Sounds like a common problem with a simple fix.
Suspect worn brushes in the alternator. The trick is to measure system voltage when the problem occurs. You'll need a cheap digital voltmeter and you'll need to know how to use it. An easy test point is the center terminal of the cigarette lighter socket.
A better choice, if you can get it to act up long enough, is to measure the voltage right between the two battery terminals. When the engine is running, the voltage must be between 13.75 and 14.75 volts. With worn brushes in the alternator, the battery voltage will stay near 12.0 volts or less. If that's what you find, the next step is to measure the voltage on the two small wires bolted to the back of the alternator. One will have full system voltage. The other one is supposed to have less than system voltage, but never 0 volts. These voltages will only be there when the engine is running, not just with the ignition switch turned on.
If you find 0 volts on the second terminal, the brushes are worn. Replace them or replace the entire alternator. It tested fine earlier because this problem always starts out being intermittent, and it was in the good state when it was tested.
If you find the exact same voltage on both wires, or roughly 4 - 11 volts on the second wire, (which is normal), when the battery voltage is below 13.75 volts, there is a different, much less common problem that will require additional tests.
The brush assembly can be replaced without removing the entire alternator from the engine, but it's hard to describe the procedure. If you can find the replacement part from a local rebuilder or parts store, I'll make an attempt at telling you how to do it. Otherwise, just replace the alternator.
Wednesday, February 24th, 2010 AT 6:49 AM