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Old 10-07-2012, 02:01 AM
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Join Date: Oct 2012
Location: East Texas
Posts: 1
Alt Warning Lamp Failure Bypass? 86 W124 300E

As everyone knows when your Alt warning light fails to come on, in the key on position, your alternator will not charge. I have this fault 90% of the time the warning lights will not light and I'll have a no charge condition. But every once and awhile I'll have the warning lamps light as normal and the alternator will charge as it should. Now I have two questions:
1) What should I be looking for as a fault. I have checked the small wire from the alternator to the connection block by the battery (x35) contuinity is good. Next I'll check the wire from the connection block to the #9 connection on the instrument panel connector. Any common faults to look for?
2) Is there any way to hot wire this intermittent connection to get the alternator to charge. I tried connecting the warning lamp wire directly to battery voltage but did not get a postive result i.e. no alternator output. Shouldn't it be possible to bypass the instrument cluster temporarily somehow?

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Old 10-07-2012, 11:10 AM
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Join Date: May 2011
Location: NYC
Posts: 5,506
Yeah, wire a 3W lamp via a fuse between positive battery voltage and the lamp terminal on the alternator.
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Old 10-07-2012, 02:53 PM
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Join Date: Jun 2012
Location: Of
Posts: 43
Welcome to the forum, Jeepingjack! You might want to post this question in the Tech Help section of this forum for additional responses!
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Old 10-08-2012, 11:41 AM
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Join Date: Jun 2012
Posts: 228
The problem with the warning light is usually a bad diode bridge from my own experience. Yes, you can bypass it like the above post mentions. But the diode bridge can be replaced if you want it to work as it should. It's not that easy and even if i've fixed hundreds of them, i would still exchange it for a rebuilt unit at your local parts store. (got spoiled)
The light is used to excite the generator and it has to go off when voltage is supplied by the generator. So you'd have to manually switch the light off after the voltage is provided.
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Old 10-10-2012, 05:19 PM
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Join Date: Apr 2010
Posts: 43
300e alternator issue

i have a 91 300e and have had ( as others have i'm sure )an alternator stop charging. The voltage regulator and brushes are a replaceable assembly mounted to the alternator - look up voltage regulator in the parts section to see what it looks like -when the brushes wear out the alternator stops charging and the dash light doesnt come on because the brushes are part of the circuit that controls the light. The warning you will get is when the voltage drops to something like10 volts the abs and airbag light come on and accessories get dim or slow. If you keerp driving the engine will die when there is not sufficient voltage. The part is easy to replace-you can even do it with the alternator still on the car

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