POWERTRAIN CONTROL MODULE (PCM)
The PCM is a digital computer that controls ignition timing, air/fuel ratio, fuel injector pulse width, ignition coil(s), spark advance, emission control devices, cooling fan, charging system, idle speed, cruise control (if equipped), fuel pump and tachometer. For PCM location, see wiring diagram in the WIRING DIAGRAMS - 2.0L article. PCM uses data from various input sources to control output devices in order to achieve optimum engine performance for all operating conditions.
PCM has voltage converters that convert battery voltage to regulated 5-volt and 8-volt outputs. The 5-volt output powers battery temperature sensor, Camshaft Position (CMP) sensor on some models equipped with Direct Ignition System (DIS) or distributor on some models without DIS, Crankshaft Position (CKP) sensor, Engine Coolant Temperature (ECT) sensor, Intake Air Temperature (IAT) sensor, logic circuits, Manifold Absolute Pressure (MAP) sensor, Throttle Position (TP) sensor and Vehicle Speed Sensor (VSS) on some models. The 8-volt output is used to power CMP sensor on some models with DIS or distributor on some models without DIS and CKP sensor. The 8-volt output may also be used to power VSS on some models.
Note. Components are grouped into 2 categories. The first category, INPUT DEVICES, includes components that control or produce voltage signals monitored by the PCM. The second category, OUTPUT SIGNALS, includes components controlled by the PCM (this is accomplished by the PCM grounding individual circuits).
INPUT DEVICES
Vehicles are equipped with different combinations of input devices. Not all devices are used on all models. To determine component location and input usage on a specific model, see wiring diagram in the WIRING DIAGRAMS - 2.0L article. Available input signals include
A/C SWITCH
Switch signals PCM that A/C has been selected. PCM then activates A/C compressor clutch relay and maintains idle speed at a scheduled RPM. This is done through control of Idle Air Control (IAC) motor.
BATTERY TEMPERATURE SENSOR
PCM uses sensor to determine battery temperature and to control battery charging rate. Temperature data along with battery voltage data, is used by PCM to vary charging rate. System voltage is higher at colder temperatures and is gradually reduced at warmer temperatures.
BATTERY VOLTAGE
PCM monitors battery voltage to determine fuel injector pulse width and generator field control. This is done to compensate for reduced current flow through injector caused by lowered voltage.
BRAKE SWITCH
This switch may also be referred to as a brake lamp/light switch or stop lamp/light switch. PCM uses this switch input to maintain idle speed at a scheduled RPM when brakes are applied. If PCM receives an input signal from brake switch when speed control system is on, PCM will turn speed control system off.
CAMSHAFT POSITION (CMP) SENSOR
On Direct Ignition System (DIS), CMP sensor reads slots in cam timing sprocket. PCM uses this information along with information from Crankshaft Position (CKP) sensor to determine if fuel injectors and ignition coils are properly sequenced for correct cylinders.
CLUTCH SWITCH
This input prevents engine from starting until clutch pedal is depressed.
CRANKSHAFT POSITION (CKP) SENSOR
CKP sensor detects sets of slots on flywheel/torque converter drive plate. PCM uses this information to determine fuel injection sequence, ignition signal and spark timing.
CRUISE CONTROL SWITCH
Cruise control switch provides PCM with 3 separate inputs. ON/OFF informs PCM that cruise control system has been activated. SET/COAST informs PCM that set vehicle speed has been selected, or if depressed will decelerate until switch is released. RESUME/ACCEL informs PCM that a previously set speed has been selected or, if depressed, will increase speed until released. PCM uses these inputs to control cruise control servo.
ENGINE COOLANT TEMPERATURE (ECT) SENSOR
ECT sensor monitors engine coolant temperature. PCM uses ECT sensor information to adjust air/fuel mixture and idle speed and to control radiator cooling fans as necessary.
HEATED OXYGEN SENSOR (HO2S)
HO2S produces a small electrical voltage (0-1) when exposed to heated exhaust gas. HO2S is electrically heated for faster warm-up. Heating element is powered through Auto Shutdown (ASD) relay.
HO2S acts like a rich/lean (air/fuel ratio) switch by monitoring oxygen content in exhaust gas. This information is used by PCM to adjust air/fuel ratio by adjusting injector pulse width.
HO2S produces low voltage when oxygen content in exhaust gas is high. When oxygen content in exhaust gas is low, HO2S produces a higher voltage.
IGNITION SWITCH
Ignition switch sends signal to PCM indicating whether switch is on, off or cranking (ST). When PCM receives ON signal, it energizes ASD relay coil and supplies power to sensors and actuators. When PCM receives ST signal, it controls fuel injection rate, idle speed, ignition timing, etc. for optimum cranking conditions.
INTAKE AIR TEMPERATURE SENSOR
Sensor measures temperature of incoming intake air. This information is used by PCM to adjust air/fuel mixture.
KNOCK SENSOR (KS)
KS detects cylinder block vibrations caused by knocking and sends a signal to PCM. PCM retards ignition timing according to strength of knocking.
MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR
MAP sensor monitors intake manifold vacuum. Sensor transmits information on manifold vacuum and barometric pressure to PCM. MAP sensor information is used with information from other sensors to adjust air/fuel mixture.
PARK/NEUTRAL (P/N) SWITCH
P/N switch, also known as transmission range switch on some models, is available on vehicles equipped with A/T only. Switch prevents engine starter from engaging if vehicle is in any gear except Park or Neutral.
P/N switch input (varied with gear selection) is used to determine idle speed, fuel injector pulse and ignition timing.
SERIAL COMMUNICATION INTERFACE (SCI) RECEIVE
SCI receive circuit is a serial communication link used when diagnosing vehicle using scan tool. PCM receives data and device activation commands from scan tool on this circuit.
THROTTLE POSITION (TP) SENSOR
TP sensor monitors opening angle of throttle blade. TP sensor will vary output voltage from about .5 volt at minimum throttle opening (idle), to about 4.5 volts at Wide Open Throttle (WOT). PCM uses this information and other sensor inputs to determine engine operation. In response, PCM will adjust fuel injection pulse width and ignition timing.
TRANSMISSION OVERDRIVE OVERRIDE (OD/OR) SWITCH (A/T MODELS)
On models with Overdrive (OD), PCM regulates 3-4 OD upshift and downshift through OD solenoid. Transmission OD/OR switch is mounted in instrument panel.
OD/OR switch is normally closed. If OD/OR switch is depressed and it opens, transmission will not enter OD. Transmission will downshift if it is in OD and OD/OR switch is depressed.
OD/OR switch circuit includes a transmission fluid temperature thermistor. If this thermistor opens, transmission will not shift into overdrive, or will downshift if already in overdrive.
VEHICLE SPEED SENSOR (VSS)
VSS generates a predetermined number of pulses per sensor revolution (4 pulses per revolution on some models, or 8 pulses per revolution on other models). VSS input is used by PCM to determine vehicle speed and distance traveled, and to maintain set speed during cruise control operation.
PCM interprets speed sensor input along with TP sensor closed throttle input. This enables PCM to determine if a closed throttle deceleration or normal throttle idle (vehicle stopped) condition exists. During deceleration, PCM controls IAC motor to maintain a desired MAP value. During idle (vehicle stopped), PCM controls IAC motor to maintain a desired idle speed.
OUTPUT SIGNALS
Note. Each vehicle may be equipped with different combinations of computer-controlled components. The following components may NOT be used on all models. To determine component location and output usage on a specific model, see wiring diagram in the WIRING DIAGRAMS - 2.0L article. For theory and operation on each output component, refer to indicated system.
A/C CLUTCH RELAY
See MISCELLANEOUS CONTROLS.
AUTO SHUTDOWN (ASD) RELAY
See AUTO SHUTDOWN (ASD) RELAY & FUEL PUMP RELAY under MISCELLANEOUS CONTROLS.
DIRECT IGNITION SYSTEM
See IGNITION SYSTEM.
ELECTRIC EGR TRANSDUCER (EET)
See EXHAUST GAS RECIRCULATION (EGR) under EMISSION SYSTEMS.
EVAPORATIVE CANISTER PURGE SOLENOID (EVAP-CPCS)
See EVAPORATIVE (EVAP) EMISSIONS SYSTEM.
EXHAUST GAS RECIRCULATION (EGR) SOLENOID
See EXHAUST GAS RECIRCULATION (EGR) under EMISSION SYSTEMS.
FUEL INJECTORS
See FUEL CONTROL under FUEL SYSTEM.
FUEL PUMP RELAY
See AUTO SHUTDOWN (ASD) RELAY & FUEL PUMP RELAY under MISCELLANEOUS CONTROLS.
GENERATOR FIELD
See MISCELLANEOUS CONTROLS.
IDLE AIR CONTROL (IAC) MOTOR
See IDLE SPEED under FUEL SYSTEM.
IGNITION COIL
See IGNITION SYSTEM.
IN-TANK FUEL PUMP
See FUEL DELIVERY under FUEL SYSTEM.
LIMP-IN MODE
See MISCELLANEOUS CONTROLS.
MALFUNCTION INDICATOR LIGHT (MIL)
Also called CHECK ENGINE light. See SELF-DIAGNOSTIC SYSTEM.
RADIATOR FAN RELAY
See MISCELLANEOUS CONTROLS.
SERIAL COMMUNICATION INTERFACE (SCI) TRANSMIT
See SELF-DIAGNOSTIC SYSTEM.
SERVICE REMINDER INDICATOR (SRI) LIGHT
Formerly called Emission Maintenance Reminder (EMR) light. See SERVICE REMINDER INDICATOR (SRI) LIGHT.
SHIFT INDICATOR LIGHT
See MISCELLANEOUS CONTROLS.
SPEED CONTROL SERVO
See MISCELLANEOUS CONTROLS.
TACHOMETER
See MISCELLANEOUS CONTROLS.
TRANSMISSION OVERDRIVE OVERRIDE (OD/OR) SWITCH INDICATOR LIGHT
See MISCELLANEOUS CONTROLS.
TRANSMISSION OVERDRIVE (OD) SOLENOID
See MISCELLANEOUS CONTROLS.
See AUTO SHUTDOWN (ASD) RELAY & FUEL PUMP RELAY under MISCELLANEOUS CONTROLS.
FUEL PRESSURE REGULATOR
Fuel pressure regulator is a mechanical device, used to maintain a constant pressure across fuel injector tip. Spring and rubber diaphragm will move from an open to closed position keeping fuel pressure constant. Excess fuel is returned to fuel tank.
Regulator may be located on fuel injector rail, outside fuel tank (between tank and rear of vehicle), or with in-tank fuel pump module. On regulators located in fuel pump module, regulator includes an internal fuel filter. Excess fuel is routed directly into fuel tank without using a return line. (Scheme 1)
Scheme 1
Fuel pump is a positive displacement, immersible pump with a permanent magnet electric motor. Fuel is drawn in through a separate filter/strainer at bottom of fuel pump and pushed through filter to fuel outlet line (to fuel injectors). Voltage to operate pump is supplied from fuel pump relay. On some models, fuel pump relay is activated by ASD relay.
On some models, a fuel pump module may include a combination fuel filter/fuel pressure regulator, fuel pump reservoir, a separate in-tank fuel filter, pressure relief/rollover valve, fuel gauge sending unit and fuel supply line. (Scheme 2)
Scheme 2
Fuel injectors are electric solenoid valves controlled by PCM. PCM determines when and length of time (pulse width) injectors should operate by switching ground path on and off. During start-up, battery voltage is supplied to injectors through ASD relay. On some models, battery voltage is supplied by charging system once engine is operating. When ground is supplied to injector by PCM, armature and pintle inside injector move a short distance against spring and open a small orifice. Since fuel is under high pressure, a fine spray is developed.
IDLE SPEED
Note. DO NOT attempt to correct a high idle speed condition by turning factory sealed throttle body throttle plate set screw. This will not change idle speed of warm engine but may cause cold start problems due to restricted airflow.
IAC motor adjusts idle speed to compensate for engine load and ambient temperature by adjusting amount of air flowing through by-pass in throttle body. PCM uses ECT sensor, VSS, TP sensor and various switch input operations to adjust IAC motor to obtain optimum idle conditions. Deceleration stall is prevented by increasing airflow when throttle is closed suddenly.
IGNITION SYSTEM
Note. To determine ignition system usage on a specific model, see the ENGINE/VIN ID article.
The PCM completely controls ignition system. During crank/start mode, PCM will set a fixed amount of spark advance for an efficient engine start. Amount of spark advance or retard is determined by inputs that PCM receives from ECT sensor, engine vacuum and engine RPM. During engine operation, PCM can supply an infinite number of advance curves to ensure proper engine operation.
DISTRIBUTORLESS IGNITION SYSTEM (DIS)
A DIS eliminates mechanical ignition components that can wear out. PCM has complete ignition control and uses a coil pack, CMP sensor and CKP sensor to control ignition timing. CMP sensor reads slots in cam timing sprocket. PCM uses this information along with information from CKP sensor to determine if fuel injectors and ignition coils are properly sequenced for correct cylinders.
Basic timing is determined by CKP sensor position and is not adjustable. One complete engine revolution may be required for PCM to determine crankshaft position during cranking.
Molded ignition coils are used. Each coil fires 2 paired spark plugs at the same time. One cylinder is on compression stroke and other cylinder is on exhaust stroke.
HALL EFFECT IGNITION SYSTEM
This system is equipped with a Hall Effect distributor. (Scheme 1) Shutter(s) attached to distributor shaft rotate through distributor Hall Effect switch, also referred to as a CMP sensor, which contains a distributor pick-up (a Hall Effect device and magnet). As shutter blade(s) pass through pick-up, magnetic field is interrupted and voltage is toggled between high and low. PCM uses this cylinder position data from CMP sensor, along with engine speed (RPM) and CKP sensor data, to control ignition timing and injector pulse width to maintain optimum driveability.
EMISSION SYSTEMS
Vehicles are equipped with different combinations of emission system components. Not all components are used on all models.
AIR INJECTION SYSTEM
This system adds a controlled amount of air to exhaust gases, through air relief valve and check valves, to assist oxidation of hydrocarbons and carbon monoxide in exhaust stream. Air is injected at catalytic converters. System does not interfere with ability of EGR system to control oxides of nitrogen (NOx) emissions.
CRANKCASE VENTILATION (CCV) SYSTEM
CCV system performs same function as a conventional Positive Crankcase Ventilation (PCV) system, but does not use a vacuum controlled valve. See POSITIVE CRANKCASE VENTILATION (PCV) .
EVAPORATIVE (EVAP) EMISSIONS SYSTEM
This system stores fuel vapors from fuel tank, preventing vapors from reaching the atmosphere. As fuel evaporates inside fuel tank, vapors are routed through vent hoses to charcoal canister where they are stored until engine is started.
Evaporative Canister Purge Control Solenoid (EVAP-CPCS)
Charcoal canister purging is controlled by PCM through an EVAP-CPCS. During engine warm-up and for a short period after hot restarts, PCM energizes EVAP-CPCS, interrupting engine vacuum signal to charcoal canister.
After engine reaches a predetermined operating temperature and PCM internal timer has expired, PCM will de-energize EVAP-CPCS, allowing engine vacuum to purge charcoal canister. EVAP-CPCS will also be de-energized during certain idle conditions so PCM can update fuel delivery calibration.
EXHAUST GAS RECIRCULATION (EGR)
EGR system allows a predetermined amount of exhaust gas to enter cylinders with air/fuel mixture. This dilution of cylinder air/fuel volume reduces oxides of nitrogen (NOx) and helps prevent spark knock by reducing peak temperatures inside combustion chamber.
EGR system is a backpressure type and uses an Electric EGR Transducer (EET). (Scheme 3) This system incorporates backpressure transducer and EGR solenoid into one unit. Backpressure transducer measures amount of exhaust gas backpressure on exhaust side of EGR valve and varies amount of vacuum applied to EGR valve.
This system allows backpressure transducer to provide proper vacuum signal to EGR valve for all engine operating conditions. EGR system is controlled by an EGR vacuum solenoid using a manifold vacuum signal from throttle body.
Scheme 3
POSITIVE CRANKCASE VENTILATION (PCV)
PCV system uses a vacuum operated valve. A closed engine crankcase breather/filter, with a hose connecting it to air filter housing, provides source of air for system. Crankcase blow-by gases are removed from crankcase through PCV valve with manifold vacuum. These gases are introduced into incoming air/fuel mixture and become part of the calibrated mixture.
A non-vacuum operated Crankcase Ventilation (CCV) system is used on some engines, see CRANKCASE VENTILATION (CCV) SYSTEM .
Note. Failure to perform part replacement or required maintenance and only reset SRI light may be a violation of federal law. Only after performing part replacement or required maintenance, should SRI light be reset.
PCM activates SRI light, or SERVICE REQ'D light, at scheduled mileage intervals to indicate need for servicing of certain emission system components. After replacing part(s) or performing required service, a scan tool is needed to reset SRI light mileage interval. Follow scan tool manufacturer's instructions for resetting SRI light.
MALFUNCTION INDICATOR LIGHT
Malfunction Indicator Light (MIL), or CHECK ENGINE light, comes on and remains on for 3 seconds as a bulb test each time ignition switch is turned to ON position. If PCM receives an incorrect signal or receives no signal from battery voltage input, charging system, ECT sensor, MAP sensor or TP sensor, MIL will come on. MIL will also come on if certain emission-related faults exist. This warns driver that PCM is in limp-in mode and immediate repairs are necessary. See LIMP-IN MODE under MISCELLANEOUS CONTROLS. MIL can also be used to display diagnostic trouble codes. For additional information, see SELF-DIAGNOSTIC SYSTEM.
The PCM monitors several different circuits of engine control system. If a problem is sensed with a monitored circuit, PCM will store a Diagnostic Trouble Code (DTC) to aid technician in diagnosis of system. The Malfunction Indicator Light (MIL), or a scan tool can be used to read DTCs. For additional information, see TESTS W/CODES - 2.0L article.
SERIAL COMMUNICATIONS INTERFACE (SCI)
SCI circuit is used by PCM to send data to and receive data and sensor activation signals from scan tool. Scan tool uses signals sent on SCI to display fault messages or DTCs, sensor voltages and device states (On/Off). Scan tool uses SCI to send solenoid and switch activation commands to PCM so that devices and circuits can be tested. SCI is also used to reset Service Reminder Indicator (SRI) light and to write SRI mileage to PCM.
MISCELLANEOUS CONTROLS
Note. Although not strictly considered part of engine performance system, some controlled devices can adversely affect driveability if they malfunction.
A/C clutch relay is controlled by PCM. When A/C or Defrost mode is selected and PCM receives A/C request signal from evaporator switch, PCM will cycle clutch on and off through A/C clutch relay. When this relay is energized during engine operation, PCM will determine correct engine idle speed through IAC motor.
When PCM senses low idle speed or wide open throttle through TP sensor, PCM will de-energize A/C clutch relay, preventing A/C operation.
AUTO SHUTDOWN (ASD) RELAY & FUEL PUMP RELAY
ASD relay and electric fuel pump relay are energized when ignition is on. These relays are controlled through PCM by switching a common ground circuit on and off. Following components are controlled by ASD and fuel pump relays
- Electric Fuel Pump
- Fuel Injectors
- Generator Field Winding
- Ignition Coil(s)
- HO2S Heating Element
When ignition switch is turned to RUN position, PCM energizes ASD relay and electric fuel pump relay which powers these components. If PCM does not receive a CMP and CKP sensor signal within one second of engine cranking (start-up), PCM will turn ground circuit off and de-energize ASD relay.
GENERATOR
Powertrain Control Module (PCM) regulates charging system voltage.
Limp-in mode is the attempt by PCM to compensate for failure of certain components by substituting information from other sources so that vehicle can still be operated. If PCM senses incorrect data or no data at all from MAP sensor, TP sensor, ECT sensor or battery voltage, system is placed into limp-in mode and Malfunction Indicator Light (MIL) on instrument panel comes on.
If faulty sensor comes back on line, PCM will resume closed loop operation. On some vehicles, MIL will remain on until ignition is shut off and vehicle is restarted. To prevent damage to catalytic converter, vehicle should NOT be driven for extended periods in limp-in mode.
Using information supplied by A/C signal, ECT sensor, transmission thermoswitch (A/T) and TP sensor, PCM controls operation of electric cooling fan. PCM operates fan through radiator fan relay by grounding or ungrounding relay circuit. PCM regulates engine idle speed through IAC motor when fan is on.
On some models, a low speed fan relay and a high speed fan relay are used. On models using multiple fan relays, one relay operates under normal (low speed) operation and other relay works under high speed or extra load (A/C on) condition.
Some models use more than one cooling fan. Second fan may function as an auxiliary cooling device when A/C is engaged or during periods of engine overheating, or high A/C refrigerant pressure.
System is electrically actuated and vacuum operated. Controls are located on steering wheel. Controls consist of 3 buttons: OFF/ON, RESUME/ACCEL and SET/DECEL. Speed control servo is controlled by PCM. System will operate at 35-85 MPH.
PCM provides signal to drive tachometer.