Circuit Description
The 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, DTC P1638 will set. When the DTC sets, the PCM will send a class 2 serial data message to the IPC to illuminate the charge indicator.
Conditions for Running the DTC
Key ON Test
| • | No generator, CKP sensors, or CMP sensor DTCs are set. |
| • | The key is in the RUN position. |
| • | The engine is not running. |
Run Test
| • | No generator, CKP sensors, or CMP sensor DTCs are set. |
| • | The engine is less than 3,000 RPM. |
Conditions for Setting the DTC
| • | During the ignition ON test, the PCM detects a PWM signal is out of range. |
| • | During the RUN test, the PCM detects a PWM signal less then 5 percent for more than 6 seconds. |
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. |
Diagnostic Aids
| • | DTCs P1637 and P1638 may set at the same time depending on the failure. |
| • | Use a scan tool in order to diagnosis the generator circuits. The scan tool should display Inactive for the L terminal and 10-40 percent for the F terminal when the ignition is ON. When the engine is operating, the display should indicate that the L terminal is Active and the F terminal is more than 10 percent. |
| • | A generator fault, such as a shorted output diode, malfunctioning regulator, open or shorted rotor, or open sense lead or battery feed may cause a low battery charge. Refer to Charging System Test for further information and generator diagnosis. |
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Engine Electrical Diagnostic System Check? | -- | Go to Step 2 | |
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 |
Important: The replacement PCM must be programmed. Replace the PCM. Refer to Powertrain Control Module Replacement in Engine Controls-4.8L, 5.3L, and 6.0L. Is the action complete? | -- | Go to Step 8 | -- |
8 |
Does the scan tool indicate that this DTC failed this ignition? | -- | Go to Step 2 | System OK |
