General Operation

Computerized Engine Control Component Tree:

The PCM determines current engine operating conditions by interpreting inputs from various sensors and switches.

The PCM then actuates various output devices to achieve the best compromise between power, fuel consumption, and emissions.

MODES OF OPERATION

Open Loop Modes

Note: In OPEN LOOP modes the PCM receives input signals and controls outputs according to preset programming. The PCM Ignores Oxygen (HO2S) sensor input during OPEN LOOP modes.

- Ignition Switch ON (Zero RPM)
- Engine Start-Up
- Engine Warm-Up
- Wide Open Throttle (WOT)
- Ignition Switch OFF
- Limp In Mode


Closed Loop Modes

Note: In CLOSED LOOP modes the PCM monitors the Oxygen Sensor (HO2S), input, and adjusts injector pulse width to compensate for operating conditions.

- Warm Idle
- Cruise
- Acceleration (Except Wide Open Throttle)
- Deceleration


SELF DIAGNOSIS OPERATION

The PCM also monitors many of some input and output circuits for faults, and if a fault is detected:

- A Diagnostic Trouble Code (DTC) is stored in memory.

- The MIL is illuminated [1].

- The vehicle enters "Limp-in Mode". SEE LIMP-IN OPERATION.

Note: DTC's can be read using the Malfunction Indicator Lamp (MIL), (previously called the Check Engine Lamp), or a scan tool. SEE Testing and Inspection.

[1] The MIL is not illuminated for every fault, always read DTC's before attempting to diagnose a driveablity or emission related problem.

INPUTS

Air Conditioning Controls
Battery Voltage (B+)
Brake Switch
Distributor (Hall Effect) Pick-up
Coolant Temperature Sensor
Manifold Absolute Pressure (MAP) Sensor
Park/Neutral Switch (Automatic Transmission)
Heated Oxygen Sensor (HO2S)
SCI Receive
Speed Control System Controls
Throttle Position Sensor (TPS)
Vehicle Distance (Speed) Sensor (VSS)

OUTPUTS

Air Conditioning Clutch Relay
Alternator Field
Idle Air Control (IAC) Motor
Automatic Shut Down (ASD) and Fuel Pump Relays
Check Engine Lamp (CEL)
Electric EGR Transducer (EET)
Emission Maintenance Reminder (EMR) Lamp
Evaporative Emission Purge Control Solenoid Valve
Fuel Injector Pulse Width
Ignition Coil
Part Throttle Unlock (PTU) Solenoid
Radiator Fan Relay
Diagnostic Connector
Speed Control (S/C) Servo Solenoids
Tachometer Output
+9 Volt Regulated Voltage for the Distributor Pickup, and Vehicle Speed Sensor
+5 Volt Regulated Voltage for the MAP and Throttle Position Sensors