PCM Controlled Generator Diagnosis

Schematic:

CIRCUIT DESCRIPTION

The PCM output (L) terminal to the alternator determines when the alternator will be turned off. The output (L) terminal is on under all normal vehicle operating conditions except during low RPM conditions. The alternator is turned off during low RPM conditions in order to remove excess load from the engine.

The PCM input field (F) terminal from the alternator provides a 400 Hz Pulse-width Modulated (PWM) signal to the PCM. The duty cycle of the F terminal mimics the duty cycle of the field coils in the alternator. Thus, the F terminal duty cycle indicates the relative electrical load on the alternator.

The alternator tell-tale lamp will illuminate any time the ignition is on and the engine is not running. The lamp will go out after the engine is running and the alternator is putting out the desired amperage.

DIAGNOSTIC AIDS
Check for the following conditions:
^ A faulty connection at the PCM or at the generator-Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connections. Use a corresponding mating terminal to test for the correct terminal tension. Refer to Testing for Intermittent and Poor Connections Connector Repairs and Wiring Repairs in Diagrams.
^ Inspect the wiring harness for damage. If the harness appears to be OK, disconnect the PCM, turn the ignition on and observe a DMM connected to the generator circuits at the PCM harness connector while moving the connectors and the wiring harnesses related to the generator control circuit. A change in voltage will indicate the location of the fault. Refer to Connector Repairs and Wiring Repairs in Diagrams.


TEST DESCRIPTION

Diagnostic Chart:

The numbers below refer to the step numbers on the diagnostic table.
10. This vehicle is equipped with a PCM which utilizes an electrically Erasable Programmable Read-only Memory (EEPROM). If the PCM is replaced, the new PCM must be programmed.