Circuit Description
The Powertrain Control Module (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 the engine is OFF. 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 below 1000 RPM for low Duty Cycle Test and the (F) terminal PWM is less then 10 percent for 30 seconds. |
| • | The engine is above 1300 RPM for High Duty Cycle test and the (F) terminal PWM is more then 95 percent for 60 seconds. |
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 10 seconds. |
OR
| • | (F) terminal PWM is less than 10 percent . |
| • | Engine RPM is less than 1000. |
| • | The above conditions are present for more then 20 seconds. |
Action Taken When the DTC Sets
| • | The PCM will illuminate the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test has been run and failed. |
| • | The PCM will store conditions which were present when the DTC set as Freeze Frame/Failure Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn OFF the malfunction indicator lamp (MIL) during the third consecutive trip in which the diagnostic has run and passed. |
| • | The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction. |
| • | The DTC can be cleared by using a scan tool. |
Diagnostic Aids
Inspect for the following conditions:
Many situations may lead to an intermittent condition. Perform each inspection or test as directed.
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.
| • | Loose terminal connection |
| - | Use a corresponding mating terminal to test for proper tension. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair. |
| - | Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair. |
| • | Damaged harness--Inspect the wiring harness for damage. If the harness inspection does not reveal a problem, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the scan tool display may indicate the location of the fault. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair. |
| • | Inspect the powertrain control module (PCM) and the engine grounds for clean and secure connections. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair. |
If the condition is determined to be intermittent, reviewing the Snapshot or Freeze Frame/Failure Records may be useful in determining when the DTC or condition was identified.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||
2 | Does the scan tool indicate that the generator PWM is within the indicated range? | 5 percent-95 percent | Go to Charging System Check . | |
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? | 0.0 V 5.0 V | ||
4 |
Is the reading affected? | -- | Go to Charging System Check . | |
5 | Test the (L) circuit for an open or short to ground and repair if necessary. Refer to Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | ||
6 | Test the (L) circuit for a short to voltage and repair if necessary. Refer to Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | ||
7 | Test the (F) circuit for an open or short to ground and repair if necessary. Refer to Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | ||
8 | Test the (F) circuit for a short to voltage and repair if necessary. Refer to Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | ||
9 | Inspect for a faulty connection at the PCM and repair if necessary. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. Did you complete the repair? | -- | ||
|
Important:: Replacement PCM must be programmed. Refer to Powertrain Control Module Replacement/Programming PCM Replacement/Programming . Replace the PCM. Did you complete the replacement? | -- | -- | ||
11 |
Does the scan tool indicate DTC P0121 failed this ignition? | -- | System OK |
