PCM Controlled Generator Diagnosis

Circuit Description

The L-terminal circuit from the generator is a discrete circuit (a discrete circuit has no splices and only one source and one destination) into the PCM. The PCM applies ignition voltage to the generator L terminal circuit. A small amount of current flows from this circuit through the generator windings to create a magnetic field which starts the generator process. When the generator is at operating speed and producing voltage, a solid state switch for the L terminal circuit in the generator opens and the PCM detects that the initial start-up current flow has stopped.

The PCM expects to detect low voltage on the L terminal circuit prior to the generator rotating at operating speed and conversely expects the circuit to be at ignition voltage potential when the generator is operational. When the PCM detects a fault (circuit shorted to ground, or circuit shorted to a voltage), the battery lamp will illuminate.

Diagnostic Aids

Important:
^ Remove any debris from the PCM connector surfaces before servicing the PCM. Inspect the PCM connector gaskets when diagnosing/replacing the PCM. Ensure that the gaskets are installed correctly. The gaskets prevent water intrusion into the PCM.
^ For any test that requires probing the PCM or component harness connectors, use the Connector Test Adapter kit J 35616-A. Using this kit prevents any damage to the harness connector terminals. Refer to Using Connector Test Adapters in Diagrams.

A generator fault, such as a shorted output diode, malfunctioning regulator, an open or shorted rotor, or an open sense lead may cause a low battery charge condition. Refer to Charging System Check in Starting and Charging.

Test Description

The numbers below refer to the step numbers on the Diagnostic Table.

2. This step determines if a fault is present.
11. A poor connection at the generator causes no output.