Diagnostic Instructions
DTC Descriptor
DTC P1638: Generator F-Terminal Circuit
Circuit/System 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 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).
Conditions for Running the DTC
| • | No generator, crankshaft position (CKP) sensors, or camshaft position (CMP) sensor DTCs are set. |
| • | The Ignition is in the RUN position. |
| • | The engine is not running. |
Conditions for Setting the DTC
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.
Action Taken When the DTC Sets
| • | The PCM will command the charge indicator and or warning message to be illuminated on the instrument panel cluster (IPC) and the driver information center (DIC), if equipped. |
| • | The PCM will not illuminate the malfunction indicator lamp (MIL). |
| • | The PCM will store conditions, which were present when the DTC set as Fail Records data only. |
Conditions for Clearing the DTC
| • | The PCM will command the message OFF after one trip in which the diagnostic test has been run and passed. |
| • | The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction. |
| • | Using the scan tool Clear DTC Information function can clear the DTC. |
Diagnostic Aids
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.
Reference Information
Schematic Reference
Starting and Charging Schematics
Connector End View Reference
Component Connector End Views
Electrical Information Reference
Scan Tool Reference
Control Module References for scan tool information
Circuit/System Verification
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.
Circuit/System Testing
- If DTC P1637 is also set diagnose that DTC first, refer to Diagnostic Aids.
- Ignition OFF, disconnect the generator harness connector.
- 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. |
- 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.
Repair Instructions
Perform the
Diagnostic Repair Verification after completing the diagnostic procedure.
