Jeep

The alternator has to tested while the light is on indicating a problem is present, not when the light is off. Worn brushes are real common and cause intermittent operation of the alternator. To test the system, when the light is on and the engine is running, use an inexpensive digital voltmeter to measure the battery voltage. It must be between 13.75 and 14.75 volts. If it is closer to 12.6 volts, measure the voltages on the two smaller wires on the back of the alternator. This MUST be done with the engine running, not just with the ignition switch turned on. One will have full battery voltage. The other one must have less but not 0 volts. 0 volts indicates worn brushes.