GM Service Manual Online
For 1990-2009 cars only

When operating normally, the generator indicator lamp will come ON when the ignition switch is in the ON position and go out when the engine starts. If the lamp operates abnormally or if an undercharged or overcharged battery condition occurs, the following procedure may be used to diagnose the charging system. Remember that an undercharged battery is often caused by accessories being left ON overnight or by a defective switch that allows a lamp, such as a trunk or glove box lamp, to stay ON.

Important: Always check the generator for output before assuming that a grounded L terminal circuit has damaged the regulator. Refer to Generator Replacement .

Diagnose the generator with the following procedure:

  1. Visually check the belt and wiring.
  2. With the ignition switch in the ON position and the engine stopped, the charge indicator lamp should be ON. If not, detach the harness at the generator and ground the L terminal in the harness with a fused, 5-ampere jumper lead.
  3. • If the lamp lights, replace the generator. Refer to Generator Replacement .
    • If the lamp does not light, locate the open circuit between the ignition switch and the harness connector. The indicator lamp bulb may be burned out.
  4. With the ignition switch in the ON position and the engine running at moderate speed, the charge indicator lamp should be turned OFF. If not, detach the wiring harness at the generator.
  5. • If the lamp goes OFF, replace the generator. Refer to Generator Replacement .
    • If the lamp stays ON, check for a short to ground in the harness between the connector and the indicator lamp.