INTRODUCTION
This article covers the basic description and operation of engine performance related systems and components. Read this article before working on unfamiliar systems.
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 PCM LOCATION table. 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 output. The 5-volt output powers battery temperature sensor, Camshaft Position (CMP) sensor on models equipped with Distributorless Ignition System (DIS) or distributor on 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.
| Application | Location |
|---|---|
| Ram Van/Wagon | On Firewall, Near Wiper Motor |
PCM LOCATION
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 appropriate wiring diagram in WIRING DIAGRAMS 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 brakelight 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 models equipped with a distributor, CMP sensor is made up of a Hall Effect switch (sync signal generator) and a rotating pulse ring (shutter) on distributor shaft. (Scheme 1) On Distributorless 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.
Scheme 1
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 sparktiming.
Cruise Control Switch
Cruise control switch provides PCM with 3 separate inputs. ON/OFF switch input informs PCM that cruise control system has been activated. SET/COAST switch input informs PCM that set vehicle speed has been selected, or if depressed will decelerate until switch is released. RESUME/ACCEL switch input 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 (IAT) Sensor
IAT sensor measures temperature of incoming intake air. This information is used by PCM to adjust air/fuel mixture.
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
This switch may also be referred to as a Park/Neutral Position (PNP) switch. P/N switch 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 sensor. If this sensor opens, transmission will not shift into overdrive, or will downshift if already in overdrive.
Transmission Temperature Sensor (A/T Models)
Transmission temperature sensor monitors transmission fluid temperature and sends an input signal to PCM. Input signal is used for controlling torque converter clutch operation, overdrive shifts, low temperature shift compensation, wide open throttle shift strategy and governor pressure. Transmission temperature sensor is located in transmission valve body, incorporated into governor pressure sensor.
If transmission fluid temperature is more than 260°F (126°C), PCM forces a 4-3 downshift and engages torque converter clutch until fluid cools. Once fluid cools to less than 230°F (110°C), PCM allows a 3-4 shift. PCM prevents torque converter clutch engagement and overdrive operation when fluid temperature is less than 50°F (10°C).
Vehicle Speed Sensor (VSS)
VSS generates 8 pulses per sensor revolution. 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 appropriate wiring diagram in WIRING DIAGRAMS 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.
Distributorless Ignition System (DIS)
See IGNITION SYSTEM.
Evaporative Canister Purge Control Solenoid (EVAP-CPCS)
See EVAPORATIVE (EVAP) EMISSIONS SYSTEM 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
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)
See SELF-DIAGNOSTIC SYSTEM.
Radiator Fan Relay
See MISCELLANEOUS CONTROLS.
Serial Communications Interface (SCI) Transmit
See SELF-DIAGNOSTIC SYSTEM.
Shift Indicator Light
See MISCELLANEOUS CONTROLS.
Speed Control Servo
See MISCELLANEOUS CONTROLS.
Tachometer
See MISCELLANEOUS CONTROLS.
Torque Converter Clutch (TCC) Solenoid
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 is located in in-tank fuel pump module. Regulator includes an internal fuel filter. Excess fuel is routed directly into fuel tank without using a return line. (Scheme 2)
Scheme 2
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.
Fuel pump module includes 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 3)
Scheme 3
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.
Modes Of Operation
As input signals to PCM change, PCM adjusts its response to output devices. Modes of operation come in 2 types, open loop and closed loop. In open loop mode, PCM is not using input from HO2S and is responding to preset programming to determine injector pulse width and ignition timing. In closed loop mode, PCM adjusts ignition timing and uses input from HO2S to fine tune injector pulse width.
The following inputs may be used to determine PCM mode
- A/C Control Positions
- A/C Switch
- Battery Voltage
- Brake Switch
- Camshaft Position (CMP) Sensor
- Crankshaft Position (CKP) Sensor
- Engine Coolant Temperature (ECT) Sensor
- Engine Speed (RPM)
- Heated Oxygen Sensor (HO2S)
- Intake Air Temperature (IAT) Sensor
- Manifold Absolute Pressure (MAP) Sensor
- Park/Neutral (P/N) Switch
- Starter Relay
- Throttle Position (TP) Sensor
- Vehicle Speed Sensor (VSS)
From these inputs, PCM determines which mode vehicle is in and responds appropriately. Not all inputs are used in all modes or by all models. Modes of operation are
- Ignition Switch On (Engine Not Running)
This is an open loop mode. PCM pre-positions IAC motor based on ECT sensor input. PCM determines atmospheric pressure from MAP sensor and determines basic fuel strategy. PCM modifies fuel strategy according to IAT sensor, ECT sensor and TP sensor inputs. PCM activates ASD relay, which in turn activates fuel pump for only 2 seconds unless engine is cranked. PCM also energizes HO2S heater element for approximately 2 seconds unless engine is cranked.
- Engine Start-Up
This is an open loop mode. When starter is engaged, PCM receives input from battery voltage, CKP sensor, CMP sensor, ECT sensor, IAT sensor, MAP sensor and TP sensor. Based on these inputs, voltage is applied to fuel injectors with PCM controlling injection sequence, rate, and pulse width. PCM provides ground for injectors to fire in proper order.
PCM determines proper ignition timing according to input received from CKP sensor. If PCM does not receive CKP sensor signal within 3 seconds after engine begins cranking, fuel injection system is shut down and a Diagnostic Trouble Code (DTC) is set in PCM memory.
- Engine Warm-Up
This is an open loop mode. PCM determines injector pulse width using input information from battery voltage, CKP sensor, CMP sensor, ECT sensor, IAT sensor, MAP sensor and TP sensor. PCM also monitors A/C request and PNP switch(A/T only) for fuel calculation. PCM controls engine idle speed through IAC motor. PCM controls ignition timing based on CKP sensor input. PCM also operates A/C compressor clutch (if A/C is requested) through A/C clutch relay. When engine reaches operating temperature, vehicle will go into idle mode and PCM will begin monitoring HO2S input and go into closed loop operation.
- Idle
When engine is at operating temperature, this is a closed loop mode. In idle mode, PCM now adds HO2S signal to array of inputs used in ENGINE WARM-UP mode. PCM maintains correct air/fuel ratio by adjusting injector pulse width and ignition timing. PCM also controls A/C clutch operation (if A/C is requested).
- Cruise
When engine is at operating temperature, this is a closed loop mode. Using information from A/C switch, battery voltage, CKP sensor, ECT sensor, IAT sensor, MAP sensor and CMP sensor. PCM also monitors A/C request and P/N switch (A/T only), TP sensor and VSS signals for fuel calculation. PCM monitors HO2S and adjusts air/fuel ratio as needed. PCM controls engine idle speed through IAC motor. PCM controls spark advance as necessary.
- Acceleration
This is an open loop mode. When PCM recognizes an abrupt increase in throttle position or manifold pressure as a demand for increased engine output, it increases injector pulse width in response to increased fuel demand. HO2S signals are ignored.
- Deceleration
This is an open loop mode when engine is at operating temperature and under deceleration. When PCM receives inputs signaling a closed throttle and an abrupt decrease in manifold pressure, it reduces injector pulse width to lean air/fuel mixture. Under certain RPM and closed throttle position conditions, HO2S signals are ignored and PCM cuts off fuel injection until idle speed is reached. PCM also drives IAC motor for smooth transition to idle mode.
- Wide Open Throttle
This is an open loop mode. When PCM senses wide open throttle, it grounds fuel injectors in sequence, it ignores HO2S input and it controls pulse width to supply a pre-determined amount of additional fuel. PCM also adjusts spark advance and disengages A/C clutch for approximately 15seconds.
- Ignition Switch Off
This is an open loop mode. PCM drives IAC motor into position in anticipation of next start-up. All outputs are turned off, no inputs are monitored and PCM shuts down.
Sequential Fuel Injection (SFI)
Individual, electrically pulsed injectors (one per cylinder) are located in intake manifold runners. These injectors are next to intake valves in intake manifold. PCM controls injection timing based on crankshaft position signal input. PCM regulates air/fuel mixture by length of time injector stays open (pulse width) based on inputs from HO2S, ECT sensor, MAP and other sensors.
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
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.
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. To determine component usage on a specific model, see EMISSION APPLICATION article.
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.
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.
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.
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.
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 article.
MALFUNCTION INDICATOR LIGHT
Malfunction Indicator Light (MIL) 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 (DTCs). For additional information, see TESTS W/CODES 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 Diagnostic Trouble Codes (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 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.
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 (if equipped), ECT sensor, and VSS, 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.
PCM provides ground for shift indicator light on models equipped with manual transmission. Based on engine speed, throttle position, and vehicle speed, PCM turns shift indicator light on to advise driver to shift to a higher gear for optimum fuel economy.
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.
PCM controls torque converter lock-up through TCC solenoid. PCM controls lock-up according to various operating conditions.
PCM controls indicator light on OD/OR switch on models equipped with overdrive automatic transmission.
On models equipped with OD transmission, PCM controls 3-4 OD upshift and downshift through OD solenoid. PCM determines optimum OD shift scheduling for all operating conditions.