P1638

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 3000 RPM.

CONDITIONS FOR SETTING THE DTC
^ During the ignition ON test, the PCM detects a PWM signal is between 10-40 percent for more than 6 seconds.
^ 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. Component Tests and General Diagnostics

TEST

Steps 1-4:

Steps 5-11:

Steps 1-3 (Applies For Year 2002):

Steps 4-8 (Applies For Year 2002):