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If the light is on Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Wed, 18 May 2005 12:28:59 In Reply to: I like your attitude: always look for the obvious,, Thomas Bielecki, Tue, 17 May 2005 11:55:27 Members do not see ads below this line. - Help Keep This Site Online - Signup |
The 12.6 volts is the normal, unloaded battery voltage. It drops down to 12.2 when the engine is running because the battery is supplying the electrical needs of the car, so the voltage drops. The alternator isn't supplying any current.
Two things come to mind-
(1) the thin green (sometimes yellow) wire to the back of the alternator is shorted to ground. This would cause the light to be on all the time, AND for the alternator not to work. Simple test - remove the wire from the connection on the back of the alternator - if the light goes out, you know it isn't shorted. Make sure it is attached to the right place.
(2) blown diode inside the alternator. Doesn't happen often, but it happens. You can drop in new VRs until the cows come home and it won't help. About the only way to be sure is to stick an oscilloscope on the alternator output - it should look like a series of connected humps. If one hump is missing - there's a gap - one of the diodes is bad. Theoretically you can remove the alternator, open it up, and replace the diode. For that amount of effort, just slap in a rebuilt one. What? You don't have an oscilloscope handy? Most automotive shops and many autoparts stores can stick a charging system tester on and diagnose a bad diode.
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