Recharge the battery on a slow rate for an hour or two, then use an inexpensive digital voltmeter to measure its voltage. If it's near 12.6 volts, it is ok. Measure again with the engine running. It must be between 13.75 and 14.75 volts. If it is low, dig down to the back of the alternator and measure the two voltages on the two small wires that are plugged in or bolted to the back. The engine must be running for these tests.
One terminal will have full battery voltage. The second one should be less, but if you find 0 volts, the brushes are worn. They can often be replaced without removing the alternator from the engine.
Saturday, December 4th, 2010 AT 7:41 PM