running, fumes are drawn into engine for burning during combustion process.

The basic components used in the evaporative emission system are an activated carbon canister (all models, open at top or bottom for fresh air intake), vacuum operated canister control valve (some Federal models), or purge control solenoid (all other models). For specific component application and vacuum hose routing, see appropriate VACUUM DIAGRAMS article.

Carbon Canister

Evaporative fumes from the 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 purge control solenoid regulates vapors through this purge line.

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

Canister Purge Control Solenoid (CPCS)

CPCS allows fuel vapor to flow from carbon canister to the engine. Solenoid is normally closed and is pulse-width modulated by the PCM to precisely control vapor flow. PCM controls flow of fuel vapors based on coolant temperature. At temperatures greater than 113°F (45°C), purge control solenoid is open. Purge control solenoid is also opened if PCM detects extreme lean air/fuel mixture ratio conditions.

POSITIVE CRANKCASE VENTILATION (PCV)

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

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

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

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

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

Note. For additional information on EGR system, see appropriate SYSTEM & COMPONENT TESTING article.

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 an EGR valve, EGR vent/EGR 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.

EGR Vent/EGR Solenoids

EGR vent/EGR solenoids are mounted at rear of engine as a single assembly. Using input from engine speed sensor and Accelerator Pedal Position (APP) sensor, PCM controls EGR by controlling amount of "on" and "off" time of EGR solenoid. When EGR is not needed, PCM energizes EGR vent solenoid to vent vacuum. Vacuum is used to control EGR valve opening.

EGR Fault Detection

The PCM 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 PCM, necessary corrections are made by the PCM. If variation is too great for PCM to correct, an error is detected. The PCM then enters into a default mode and sets a related diagnostic trouble code in memory.

Vacuum Pump

Vacuum pump provides vacuum for operating emission control components (some "C", "G" and "K" series vehicles), cruise control and A/C-heater servos. The vacuum pump is driven by either a belt or gear.

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

The gear-driven pump is mounted on the top rear of 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)

CDR valve, located on right-side valve cover, is used on all diesel engines. Valve prevents crankcase pressure from accumulating during idle by regulating (metering) crankcase pressure back into the engine. Intake manifold vacuum (only slight vacuum is present) 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 PCM is equipped with a self-diagnostic system which detects system failures or abnormalities. When a malfunction occurs, PCM will illuminate the Malfunction Indicator Light (MIL) located on instrument cluster. When a malfunction is detected and MIL is turned on, a corresponding Diagnostic Trouble Code (DTC) will be stored in PCM memory. Malfunctions are designated as either "emission related" or as "non-emission related", and are divided into 4 code types to identify type of fault. The 4 code types are defined as follows

1. Type "A" Emission related faults that illuminate MIL at first occurrence of a fail condition.
2. Type "B" Emission related faults that illuminate MIL if a fault occurs in 2 consecutive ignition cycles.
3. Type "C" Non-emission related faults that illuminate MIL only when fault is present. MIL will turn off 3 seconds after engine start if fault is no longer present, but a record of fault will remain stored in memory.
4. Type "D" Non-emission related faults which do not illuminate MIL.

Emission related DTCs (type "A" or "B") cause MIL to illuminate and remain on until the malfunction is repaired. On models using digital display on dash to indicate DTCs, DTCs may be accompanied by a "current" or "history" indication for intermittent and hard failures. If MIL comes on and remains on during vehicle operation, cause of malfunction must be determined using appropriate diagnostic procedure for affected DTC located in appropriate SELF-DIAGNOSTICS article. If a sensor fails, PCM will use a substitute value in its calculations to continue engine operation. In this condition, vehicle is functional but loss of good driveability is likely.

Non-emission related DTCs (type "C") cause MIL to flicker or glow and go out about 10 seconds after the intermittent fault goes away. The corresponding DTC, however, will be retained in PCM memory. On models using digital display on dash to indicate DTCs, DTCs may be accompanied by a "current" or "history" indication for intermittent and hard failures. If related DTC does not reoccur within 50 engine restarts, related DTC will be erased from PCM memory. Intermittent failures may be caused by sensor, connector or wiring related problems. See appropriate TROUBLE SHOOTING - NO CODES article.

As a bulb and system check, MIL will illuminate when ignition switch is turned to ON position and engine is not running. When engine is started, MIL should go out. If MIL does not go out, a malfunction has been detected in the computerized engine control system or MIL circuit is faulty. MIL may be used on some models to display a stored Diagnostic Trouble Code (DTC). To access DTCs, see appropriate SYSTEM & COMPONENT TESTING article.

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

MISCELLANEOUS PCM/VCM CONTROLS

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

A/C CLUTCH

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

Refrigerant pressure is sensed through the monitoring of high and low pressure switches or a pressure sensor which registers either high or low pressure levels. Hot restart is monitored through the coolant temperature sensor. For component application and related wiring, see appropriate A/C COMPRESSOR CLUTCH CONTROLS article in AIR CONDITIONING.

A/C Pressure Switches

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

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

CRUISE CONTROL

On models equipped with cruise control, the system is operated by the PCM. PCM receives inputs from VSS, servo diaphragm position sensor, cruise control switch and brake release switch. Based on these inputs, PCM controls position of cruise control stepper motor. PCM prevents system engagement at speeds of less than 25 MPH. PCM is not serviceable; if defective, it must be replaced. A system fault is stored as a Diagnostic Trouble Code (DTC) in PCM memory.

The transmission/transaxle TCC eliminates power loss of torque converter stage when vehicle is in a cruise condition, allowing driver the convenience of an automatic transmission while providing the fuel economy of a manual transmission. Fused battery ignition is supplied to TCC 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 PCM, without sharing power or ground with TCC solenoid. For wiring reference, see appropriate ELECTRONIC CONTROLS article in AUTOMATIC TRANSMISSIONS.

The TCC 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), PCM energizes TCC solenoid mounted in transmission, allowing torque converter to directly connect engine to the transmission. When operating conditions indicate transmission should operate as normal, TCC solenoid is de-energized, allowing transmission to return to normal automatic operation. Since power for the TCC solenoid is delivered through the brake switch, transmission also returns to normal automatic operation when brake pedal is depressed. To check function of TCC system, perform functional check of system. See MISCELLANEOUS PCM/VCM CONTROLS in appropriate SYSTEM & COMPONENT TESTING article.

Electronic Transmission

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

1. Shift Solenoid "A" (1st-2nd) Shift solenoid "A" is attached to the valve body and is a normally-open exhaust valve. PCM 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.
2. Shift Solenoid "B" (2nd-3rd) Shift solenoid "B" is attached to the valve body and is a normally-open exhaust valve. PCM 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.
3. Force Motor (Pressure Control Solenoid) Force motor is attached to valve body and controls line pressure by moving a pressure regulator valve against spring pressure. Force motor replaces throttle valve or vacuum modulator used on past transmissions. PCM varies line pressure based upon engine load. Engine load is calculated from various inputs, especially TP sensor. Line pressure is actually varied by changing amperage applied to force motor from zero (high pressure) to 1.1 amps (low pressure). Force motor is periodically pulsed to prevent fluid contamination or varnish 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 PCM supplies a ground circuit for bulb. For wiring reference, see wiring schematic under MISCELLANEOUS PCM/VCM CONTROLS in appropriate SYSTEM & COMPONENT TESTING article.

See also:
• MODEL IDENTIFICATION
• MISCELLANEOUS PCM/VCM CONTROLS
• SELF-DIAGNOSTIC SYSTEM
• TURBOCHARGER