PCM Controlled Generator Diagnosis

Diagnostic Chart:

Schematic:

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

The PCM input field (F) terminal from the alternator provides a 400 Hz 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 telltale lamp will illuminate any time the ignition is ON and engine is not running. The telltale will go out after the engine is running and the alternator is putting out the desired amperage.

DIAGNOSTIC AIDS
Check for the following conditions:
^ Poor connection at the PCM or generator.
Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection. Use a corresponding mating terminal to test for proper terminal tension. Refer to Testing for Intermittent and Poor Connections Connector Repairs and Wiring Repairs in Diagrams.
^ Damaged harness.
Inspect the wiring harness for damage. If the harness appears to be OK, disconnect the PCM, turn the ignition ON and observe voltmeter connected to the generator circuits at the PCM harness connector while moving connectors and 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 Wiring Repairs.

TEST DESCRIPTION
The numbers below refer to the step numbers on the diagnostic table:
10. This vehicle is equipped with a PCM which utilizes and Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is being replaced, the new PCM must be programmed.