'77 240D W123 warning lamps out and not charging. - Mercedes-Benz Forum

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post #1 of 3 (permalink) Old 01-03-2013, 11:38 AM Thread Starter
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Date registered: Sep 2012
Vehicle: 1977 240D
Location: Michigan
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Unhappy '77 240D W123 warning lamps out and not charging.

My 77 240D (W123) is currently not charging, and none of the warning lights turn on for lamp test when I turn the key to any position. The glow plug light comes on, fyi. I know the warning lights were working not long ago but can't say when they stopped. I've been working on the car and this may complicate the diagnosis. I put a new battery in a few weeks ago because I knew it was old and as the temperature was dropping, it was cranking slower. Warning lights were still working at that time. A few days after replacing the battery, i replaced the voltage regulator, as I'd pulled it about a month previously and knew that the brushes were fairly worn down so I ordered a replacement.

I'm not sure if after replacing the regulator the warning lights still worked or not. I started the car but didn't check the alternator output. (crap!)

About 3 days later, (driving daily) I blew the cigar lighter fuse when my GPS charger cord shorted. I replaced the fuse but still didn't notice whether or not the warning lights were working or not. I drove the car the next day, but on the second morning following the blown fuse, the car cranked over very slowly and wouldn't start. I checked the voltage, after boosting the car and found the battery voltage was 11.91 and no increase with rpms.

I did some forum checking and found that the charging lamp was required to excite the alternator field so I replaced that, but still no charging voltage.

So, I currently have no charging from the alternator and the brake warning/charging lights fail to come in in self test, and the charging light doesn't come on either. Fuses are all good. Alternator belt is tight.
Any idea as to what may be wrong, or what i should check next?

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post #2 of 3 (permalink) Old 01-04-2013, 08:22 PM
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One thing that comes to mind is that your alternator is not charging. There is a wire from the ignition switch to the alternator (usually green, and a smaller guage) that provides battery voltage to the field windings of the alternator. If you don't have the voltage, the alternator will not create any voltage. People have seen this when the ignition switch is failing.

The switch assembly is 3 major parts, the lock core, the steering wheel lock mechanism, and the electrical portion of the switch that is plugged into the wiring harness. The switch can be replaced with a bit of care and a very short flat head screwdriver by removing the steering wheel and the instrument cluster. (Be glad you're not trying to do this on a 107 with big hands.) There are detailed procedures and even a video in the Encyclpedia Germanica v107 sticky of the 107 forum. Go over them first, as there are some things to watch out for when doing this job. Your process should be very similar to a 107, just with more room to work in.


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post #3 of 3 (permalink) Old 01-07-2013, 06:59 AM Thread Starter
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Date registered: Sep 2012
Vehicle: 1977 240D
Location: Michigan
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Thanks RadioTek! I found the write up in the 107 manual, but couldn't find the video you referred to. I did find a W123 online manual but it only shows a picture of the lock mechanism with no instructions on how to get at it, so the pointer you gave is much appreciated. I'm trying to figure out from the schematic just what is going on here, but it looks to me like your diagnosis of the switch being bad could well explain why there are no warning lights and no charging, since the alternator field would not be excited.

Thanks for your help! Now all I have to do is wait until the Michigan weather warms up enough to let me get into the guts of the matter!

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