Here's a copy of a reply I just posted for someone else with the same problem.
Unplug the regulator and run the engine to see if it still overcharges. If it does not charge at all, the regulator is shorted.
If it still is overcharging, the green wire between the alternator and regulator is grounded. Unplug the green wire from the alternator and try it. If it's still overcharging, look at that terminals on the alternator. You'll see that the other terminal has a fiber washer under the screw holding the terminal / brush onto the alternator case. The terminal where the green wire came from has one of two possible problems. If your alternator is from a 1969 or older vehicle, there isn't actually a second terminal. That brush is grounded by the mounting bolt and will not work on a 1970 or newer system.
The second, more likely possibility is that instead of a fiber insulating washer, there is a metal washer under the screw holding the brush and terminal where the green wire went to. That metal washer allows the 1970 and newer alternator to be used on 1969 and older vehicles. You simply need to remove that metal washer and install a fiber one. You'll find a bazillion of them in the salvage yards. It's the metal ones that are hard to find.
Since you're smart enough to use a voltmeter, I'm betting you know how to use an ohm meter too. When both terminals have the fiber washers properly installed, your ohm meter will read about 4 ohms between the two terminals, If not, turn the pulley just a little to irritate the brushes. It's common to have to do that to get a good reading when it's not spinning. Next, measure from either terminal to the case of the alternator. There must be an open circuit. When these proper measurements are found, you can plug in the blue and green wires on either terminal. Switching them will have no effect on alternator operation.
This is by far the world's simplest, most reliable, and easiest to understand charging system. Feel free to ask any questions about it.
Tuesday, December 8th, 2009 AT 5:37 AM