Powertrain Control Module (Pcm)
The PCM regulates the ignition system, see Fig 1. The PCM supplies battery voltage to the ignition coil through the Automatic Shutdown (ASD) Relay. The PCM also controls the ground circuit for the ignition coils. By switching the ground path for the coil on and off, the PCM adjusts ignition timing to meet changing engine operating conditions. During the crank-start period the PCM advances ignition timing a set amount. During engine operation the following inputs determine the amount of spark advance provided, by the PCM:
- Intake Air Temperature
- Coolant Temperature
- Engine RPM
- Available Manifold Vacuum
The Powertrain Control Module (PCM) is a digital computer containing a microprocessor. 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
- Brake Switch
- Camshaft Position Sensor
- Crankshaft Position Sensor
- Engine Coolant Temperature (ECT) Sensor
- Fuel Level Sensor
- Ignition Circuit Sense
- Intake Air Temperature Sensor
- Manifold Absolute Pressure (MAP) Sensor
- Oxygen Sensors
- SCI Receive
- Throttle Position (TPS) Sensor
- Vehicle Speed Sensor
- Fuel Level Sensor
- Heated Oxygen Sensors (O2 Sensor)
- ASD Sense Circuit
- Clutch Pedal Position Switch
- Sensor Return
- CCD To Security System
Based on these inputs, the PCM adjusts various engine and vehicle operations through devices referred to as PCM Outputs.
PCM Outputs:
- Air Conditioning WOT Relay
- Auto Shutdown (ASD) Relay
- CCD To Security System
- Data Link Connector
- Duty Cycle EVAP Canister Purge Solenoid
- EGR Solenoid
- Fuel Injectors
- Fuel Pump Relay
- Idle Air Control (IAC) Motor
- Ignition Coils
- Malfunction Indicator (Check Engine) Lamp
- Radiator Fan Relay
- Tachometer
- Self Diagnostic System
- Radiator Fan High/Low Relay
- Skip Shift Solenoid
- Reverse Lockout Solenoid
- SCI Transmit
- First To Fourth Gear Shift Indicator Lamp
- Cooling Fan
- Fuel Control
- Fuel Pressure Regulator
Based on inputs it receives, the PCM adjusts fuel injector pulse width, idle speed, ignition spark advance, ignition coil dwell and EVAP canister purge operation. The PCM regulates the cooling fan, and air conditioning. The PCM changes generator charge rate by adjusting the generator field. The PCM also performs diagnostics.
The PCM adjusts injector pulse width (air/fuel ratio) based on the following inputs:
- Battery Voltage
- Coolant Temperature
- Intake Air Temperature
- Exhaust Gas Content (Oxygen Sensors)
- Engine Speed (Crankshaft Position Sensor)
- Manifold Absolute Pressure
- Throttle Position
The PCM also adjusts ignition timing based on the following inputs:
- Coolant Temperature
- Intake Air Temperature
- Engine Speed (Crankshaft Position Sensor)
- Manifold Absolute Pressure
- Throttle Position
The PCM also adjusts engine idle speed through the idle air control motor based on the following inputs:
- Air Conditioning Sense
- Battery Voltage
- Brake Switch
- Coolant Temperature
- Engine Speed (Crankshaft Position Sensor)
- Engine Run Time
- Manifold Absolute Pressure
- Throttle Position
- Vehicle Distance (Speed)
The Auto Shutdown (ASD) and fuel pump relays are mounted externally, but turned on and off by the PCM.
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.
The PCM contains a voltage converter that changes battery voltage to a regulated 5 volts direct current to power the camshaft position sensor, crankshaft position sensor and vehicle speed sensor. The PCM also provides a 5 volt direct current supply for the manifold absolute pressure sensor and throttle position sensor.