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
- (L) terminal voltage is low while the PCM is commanding the generator on.
- The above condition is present for more than 30 seconds.
OR
- (F) terminal PWM is less than 5%.
- Engine RPM is less than 2500.
- 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 clared 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 out 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 below refers to the step number on the diagnostic table.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Was the On-Board Diagnostic (OBD) System Check performed? | -- | ||
2 | Does the scan tool indicate that the Generator PWM is within the indicated range? | 5% - 95% | Go to Charging System Check in Engine Electrical | |
3 |
Command the Generator ON with the scan tool while monitoring the DMM. Does the voltage change from the first indicated value to the second indicated value? | 0V 5V | ||
4 |
Is the reading affected? | -- | Go to Charging System Check in Engine Electrical | |
5 | Test the (L) circuit for an open or short to ground and repair if necessary. Refer to Wiring Repairs in Wiring Systems. Was a problem found? | -- | ||
6 | Test the (L) circuit for a short to voltage and repair if necessary. Refer to Wiring Repairs in Wiring Systems. Was a problem found? | -- | ||
7 | Test the (F) circuit for an open or short to ground and repair if necessary. Refer to Wiring Repairs in Wiring Systems. Was a problem found? | -- | ||
8 | Test the (F) circuit for a short to voltage and repair if necessary. Refer to Wiring Repairs in Wiring Systems. Was a problem found? | -- | ||
9 | Check for a poor connection at the PCM and repair if necessary. Refer to Intermittents and Poor Connections Diagnosis and Repairing Connector Terminals in Wiring Systems. Was a problem found? | -- | ||
|
Important: Replacement PCM must be programmed. Refer to Powertrain Control Module Replacement/Programming .Replace the PCM. Is the action complete? | -- | -- | ||
11 |
Does the scan tool indicate DTC P0121 failed this ignition? | -- | System OK |
