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

^ The engine is running.
^ 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 a scan tool.

Diagnostic Aids

Check for the following conditions:

Important: Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.

^ Poor terminal connection.
Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Use a corresponding mating terminal to test for proper tension. Refer to Diagrams.
^ Damaged harness.
Inspect the wiring harness for damage. If the harness appears to be OK, observe the sensor display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the sensor display may indicate the location of the fault.
^ Inspect the PCM and the engine grounds for clean and secure connections.

If the DTC is determined to be intermittent, reviewing the Fail Records can be useful in determining when the DTC was last set.

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.