Hello Peach Parts Forum! I'm new here and new to both Mercedes and diesels. FWIW, I've been building classic cars for about 15 years now. I'm handy with engine and transmission building, rear ends, 12v electrical, fabrication, etc.

I just purchased my first Mercedes, a 1984 300SD for $400, and I'm stoked about it! The car runs, drives, and stops well at this point. The previous owner claims everything worked great until recently when it developed some electrical issues. Currently, no interior lights, gauge cluster lights, sunroof, oil pressure gauge, A/C, cruise control, or tachometer work. I'll work through those little by little over time.

But the issue I'm most concerned about currently is the tail lights. Headlights, turn signals, and brake lights work fine, but when the headlights are turned on, the tail light fuses pop immediately. Obviously there's a short somewhere, and I can start at the switch and probe my way through the car and out to the front and rear until I find it, but before I start tearing things apart, perhaps someone here has a few suggestions based on common issues that I can focus my efforts on first. Hoping for a simple fix of a common issue!

Thanks in advance, friends!

-Josh in Oceanside, CA

I have not read of this being problem that comes up often on any of the older Mercedes. If it was me I would inspect the Bulb sockets and wiring.

I worked as a mechanic for a school district for a few months and worked on 3 different cares that had been blowing fuses and I missed connecting one of the signs to the problem.
One of the Turn Signal Lights had Water in it. More then condensation would have allowed.
After that job I went back to the place I had previously worked and was telling my Old Boss about the electrical issues. He had been hired to once in awhile trouble shoot vehicles over at an Oil Company. He told me the Cars provide by the Oil Company for employees were having similar issues and it was because the employees were required to keep the vehicles looking presentable and used a Hose with a nozzle to wash off the Cars.
The pressurized water was forcing in past old tail light and other seals.

If the Trunk lining is missing it could be something in the trunk did in one of more of the Wires.

Concerning the Instrument Cluster somewhere behind it is ground point that often needs attention.
There used to be some free online manual sites where you might be able to bring up the wiring diagrams.
See if this online manual works for you:

http://www.startekinfo.com/StarTek/outside/12253/?requestedDocId=12253

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84 300D, 82 Volvo 244Gl Diesel

Josh,


Welcome and congrats on the SD.
A few general comments about the electrical gremlins.

Clean and tighten every ground you can find on the car. Other than the black negative at the battery, the grounds will be brown and there should be a braided strap from the block to the chassis.

Remove every fuse, clean the contacts or replace the fuses,

It is common to find the wrong bulbs e.g., 1156 and 1157's in these cars. The US bulbs burn the lenses and drive the cruise control out of its mind.

Re: the short blowing fuses, I'd pull the rear bulbs and with the headlight switch off test for short to ground on the positive side of the bulb, if yes disconnect the switch and see if the circuit opens. If no shorted wire, if yes bad switch.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Welcome Josh! I see you're in Oceanside, I am down in North Park with a W123. Congrats on your new car, neighbor!

I purchased my 300D in Encinitas a few months back, the PO said the window and headlight fuses would pop often. I did a deep clean on my fuse box, and that seemed to fix the issue. (even though I didn't see excessive corrosion beforehand)

You're in luck Josh ~ there's a Pick-A-Part junkyard in Ocea side, it almost always has several old Mercedes' like yours .

Agreed, pull all the taillights and see if the short is gone .

You can make a simple short detector by soldering a horn or beeper onto an old fuse ~ insert in the fuse box, energize the circuit and begin looking ~ every time the circuit goes to ground the noise maker will sound .

Most often this is bad bulbs or water in the MPC .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

Nate...great idea!!! A horn or beeper. I've used a test light which draws attention away from being able to focus on the problem location/short.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer