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Old 10-16-2014, 01:30 AM
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Join Date: Oct 2014
Posts: 3
1976 240D alternator charging occasionally

Hey all. I know I've seen similar topics on this before about alternators on this particular model not charging, but I've seen nothing like what I'm experiencing.

I've got a 1976 240D 115 chassis, brand new Bosch alternator. I had intermittent connection issues with it before, and that was from the engine shaking lose the connection to the alternator, therefore making the "battery not charging" light FLICKER on and off. I fixed that. NOW however, my light comes on after I start it, but FADES as RPM in the engine increase. Sometimes, if RPMs hit a certain height, it'll "stick" and the light will stay off for a while, which means it's charging like it should. But, low and behold, it will turn back on and do the fade thing in relation to engine RPM again.

I've tested my battery, being a new battery, and it holds a good charge. I've measured the voltage after starting the engine, and when the light is OFF, which means charging, it's an expected ~14V. Whenever the light is on, it's at ~12.6V, which means not charging. I have no idea what's going on here, I've tried damn near everything. I've tightened the belt, it's never squealed once, when I look at the alternator, I can see it spinning fine.

The ONE thing that may be an issue that I'm just not understanding could affect anything: there is a small crack on the alternator bracket on the side where the teeth meet the star bolt tightening system thingy. It's not NEAR the teeth at all just on the same side, it's where the bracket is bolted and can pivot up and down off of the block. I personally didn't think this could cause what is going on, but hey, I'm here asking for help now. What do you think? Simple belt tension stuff due to the crack? Or electrical stuff being weird? Thanks in advance!!

Last edited by RoachTaylor08; 10-16-2014 at 01:31 AM. Reason: changed on to off
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Old 10-16-2014, 02:04 AM
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Join Date: Apr 2012
Location: San Luis Obispo, CA
Posts: 1,512
It sounds electrical to me, not mechanical. I had similar problems in my w115 and it was the solder connections at the alternator plug. I had to resolder those wires and it resolved the problem. So perhaps unplug your alternator and take apart the plug. On my older version of your car, there was just a small back cover on the plug which unclips, and then the three soldered connections are easily visible. Not sure if your car would have the same alternator connector though.
1968 220D, W115, /8, OM615, Automatic transmission.
1987 300TD, W124, OM603, Automatic transmission.
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Old 10-16-2014, 08:35 AM
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Join Date: Apr 2005
Location: Lafayette Indiana
Posts: 33,450
sounds like worn brushes.
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual. It still needs upholstery redone...I also have a 427 Cobra replica with an aluminum chassis.
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Old 11-25-2014, 03:33 PM
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Join Date: Oct 2014
Posts: 3
Ended up being a half worn down voltage regulator. Thanks for your input!
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