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)
PCM is a digital computer that controls ignition timing, air/fuel ratio, emission control devices, cooling fan, charging system, idle speed, cruise control (if equipped), fuel pump and tachometer. The control unit is located in engine compartment. 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 8-volt output powers vehicle speed sensor, Camshaft Position (CMP) sensor and Crankshaft Position (CKP) sensor. The 5-volt output powers coolant temperature sensor, throttle body temperature sensor, Manifold Absolute Pressure (MAP) sensor and Throttle Position Sensor (TPS).
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 input usage on a specific model, see appropriate wiring diagram in WIRING DIAGRAMS section at the end of this 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 Voltage
PCM monitors battery voltage to determine fuel injector pulse width and alternator field control. This is done to compensate for reduced current flow through injector caused by lowered voltage.
Brake Switch
PCM uses this 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
CMP sensor is located inside distributor and is made up of a Hall Effect device (sync signal generator) and a rotating pulse ring (shutter) on distributor shaft. This 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.
Coolant Temperature Sensor (CTS)
The CTS monitors engine coolant temperature. Sensor is mounted in water passage near thermostat housing. PCM uses coolant temperature information to adjust air/fuel mixture and idle speed and to control cooling fans as necessary.
Crankshaft Position (CKP) Sensor
CKP sensor is mounted on top of cylinder block near right rear cylinder head. PCM uses this information to determine fuel injection sequence, ignition signal and spark timing.
Ignition Sense Circuit
Ignition sense circuit informs PCM that ignition circuit has been activated. PCM then prepares for engine operation.
Intake Air Temperature Sensor
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 is mounted on throttle body unit. 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.
Magnetic Ignition System (Hall Effect)
See IGNITION SYSTEM .
Optical Distributor
Optical distributor provides engine speed and crankshaft position signals. The pick-up unit operates by detecting light passing through a rotating ring. PCM uses this information to control fuel injection, ignition timing and idle speed. For more information, See IGNITION SYSTEM .
Heated Oxygen Sensor (HO2S)
O2S sensor produces a small electrical voltage (0-1 volt) when exposed to heated exhaust gas. O2S sensor is electrically heated for faster warm-up. Heating element is powered through Auto Shutdown (ASD) relay.
O2S sensor 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.
O2S sensor produces low voltage when oxygen content in exhaust gas is high. When oxygen content in exhaust gas is low, O2S sensor produces a higher voltage.
Park/Neutral (P/N) Switch
P/N switch is available on automatic transmission vehicles only. P/N switch is located on transmission housing. Switch prevents engine starter from engaging if vehicle is in any gear except Park or Neutral. Also see TRANSMISSION GEAR SELECTION.
Serial Communication Interface (SCI) Receive
SCI receive is serial communication link used when diagnosing vehicle using DRB. PCM receives data and device activation commands from DRB on this circuit.
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.
Throttle Position Sensor (TPS)
TPS is mounted on throttle body and monitors opening angle of throttle blade. TPS will vary output voltage, approximately 1-5 volts, according to angle of throttle blade opening. 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 Switch (A/T Models)
On models with Overdrive, PCM regulates 3-4 overdrive upshift and downshift through overdrive solenoid. Transmission overdrive override switch is mounted in instrument panel.
Switch is normally closed. If switch is depressed and switch opens, transmission will not enter Overdrive. Transmission will downshift if it is in Overdrive and switch is depressed.
Switch circuit contains 2 other switches: a transmission thermo-switch and a coolant temperature switch. When either switch opens, transmission will not shift into overdrive, or will downshift if already in overdrive.
Vehicle Speed Sensor (VSS)
VSS generates 8 pulses per sensor revolution. VSS input is used by PCM to determine vehicle speed and distance traveled. PCM interprets speed sensor input along with TPS 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 Idle Air Control (IAC) motor to maintain a desired MAP value. During idle (vehicle stopped), PCM controls IAC motor to maintain a desired idle speed. PCM also uses VSS signal to maintain set speed during cruise control operation.
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. For theory and operation on each output component, refer to indicated system.
A/C Clutch Relay
See MISCELLANEOUS CONTROLS .
Alternator Field
See MISCELLANEOUS CONTROLS .
Auto Shutdown (ASD) Relay
See MISCELLANEOUS CONTROLS .
Canister Purge Solenoid
See EVAPORATIVE EMISSIONS SYSTEM under EMISSION SYSTEMS.
Direct Ignition System
See IGNITION SYSTEM .
Electric EGR Transducer (EET)
See EXHAUST GAS RECIRCULATION (EGR) under EMISSION SYSTEMS.
Exhaust Gas Recirculation (EGR) Solenoid
See EXHAUST GAS RECIRCULATION (EGR) under EMISSION SYSTEMS.
Fuel Injectors
See FUEL CONTROL under FUEL SYSTEM.
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 .
Overdrive Override Switch Indicator Light
See MISCELLANEOUS CONTROLS .
Overdrive Solenoid
See MISCELLANEOUS CONTROLS .
Radiator Fan Relay
See MISCELLANEOUS CONTROLS .
Serial Communication 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 Solenoid
See MISCELLANEOUS CONTROLS .
See AUTO SHUTDOWN (ASD) RELAY under MISCELLANEOUS CONTROLS.
Fuel Pressure Regulator
Fuel pressure regulator is a mechanical device, used to maintain a constant pressure across fuel injector tip. A spring loaded rubber diaphragm is used to cover a fuel return port. When fuel pressure reaches a preselected pressure, the rubber diaphragm is forced off of fuel return port and excess fuel pressure is routed back to tank. Spring and rubber diaphragm will move from an open to closed position and back keeping fuel pressure constant. See the GAS FUEL PUMP REGULATED PRESSURE. table. Pressure regulator includes an internal fuel filter. Fuel filter/pressure regulator is located on top of in-tank fuel pump module. Excess fuel is routed directly into fuel tank without using a fuel return line. (Scheme 1)
| Application | Pressure - psi (kPa) |
|---|---|
| 3.9L | 35-45 (241-310) |
| (1) Fuel pressure taken at engine idle (no load). | |
| (1) | Fuel pressure taken at engine idle (no load). |
GAS FUEL PUMP REGULATED PRESSURE (1)
Scheme 1
In-Tank Fuel Pump (MFI)
Fuel pump is located in fuel pump module. Fuel pump module is located in top of fuel tank. 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. On 5.9L HD and 8.0L engines and all cab-chassis models, an auxiliary (capped) non-pressurized fuel supply fitting is located on top of fuel module.
Fuel pump is a positive displacement, immersible pump with a permanent magnet electric motor. Fuel is drawn in through a separate filter at bottom of fuel pump module and pushed through electric motor to fuel filter/fuel pressure regulator. Voltage to operate pump is supplied from fuel pump relay controlled by ASD relay. Fuel pump is not serviceable. If fuel pump is faulty, fuel pump module must be replaced.
In-Tank Fuel Pump (TBI)
Fuel pump is located in fuel pump module. Fuel pump module is located in top of fuel tank. Fuel pump module includes a fuel pump reservoir, a separate in-tank fuel inlet strainer, fuel gauge sending unit, a fuel return line and a fuel supply line. Fuel pump is an immersible electric pump with permanent magnet motor. Pump incorporates an inlet strainer attached to pump pick-up.
Voltage to operate pump is supplied through fuel pump relay.
Fuel injectors are electric solenoid valves controlled by PCM. PCM determines when and length of time injectors should operate by switching ground path on and off. During start-up, battery voltage is supplied to injectors through Automatic Shutdown (ASD) relay. Battery voltage is supplied by charging system after 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.
Multi-Port Fuel Injection (MFI)
Individual, electrically pulsed injectors (one per cylinder) are located in intake manifold fuel rails. These injectors are next to intake valves in intake manifold. MFI system features sequential injection. PCM controls injection timing based on crankshaft position signal input. Air/fuel mixture is regulated by length of time injector stays open (pulse width) based on oxygen sensor, temperature sensor, manifold pressure and other sensor inputs.
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 oxygen sensor to adjust fuel mixture. In open loop mixture and timing may be fixed or following a pre-programmed profile.
In closed loop mode, PCM is adjusting ignition timing and using input from oxygen sensor to fine tune injector pulse width.
The following inputs are used to determine PCM mode
- A/C Control Positions
- A/C Switch
- Battery Voltage
- Brake Switch
- Camshaft Position Sensor
- Coolant Temperature Sensor (CTS)
- Crankshaft Position Sensor
- Engine Speed
- Park/Neutral Position Switch
- Idle Contact Switch
- Manifold Absolute Pressure (MAP) Sensor
- Heated Oxygen Sensor (HO2S)
- Starter Relay
- Throttle Position Sensor (TPS)
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 In ON Position (Zero RPM)
When ignition switch is in ON position and PCM does not detect a crankshaft position signal (distributor signal), PCM will de-energize Automatic Shutdown (ASD) Relay. When ASD relay is not energized, battery power is cut from fuel injectors, fuel pump and coil.
- Engine Start-Up
This is an open loop mode. When starter is engaged, PCM receives distributor signal and energizes Auto Shutdown (ASD) relay. Once ASD relay is energized, PCM determines proper injector pulse width and ignition timing from input signals. During engine start-up, PCM energizes all injector(s) simultaneously until it determines crankshaft position.
- Engine Warm-Up
This is an open loop mode. PCM determines injector pulse width using information from various inputs and controls each injector sequence and pulse width. PCM controls engine idle speed, throttle stop angle and ignition timing.
- Cruise Or Idle
When engine is at operating temperature, system is in closed loop mode. Using information from various inputs, PCM determines injector pulse width. PCM controls engine idle speed (using IAC motor), throttle stop angle and ignition timing. PCM determines proper air/fuel ratio using input from oxygen sensor(s).
- Acceleration
This is a closed loop mode ON all FWD models. All other models are in open loop mode. When PCM recognizes an abrupt increase in throttle position or MAP pressure as a demand for increased engine output, it increases injector pulse width in response to increased fuel demand.
- Deceleration
When engine is at operating temperature, this is an open loop mode. On hard deceleration, with proper RPM and closed throttle conditions, PCM will ignore O2S input and enter a fuel cut-off strategy. If a hard deceleration does not exist, PCM will maintain proper injector pulse width and continue injection. PCM disengages A/C compressor clutch until vehicle is no longer under deceleration.
- Wide Open Throttle (WOT)
This is an open loop mode. When PCM senses wide open throttle, it grounds EGR and EVAP purge solenoids to prevent EGR and EVAP purge functions. Oxygen sensor input is not utilized and PCM adjusts injector pulse width to supply a predetermined amount of additional fuel.
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.
The 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 coolant temperature, distance (speed) sensor, throttle position 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 timing is controlled by Powertrain Control Module (PCM). PCM uses engine RPM data from Hall Effect switch (also referred to as Camshaft Position (CMP) sensor) to control ignition timing.
A single-blade (180-degree) shutter is used with Hall Effect device. This information is used in conjunction with CKP signal to differentiate between fuel and ignition events. CKP provides engine speed and crankshaft position to PCM. CKP is located on top of block near the rear of right side head.
PCM applies and monitors voltage to Hall Effect switch. In distributors, as shutter blade(s) pass through pick-up, magnetic field is interrupted and monitored voltage is toggled between high and low. Same effect is generated when windows in flywheel/drive plate pass magnet in crankshaft position sensor.
IGNITION TIMING CONTROL SYSTEM
The Powertrain Control Module (PCM) completely controls ignition system. During crank/start mode, PCM will set a fixed amount of spark advance for an efficient engine start.
Ignition Timing Advance Control
Amount of spark advance or retard is determined by inputs that PCM receives from coolant temperature, engine vacuum and engine RPM. During engine operation, PCM can supply an infinite number of advance curves to ensure proper engine operation.
EVAPORATIVE 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 inside vent hoses to charcoal canister, located in wheelwell area, where they are stored until engine is started.
Charcoal canister purging is controlled by a canister purge solenoid. Canister purge solenoid is controlled by PCM. During engine warm-up and for a short period after hot restarts, PCM grounds canister purge solenoid winding causing solenoid to energize.
When canister purge solenoid is energized, engine vacuum signal to charcoal canister is interrupted. After engine reaches a predetermined operating temperature and PCM internal timer has expired, PCM will de-energize canister purge solenoid. This will allow engine vacuum to purge charcoal canister. Canister purge solenoid 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 cylinder 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. Backpressure transducer measures amount of exhaust gas backpressure on exhaust side of EGR valve and varies amount of vacuum applied to EGR valve. (Scheme 2)
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.
EGR system uses an Electric EGR Transducer (EET). This system incorporates backpressure transducer and EGR solenoid into one unit. (Scheme 2)
Scheme 2
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, coolant temperature sensor, manifold absolute pressure sensor or throttle position sensor, MIL will glow. MIL will also glow if certain emission-related faults exist. This warns driver that PCM is in limp-in mode and immediate repairs are necessary. MIL can also be used to display fault messages. For additional information, see TESTS W/CODES - 3.9L article in the ENGINE PERFORMANCE section.
POSITIVE CRANKCASE VENTILATION (PCV)
All gas engines, except 8.0L engine, use a closed crankcase ventilation system with a Positive Crankcase Ventilation (PCV) 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.
SELF-DIAGNOSTIC SYSTEM
The Powertrain Control Module (PCM) monitors several different circuits of engine control system. If a problem is sensed with a monitored circuit, PCM will store a trouble code to aid technician in diagnosis of system. The Malfunction Indicator Light (MIL), also called the CHECK ENGINE light, or Diagnostic Readout Box (DRB) can be used to read trouble codes.
SERIAL COMMUNICATIONS INTERFACE (SCI)
SCI circuit is used by PCM to send data to and receive data and sensor activation signals from DRB tester. DRB uses signals sent on SCI to display fault messages, sensor voltages and device states (ON/OFF). DRB uses SCI to send solenoid and switch activation commands to PCM so that devices and circuits can be tested.
MISCELLANEOUS CONTROLS
Note. Although not strictly considered part of engine performance system, some controlled devices can adversely affect driveability if they malfunction.
On gas engines, 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.
On all models, when PCM senses low idle speed or wide open throttle through throttle position sensor, PCM will de-energize A/C clutch relay, preventing A/C operation.
ALTERNATOR
Powertrain Control Module (PCM) regulates charging system voltage to 12.9-15.0 volts.
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
- Alternator Field Winding
- Electric Fuel Pump
- Fuel Injectors
- 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 camshaft and crankshaft position sensor signal within one second of engine cranking (start-up), PCM will turn ground circuit off and de-energize ASD relay.
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, throttle position sensor, coolant temperature sensor or battery voltage, system is placed into limp-in mode and Malfunction Indicator Light (MIL) on instrument panel is illuminated.
If faulty sensor comes back on line, PCM will resume closed loop operation. On some vehicles, MIL will remain illuminated 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.
PCM controls indicator light on overdrive override switch on models equipped with overdrive automatic transmission.
On models equipped with overdrive transmission, PCM controls 3-4 overdrive upshift and downshift through overdrive solenoid. PCM determines optimum overdrive shift scheduling for all operating conditions.
Powertrain Control Module (PCM) controls radiator fan relay(s).
Radiator fan relay(s) are energized during the following conditions
- When A/C clutch is engaged.
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 clutch solenoid. PCM controls lock-up according to various operating conditions.
