P0620

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.

Conditions for Running the DTC

1. The engine is running.

2. The Voltage telltale is on.

Conditions for Setting the DTC

1. (L) terminal voltage is low while the PCM is commanding the generator on.

2. The above condition is present for more than 30 seconds.

OR

1. (F) terminal PWM is less than 5%.

2. Engine RPM is less than 2500.

3. The above conditions are present for more then 30 seconds.

Action Taken When the DTC Sets

^ The PCM will not illuminate the Malfunction Indicator Lamp (MIL).

^ The PCM will store conditions which were present when the DTC set as Fail Records data only. This information will not be stored as Freeze Frame data.

Conditions for Clearing the MIL/DTC

^ A history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.

^ DTC can be cleared by using the scan tool.

Diagnostic Aids

Check for the following conditions:

^ Poor connection at the PCM or Generator: Inspect the harness connectors for backed but terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection. Refer to Intermittents and Poor Connections Diagnosis Connector Repairs and Wiring Repairs.

^ 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 circuit(s) 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 Intermittents and Poor Connections Diagnosis Connector Repairs and Wiring repairs.

Test Description

The number(s) below refer to the step number(s) 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 replaces, the new PCM must be programmed.