General System Description

PURPOSE
The computerized engine control system controls:

- Air/Fuel Mixture
- Idle Speed
- Spark Advance
- Ignition Dwell
- EGR System Operation
- EVAP Canister Purge
- Generator Field Voltage
- Cooling Fan Operation
- Speed Control System
- Air Conditioning System Operation

OPERATION
The engine control system is built around a Powertrain Control Module (PCM).

- 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 engine performance, fuel consumption, and tailpipe emissions.

- The PCM also monitors input and output circuits for malfunctions (circuit operation outside of expected parameters, i.e. current or voltage too high for an extended period). See SELF DIAGNOSIS below.

SELF DIAGNOSIS
Diagnostic Trouble Code (DTC)

If a monitored circuit excedes the limits set for that circuit, the PCM stores a Diagnostic Trouble Code (DTC) in memory. (DTC information can be read using the MIL or a scan tool. SEE Testing and Inspection Procedures.)

Malfunction Indicator Lamp (MIL)
For most faults, the PCM illuminates the Malfunction Indicator Lamp (MIL), (previously called the Check Engine Lamp), in the instrument panel. SEE Powertrain Management/Computers and Control Systems/Testing and Inspection/Procedures.

Limp-In Mode
Depending on the fault detected, the PCM may also enter LIMP-IN mode.

- If the MIL is illuminated during vehicle operation, the vehicle has entered limp-in.

- Limp-in mode is a strategy programmed into the PCM, which allows reduced vehicle operation, if one or more inputs fail. (The vehicle should be driven to a service facility as soon as possible.)

- In Limp-in mode the PCM substitutes the input of an alternate sensor, for that of the failed sensor, while continuing to monitor the faulty sensor input.

- If the faulty sensor input returns within limits, the PCM will return to normal operation.