Idiot light. - Mercedes-Benz Forum
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post #1 of 11 (permalink) Old 04-11-2019, 04:46 PM Thread Starter
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Idiot light.

The battery idiot light on my 88 SEC comes on when the car is turned off, not on when it's running. My brother immediately said on American cars it is almost always a diode gone bad in the alternator, any ideas gentlemen?
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post #2 of 11 (permalink) Old 04-11-2019, 11:07 PM
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Correct, your alternator is faulty.

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post #3 of 11 (permalink) Old 04-13-2019, 03:54 AM
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you are lucky to get the light. Take it to local auto store (the alternator) and have it tested. Often the battery is drained so bad there is no juice to tell you via the light.


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post #4 of 11 (permalink) Old 04-13-2019, 08:53 AM
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Most Bosch alternators of the era have what are called exciter diodes that feed the light. [s]I recall they are in the voltage regulator/brush block. Worth trying...[/s] They're in the alternator body itself. It's a common failure. We used to make up a diode pack from 3 1n4004 diodes and replace it without overhauling the whole thing. (on the alternators that were not internally regulated.

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Last edited by John350; 04-13-2019 at 12:08 PM. Reason: I was wrong...
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post #5 of 11 (permalink) Old 04-13-2019, 03:28 PM
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I dont beleive any of that, yes when the key is on and car is not running battery light will be on.
The battery light has Battery positive to one side all the time, and when it is running battery positive to the other side. When it stops charging, alternator not spinning or no output it supplies ground to the light to illuminate it. That being said, check your alternator to be sure.
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post #6 of 11 (permalink) Old 04-13-2019, 05:33 PM
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Originally Posted by 29Hersey View Post
The battery light has Battery positive to one side all the time,
Yes, and the exciter diode pack keeps the battery from discharging through the idiot light when the key is not supplying 12 volts to the other side. Otherwise current will flow from the battery through the light and then through anything else connected to that circuit on the ignition switch, to ground.

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post #7 of 11 (permalink) Old 04-14-2019, 06:08 AM
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My bad... Idiot light on when key off indicates a leaky B+ diode. A false discharge indication when the engine is running idndicates a leaky exciter diode. Chalk it up to oldtimers disease...

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post #8 of 11 (permalink) Old 04-14-2019, 10:13 AM Thread Starter
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I ordered a new alternator today will most likely install it tomorrow and I'll post an update.
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post #9 of 11 (permalink) Old Yesterday, 02:06 AM
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Hope you got a good one, the quality of everything has gone downhill these days. Bosch is still good but not necessarily as good as it was. MB Bosch was even better, but even MB parts are not what they were. (Still the best though I think.)

With another car I have resorted to using a great little rebuild place, I even get old dead units from club members to get rebuilt rather than buy new.

I would recommend having your original rebuilt as a spare perhaps.

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post #10 of 11 (permalink) Old Yesterday, 05:35 PM Thread Starter
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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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