PCM Operation - General Info
The PCM is a pre-programmed dual microprocessor ditital
computer, which monitors various input signals from engine sensors, and then
feeds its response to the engine control devices. The PCM operates in
two modes: Open Loop and Closed Loop. Ignition timing and ignition coil
dwell are adjusted in response to engine rpm, manifold absolute pressure, engine
coolant temperature, throttle position, transmission gear, vehicle speed, and
the brake switch. The PCM also controls the battery charge rate and the AC clutch,
and provides speed control operation.
- During open loop operation, the PCM receives input signals and responds
according to preset PCM programming. Input from O2 sensors is ignored.
- During closed loop operation, the PCM monitors O2 sensor signals and adjusts
the injector puls based on the response from the O2 sensors. The PCM attempts
to maintain the ideal 14.7:1 air/fuel ration by fine tuning fuel injection.
This produces the best fuel economy and lowest emissions for a given engine
The fuel injection has these operating modes:
- Ignition switch ON - open loop
- Engine crank (start-up) - open loop
- Engine warm-up - open loop
- Idle - closed loop
- Cruise - closed loop
- Acceleration - open loop
- Deceleration - open loop
- Wide open throttle (WOT) - open loop
- Ignition switch off
- Generator output
- A/C request
- A/C select
- Auto shutdown (ASD) sensor
- Intake manifold air temperature
- Battery voltage
- Brake switch
- Engine coolant temperature
- Crankshaft position
- Ignition switch RUN position
- Manifold Absolute Pressure (MAP)
- Overdrive/Override switch
- Oxygen sensors
- Park/neutral switch
- SCI receive (DRB scan tool connection)
- Speed control resume switches
- Camshaft position
- Throttle position (TPS)
- Vehicle speed sensor
- Sensor return
- power ground
- Signal ground
- A/C clutch relay
- Idle Air Controller (IAC) motor
- Auto Shutdown (ASD) relay
- Generator field
- Malfunction indicator (check engine lamp)
- EGR valve control solenoid
- Fuel injectors
- Fuel pump relay
- Ignition coils
- EVAP canister purge solenoid
- SCI transmit (DRB scan tool connection)
- Speed control vacuum solenoid
- Speed control vent solenoid
The PCM also contains two 5 volt regulators
to power sensors.
There are several families of PCM's that were developed to meet EPA regulations.
These various versions are not interchangeable.
- 1994 and 1995PCM's are OBD compliant.
- 1996 and 1997 PCM's meet OBD II specifications.
- 1998 and later PCM's meet OBD III specifications.
Last update: June 19, 2001