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Old 03-27-2013, 09:05 PM
Registered User
Join Date: Mar 2013
Posts: 379
Electrical Challenges


I am new to this forum. I did some rough searching through the forums and could not find an answer to my challenges. I own a W116 1980 300SD MB Turbo Diesel (love her)

I am currently working on squaring away the electrical system before I tackle some other areas. After reconnecting all the wires and grounds I could find (previous 2nd owner decided to become a fly by night electrician) he had tied wires together, allowing for green corrosion build up on the connections ( have cleaned it off) before I took about repairing his jigger rigged setup my lights worked as followed:
The only way my brake lights work now is if I have my light switch turned to the first or second click to the right, other wise only my left headlight turns on with the first click to the right (explained below) as well as the front passenger right and rear passenger left windows only work with the light switched turned to the first or second click to the right and only the rear passenger left window will go up, the front passenger window only will go down and wont go up (I took the driver switch and flipped it to fit the passenger window to get the window rolled up and it worked.)

My light systems works as follows

Put key in ignition and turn to accessory, left headlight goes on, turn car on left headlight stays on. Turn first click of light dial (while car is running, off or accessory) to the right and left headlight, left running light (inside turn signal housing) and rear backup?(middle light) light go on.
Turn the light dial two clicks to the right Both headlights go on, left running light (inside turn signal housing) and rear backup?(middle light) light go on.

My instrument cluster lights and climate control lights, license plate lights, right running light (inside turn signal housing) and rear backup?(middle light), as well as the rear side running light do not go on. ( I previously had all these lights working before I removed the jigger rigged setup)

My brights, highs, and brake lights work without problem, none of my fuses are blown.

I am missing a blub and a housing + blub for my fog lights so I can not test those. There was previously a wire connecting the power for both of these lights together, I removed that due to massive corrosion and clipped the wire and reconnected them.

I believe that is all for my electrical, if anyone has an suggestion or advice please let me know, I'll do my best to offer my experiences for others.

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Old 04-01-2014, 03:04 AM
gearhead counselor
Join Date: Feb 2012
Location: Los Angeles
Posts: 18
You absolutely need a wire diagram. Someone told me that the factory manual is posted on Mercedes' website now, or something like that. I'm pretty sure you can download it for free - I bought mine for 50 bucks 5 years ago before it became available.

Just trace all the circuits, and double check where all of the physical locations are (the diagram will tell you), and start from there. It will be tedious, but methodical work and patience will yield great results. I'm not sure if you are an electrical whiz, but I most certainly am not; but with a diagram, it's totally doable.
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Old 04-01-2014, 07:19 PM
Registered User
Join Date: Jun 2004
Location: West of Ft. Worth. TX
Posts: 4,186
Here is a link to the FSM for the W126. It is likely to be very similar in that area.

Mercedes Service Manual Source, CD/DVD/Paper

84 300SD 350K+ miles ( Blue Belle )
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Old 04-01-2014, 08:31 PM
Registered User
Join Date: Jul 2008
Posts: 3,740
This will also work, you have a om617 turbo....go through the wiring diagram.

Model 123 Maintenance Manual Index

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