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 ignition 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 5-100 percent. The PCM monitors the PWM signal using a ignition 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 turn ON the charge indicator in the instrument panel cluster (IPC).

The PCM detects a out of range PWM signal during the Ignition ON test the PCM detects, a PWM signal greater than 65 percent for at least 5 seconds or during the RUN test a PWM signal less than 5 percent for at least 15 seconds.

On vehicles equipped with a sense circuit connected to the voltage regulator. This circuit must have voltage for the charging system to operate properly. This circuit is used as a reference to control system voltage. All circuits must be tested at the generator before replacing any components. refer to system schematic to confirm if the vehicle being serviced uses the sense circuit as part of the charging system.

Schematic Reference

Starting and Charging Schematics

Connector End View Reference

Engine Electrical Connector End Views

Electrical Information Reference

Scan Tool Reference

With the scan tool installed, ignition ON and the engine running, observe the GEN-F Terminal Signal parameter in the PCM data list. The GEN-F Terminal Signal parameter should read between 5 and 95  percent.

1. If DTC  P1637 is also set diagnose that DTC first, refer to Diagnostic Aids.
2. Ignition OFF, disconnect the generator harness connector.
3. The ignition ON, engine OFF, connect a test lamp to battery positive voltage and repeatedly probe the generator field duty cycle circuit, harness side while monitoring the GEN-F Terminal Signal Parameter in the data list, it should change from less than 5   percent to above 95  percent.

⇒	If the GEN-F Terminal Signal parameter was not affected by the test lamp, test the GEN-F Terminal Signal circuit for a short to ground, a high resistance, an open circuit. If the circuit tests normal replace the PCM.

4. If the circuit tests normal during the Ignition ON / Run tests, and the PCM parameter changed from less than 5 percent to above 95  percent replace the generator.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.