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.

The engine is running.

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.

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

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

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

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.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.