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Old 05-17-2016, 02:59 PM
Diseasel300's Avatar
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'86 300SDL Popping Fuse 11

I'd have this problem licked already if MB supplied proper wiring diagrams, however so far I've been unable to find a circuit-based wiring diagram for this car.

I know that the seat belt relay is bad (had courtesy light wires slammed in door) with all the usual symptoms of a failed relay, however all of the lighting is working normally from the passenger door, rear doors, and trunk lid.

Fuse box says Fuse # 11 controls Parking/Tail light right, warning buzzer, headlamp cleaning unit, illumination, license plate, instrument control elements.

Fuse #11 is fine, until I switch on the headlight switch, then it immediately blows, I can confirm a short to ground probing the fuse box with the headlight switch turned on. So far though, I haven't found the actual short!

I have the FSM on CD, but I'm not finding what I need there.

Is it possible for the seatbelt relay to be the culprit? I haven't pulled it yet, but it seems to be the only common link. I haven't read of anyone else having it cause a short or pop a fuse though. Before the wires were slammed in the door, I didn't have this issue, but a lot of other work has also been done to the car (window regulators, seat controls, etc), but nothing related to this #11 circuit.

I have disconnected the tail light assemblies and the license plate light wire harness. No change. I've also disconnected (removed) the warning light module in the fuse box with no change (supposedly this is a common failure point).

Has anyone else ever had this type of issue? Know where to find a better wiring diagram? Things to check?

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Old 05-31-2016, 10:45 PM
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I figured I'd follow up with this issue in case anyone in the future ever has the same problem. I managed to get my hands on an electronic copy of the ETM (Electrical Troubleshooting Manual) for the W126 and it provided all the diagrams I needed to isolate and find the source of the mysterious Fuse 11 popping.

After studying the diagrams and figuring out what Fuse 11 controls, I was able to rule out the interior and instrumentation lighting since the fuse did not blow with the light switch in "L" standing mode, but did in "R", "Park" and "Head" positions (see diagrams for switching arrangements). This led me to narrow it down to wiring or bulb holder in the right front parking light or right rear parking light. (Circuit 58R). Pulling the plugs and probing the connectors showed the wiring (grey wire) coming up to the front right parking light is grounded. I later traced this to a chafed wire behind the false firewall.

I've colored the diagrams and am attaching them to this reply in case it ever helps someone else out in the future. Very frustrating!

Last edited by Diseasel300; 06-19-2021 at 10:06 PM.
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Old 05-31-2016, 11:19 PM
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Thanks for the follow up!

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