Diagnostic Instructions
DTC Descriptor
DTC P1637 : Generator L-Terminal Circuit
Circuit/System Description
The powertrain control module (PCM) uses the generator turn on signal circuit to control the load of the generator on the engine. A high side driver in the PCM applies a voltage to the voltage regulator. This signals the voltage regulator to turn the field
circuit ON and OFF. When the PCM turns ON the high side driver, the voltage regulator turns ON the field circuit. When the PCM turns OFF the high side driver, the voltage regulator turns OFF the field circuit.
The PCM monitors the state of the generator turn on signal circuit. The PCM should detect a low generator turn on signal circuit voltage when the ignition is ON and the engine is OFF, or when the charging system malfunctions. With the engine running, the
PCM should detect a high generator turn on signal circuit. The PCM performs ignition ON and RUN tests to determine the status of the generator turn on signal circuit. During the ignition ON test, if the PCM detects a high generator turn on signal circuit voltage,
DTC P1637 will set. DTC P1637 will also set if, during the RUN test, the PCM detects a low generator turn on signal circuit. When the DTC sets, the PCM will turn on the charge indicator in the instrument panel cluster (IPC).
Conditions for Running the DTC
Ignition ON Test
| • | No generator, crankshaft position (CKP) sensors, or camshaft position (CMP) sensor DTCs are set. |
| • | The ignition is in RUN position. |
| • | The engine is not running. |
RUN Test
| • | No generator, CKP sensors, CMP sensor DTCs are set. |
Conditions for Setting the DTC
During the ignition ON test, the PCM detects high voltage on the charge indicator control circuit for at least 5 seconds. OR during the RUN test the PCM detects low voltage on the charge indicator control circuit 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 history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
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 Testing
- Ignition OFF, test for B+ between the generator B+ output terminal (stud) to ground.
| ⇒ | If less than the specified value , test the circuit for a short to ground, an open or a high resistance. |
- Ignition OFF, disconnect the harness connector at the generator.
- Ignition OFF, test for B+ between the battery positive voltage circuit terminal D and ground.
| ⇒ | If less than the specified value , test the circuit for a short to ground, an open or a high resistance. |
- Engine Running, test for greater than 3.5 volts between the generator turn on signal circuit terminal B and ground.
| ⇒ | If less than the specified range, test the circuit for a short to ground, an open or a high resistance. If the circuit tests normal, replace the PCM. |
- If the circuit test normal, replace the generator.
Repair Instructions
Perform the
Diagnostic Repair Verification after completing the diagnostic procedure.
