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Topic: Idiot light.
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post #10 of (permalink) Old 04-16-2019, 05:35 PM Thread Starter
bflobob8
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Date registered: Jun 2012
Vehicle: 1991 560 SEC Black on Grey
Location: Leesburg, Va. 20175
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Update: Got an education on alternators today. I took out the original alternator and put in the new one and the light was not on anymore. I switched the old one back in and the light was back on with the key out so that's figured out, bad alternator. I took it to a rebuild shop that is run by this guy that has to be 70+. What a trip, you don't find places like this around here anymore. He was from Greece I believe and said he had been at the same one man shop for 30 years and had a masters from Europe in Auto electronics, by the look of his shop he hadn't thrown out a part or swept the floor or cleaned off a surface the entire time he was there. There were what had to be a couple thousand starters and alternators and whatnot everywhere you looked. Anyways he put in it a homemade contraption hooked up to what I think was about a 70s occiloscope and said after two seconds it had a bad diode. Took it to another tester and said one of the positive diodes was bad and the stator was distressed as the windings ? were different colors which he said meant a tough load at low RPM such as idling for extended periods and could also be from sitting for extended periods and crappy not fully charged batteries or a combination of things. He said the problem with the light was very unusual at least in his experience and said the bad diode had to have failed in a closed position ? to let the current flow both ways. I'm not 100% sure about that last part as my brain was glazed over by all the schematics he was drawing and the explanation was a bit over my head.
He said these alternators are about as simple and reliable as they come and really aren't worth rebuilding as he could sell me a new Bosch with the same components he would put in if he rebuilt it for $100 and it should last another 30 years. He showed me that he pays $68 for them.
Anyways so that answers the question, "why is my battery light on when the key is out"?
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