PCM Operation

Fig. 100 Powertrain Control Module:

Fig. 3 Data Link Connector:

The Powertrain Control Module (PCM) is a digital computer containing a microprocessor (Fig. 2). The PCM receives input signals from various switches and sensors that are referred to as PCM Inputs. Based on these inputs, the PCM adjusts various engine and vehicle operations through devices that are referred to as PCM Outputs.

PCM Inputs:
- Air Conditioning Controls
- Battery Voltage
- Battery Temperature Sensor
- Brake Switch
- Camshaft Position Sensor
- Crankshaft Position Sensor
- Engine Coolant Temperature Sensor
- Fuel Level Sensor
- Ignition Switch (A21)
- Intake Air Temperature Sensor
- Knock Sensor
- Manifold Absolute Pressure (MAP) Sensor
- Oxygen Sensors
- Power Steering Pressure Switch
- SCI Receiver
- Speed Control Switches
- Throttle Position Sensor
- Transmission Park/Neutral Switch (Automatic Transmission)
- Vehicle Speed Sensor

PCM Outputs:
- Air Conditioning Clutch Relay
- Air Conditioning WOT Relay
- Auto Shutdown (ASD) Relay
- Charging Indicator Lamp
- Data Link Connector
- Duty Cycle EVAP Canister Purge Solenoid
- EGR Solenoid
- Fuel Injectors
- Fuel Pump Relay
- Generator Field
- Idle Air Control Motor
- Ignition Coils
- Malfunction Indicator (Check Engine) Lamp
- Radiator Fan Relay
- Speed Control Solenoids
- Tachometer
- Torque Convertor Clutch Solenoid

Based on inputs it receives, the PCM adjusts:
- Fuel Injector Pulse Width
- Idle Speed
- Ignition Spark Advance
- Ignition Coil Dwell
- EVAP Canister Purge Operation.

The PCM Regulates:
- Cooling Fan
- Air Conditioning
- Speed Control System
- Generator Charge Rate -- by Adjusting The Generator Field

The PCM Also Performs Diagnostics

The PCM adjusts injector pulse width (air-fuel ratio) based on:
- Battery Voltage
- Coolant Temperature
- Exhaust Gas Content (Oxygen Sensor)
- Engine Speed (Crankshaft Position Sensor)
- Intake Air Temperature
- Manifold Absolute Pressure
- Throttle Position

The PCM adjusts ignition timing based on:
- Coolant Temperature
- Engine Speed (Crankshaft Position Sensor)
- Intake Air Temperature
- Knock Sensor
- Manifold Absolute Pressure
- Throttle Position
- Transmission Gear Selection (Park/Neutral Switch)

The PCM also adjusts engine idle speed through the idle air control motor based on:
- Air Conditioning Sense
- Battery Temperature
- Battery Voltage
- Brake Switch
- Coolant Temperature
- Engine Speed (crankshaft Position Sensor)
- Engine Run Time
- Manifold Absolute Pressure
- Power Steering Pressure Switch
- Throttle Position
- Transmission Gear Selection (Park/Neutral Switch)
- Vehicle Distance (Speed)

ASD and Fuel Pump Relay Control
The Auto Shutdown (ASD) and fuel pump relays are mounted externally, but turned on and off by the PCM.

Crankshaft Position (Engine Speed) Signal
The crankshaft position sensor signal is sent to the PCM. If the PCM does not receive the signal within approximately one second of engine cranking, it deactivates the ASD relay and fuel pump relay When these relays deactivate, power is shut off from the fuel injectors, ignition coils, heating element in the oxygen sensors and the fuel pump.

PCM Regulated Output Supply Voltages
The PCM contains a voltage converter that changes battery voltage to a regulated 8 volts direct current to power:
- Camshaft Position Sensor
- Crankshaft Position Sensor
- Vehicle Speed Sensor

The PCM also provides a 5 volt direct current supply for:
- Manifold Absolute Pressure Sensor
- Throttle Position Sensor