Circuit Description
The powertrain control module (PCM) uses the generator field duty cycle signal circuit to monitor the duty cycle of the generator. The generator field duty cycle signal circuit connects to the high side of the field winding in the generator. A pulse width modulated (PWM) high side driver in the voltage regulator turns the field winding ON and OFF. The PCM uses the PWM signal input to determine the generator load on the engine. This allows the PCM to adjust the idle speed to compensate for high electrical loads.
The PCM monitors the state of the generator field duty cycle signal circuit. When the key is in the RUN position and the engine is OFF, the PCM should detect a duty cycle near 0 percent. However, when the engine is running, the duty cycle should be between 5 percent and 100 percent. The PCM monitors the PWM signal using a key ON test and a RUN test. During the tests, if the PCM detects an out of range PWM signal, the DTC will set. When the DTC sets, the PCM will send a class 2 serial data message to the instrument panel cluster (IPC) to illuminate the charge indicator.
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P1638 Generator F-Terminal Circuit
Conditions for Running the DTC
The engine is operating.
Conditions for Setting the DTC
| • | The F-Terminal indicates a duty cycle less than 10 percent with the engine running. |
| • | All conditions met for 6 seconds. |
Action Taken When the DTC Sets
| • | The PCM stores the DTC information into memory when the diagnostic runs and fails. |
| • | The PCM records the operating conditions at the time the diagnostic fails. The PCM stores this information in the Failure Records. |
| • | The malfunction indicator lamp (MIL) will not illuminate. |
Conditions for Clearing the DTC
| • | A last test failed, or current DTC, clears when the diagnostic runs and does not fail. |
| • | A history DTC will clear after 40 consecutive warm-up cycles, if no failures are reported by this or any other non-emission related diagnostic. |
| • | Use a scan tool in order to clear the DTC. |
Step | Action | Values | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | Go to Diagnostic System Check - Vehicle in Vehicle DTC Information |
2 |
Does the scan tool indicate that the GEN - F Terminal parameter is within the specified range? | 5-95% | Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems | Go to Step 3 |
3 |
Is the Generator PWM display affected? | -- | Go to Step 5 | Go to Step 4 |
4 | Test the generator field duty cycle signal circuit for a short or open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 8 | Go to Step 6 |
5 | Inspect for poor connections at the harness connector of the generator. Refer to Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 8 | Go to Charging System Test |
6 | Inspect for poor connections at the harness connector of the PCM. Refer to Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 8 | Go to Step 7 |
7 | Replace the PCM. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming. Did you complete the repair? | -- | Go to Step 8 | -- |
8 |
Does the scan tool indicate that this DTC failed this ignition? | -- | Go to Step 2 | System OK |
