P1638

Circuit Description
The generator has an input to the PCM called the F Terminal to indicate the percentage of total capacity that the generator is producing. This signal is detected by the PCM as a duty cycle from the generator and displayed on the scan tool as a percentage. The PCM can monitor the generators output under all conditions to determine if it is functioning normally.
When there is low demand from the electrical system on the generator, a low duty cycle percentage will be displayed. As more accessory load is placed on the generator, the duty cycle output detected by the PCM will approach 100%. A normally functioning generating system will never reach 100% as indicated on the scan tool.

Conditions for Running the DTC
The ignition switch in the ON position or the engine is operating.

Conditions for Setting the DTC
^ The F Terminal indicates a duty cycle not between 10 and 40% with the ignition ON and the engine OFF or the duty cycle is less than 5% 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 Malfunction Indicator Lamp (MIL) will not illuminate.
^ The PCM records the operating conditions at the time the diagnostic fails. The PCM stores this information in the Failure Records.
^ A message is displayed on the Drivers Information Center.

Conditions for Clearing the MIL/DTC
^ A History DTC will clear after forty consecutive warm-up cycles, if no failures are reported by this or any other non-emission related diagnostic.
^ A last test failed (Current DTC) will clear when the diagnostic runs and does not fail.
^ Use a scan tool in order to clear the MIL/DTC.

Diagnostic Aids

Important:
^ Remove any debris from the PCM/TAC module connector surfaces before servicing the PCM/TAC module. Inspect the PCM/TAC module connector gaskets when diagnosing/replacing the modules. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion into the PCM/TAC modules.
^ For any test that requires probing the PCM or a component harness connector, use the Connector Test Adapter Kit J 35616-A. Using this kit prevents damage to the harness/component terminals.
^ The following may cause an intermittent:
- Poor Connections
- A mis-routed harness
- A rubbed through wire insulation
- A broken wire inside the insulation
^ DTCs P1637 and P1638 may set at the same time depending on the failure. Also, the L-terminal circuit can cause a DTC P1638 to set. This diagnostic table diagnoses both DTCs P1637 and P1638.
^ 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 greater than 5 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.
^ Using the Freeze Frame and/or Failure Records data may aid in locating an intermittent condition. If you cannot duplicate the DTC, the information included in the Freeze Frame and/or Failure Records data can help determine how many miles since the DTC set. The Fail Counter and Pass Counter can also help determine how many ignition cycles the diagnostic reported a pass and/or a fail. Operate the vehicle within the same freeze frame conditions (RPM, load, vehicle speed, temperature etc.) that you observed. This isolates when the DTC failed.
^ For an intermittent condition, refer to Symptoms. Symptom Related Diagnostic Procedures

Test Description
The numbers below refer to the step numbers on the Diagnostic Table.
2. This step verifies the fault is current.
4. Inspect the circuit for an open or a short to ground if the test lamp did not illuminate.
5. If the scan tool displays Active, this indicates the circuit is open.
6. This step tests the F terminal for being shorted to a voltage. With the ignition ON, the F terminal should display between 10-40 percent. If the F terminal displays greater than 40 percent, inspect the circuit for a short to voltage.
7. When the engine is started the L terminal should be active.
8. This step is testing the L terminal for an open. If the L terminal displays Active when the circuit is grounded, this indicates the circuit is open.
10. This step is testing the F terminal circuit for an open or a short to ground. If the F terminal parameter goes to 99 percent the condition is within the generator.