It's common for the brushes to wear in the alternator but the fact the voltage rises above 12.6 volts proves that isn't the problem. It sounds more like there may be a shorted diode inside the alternator. That will cause it to be able to supply only very near one third of its rated capacity, but more importantly, you will lose one of the three phases of output current. That will cause a lot of ripple which can interfere with sensor readings and computer operation. You can prove that by charging the battery first, then starting the engine, and while it's running poorly, disconnect the small wires on the back of the alternator. That's easy to do if they plug in; not so easy when they go through the black plastic block that bolts on with two small tabs. For that type, I unbolt them first, reconnect them with clip leads, then it's easy to unhook one of them while the engine is running. You can connect them in any order, and there won't be any voltage on them to worry about when the engine isn't running.
If the running problem clears up when the field wires are disconnected, have a load test performed on the alternator. Most commercial testers will also measure ripple. If it's high, and you can only get around 30 amps out of the unit, suspect a bad diode.
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Saturday, May 21st, 2011 AT 7:03 AM