First verify it really is over-charging. Use an inexpensive digital voltmeter to measure battery voltage while the engine is running. It should be between 13.75 and 14.75 volts. If it is higher than 15.0 volts, the problem is with the voltage regulator circuit, not the alternator. He is just doing what he is told to do.
Measure the voltages on the two small terminals on the back of the alternator while the engine is running. One will have full battery voltage. The other one is the secret. It must be lower but not 0 volts. If you find 2 - 6 volts, the voltage regulator inside the Engine Computer is shorted. It is not possible for it to draw that voltage all the way down to 0 volts.
If you find 0 volts on that second terminal, the green wire between the alternator and computer is grounded.
Monday, April 11th, 2011 AT 9:38 PM