For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• | Diagnostic Procedure Instructions provides an overview of each diagnostic category.

### DTC Descriptor

DTC 1668: : Generator L-Terminal Control Circuit
DTC 2500: : Generator L-Terminal Circuit Low Voltage
DTC 2501: : Generator L-Terminal Circuit High Voltage

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
Generator L-Terminal Circuit | P2500 | P2500 | P2501 | P1668

### Circuit/System Description

The engine control module (ECM) uses the alternator turn ON signal circuit to control the load of the alternator on the engine. A high side driver in the ECM applies a voltage to the voltage regulator. This signals the voltage regulator to turn the field circuit ON and OFF. The ECM monitors the state of the alternator turn ON signal circuit. The ECM should detect low voltage on alternator turn on signal circuit when the ignition is ON and the engine is OFF or when the charging system malfunctions. With the engine running, the ECM should detect high voltage on the alternator turn on signal circuit. The ECM performs key ON and RUN tests to determine the status of the alternator turn on signal circuit.

### Conditions for Running the DTC

The engine is running.

### Conditions for Setting the DTC

With the engine running, the ECM detects low/high compared to a predetermined voltage on the alternator turn on signal circuit for at least 15 seconds.

### Action Taken When the DTC Sets

DTCs P1668, P2500 or P2501 are type C store only DTCs.

### Conditions for Clearing the DTC

DTCs P1668, P2500 or P2501 are type C store only DTCs.

### Reference Information

#### Schematic Reference

Starting and Charging Schematics

#### Connector End View Reference

Engine Electrical Connector End Views

#### Description and Operation

Charging System Description and Operation

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

### Circuit/System Testing

1. With ignition ON and the engine OFF, disconnect the alternator harness at the alternator and measure the voltage of the alternator turn on signal circuit should read near 0 volts.

⇒ | If greater than 0 volts test the alternator turn on signal circuit for a short to voltage. If circuit tests normal replace the ECM.

2. With ignition ON and the engine running, the voltage of the alternator turn on signal circuit should read over 3.5 volts.

⇒ | If no voltage is measured, test the alternator turn on signal circuit for a short to ground an open/high resistance. If circuit tests normal replace the ECM.

3. If all circuits test normal, refer to Testing for Intermittent Conditions and Poor Connections.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.