One fog light?!?

[Phillytwotank]

Howdy,

Been working on “restoring” my ‘92 300D. I’ve recently come to realize that the right side fog light is not getting power at the plug and the light is not working. Left side fog works as it should. I’ve tried a couple things with no luck and I’ve done a bit of searching but I haven’t found anything very helpful.
Anybody have any ideas on what to check? Thanks

[87tdwagen]

Trace the wires from the fog lamp receptacle to the lighting harness plug end and keep going until you find power again. Check for breaks in the connection point along the way.

Good luck

[Phillytwotank]

I’m at the lighting harness plug on the left side. No power there. Looks like it comes arround from the other side along the front?

[jsb357]

check the grounding post near the RH headlight make sure it's clean and tight.

when I got my w123 I had a weird headlight issue.

turns out at some point someone took apart the
LH head light connector and the wiring
order was messed up causing MY problem

[vstech]

The 124 lighting and much electrical passes through the junction area under the dashboard. It’s a relay and connector panel iirc…

[jay_bob]

Actually *every* exterior light bulb passes independently through the bulb out detector relay. Only exception is the turn signal lamps, they come off the turn signal switch, there is a line to the detector relay from the turn flasher, that looks for the 'fast flash' as an indication you have a bad turn signal. But every other filament has a dedicated line back to that relay.

This is the extra large relay in the control box behind the fuses. They are known to develop internal bad connections as the current for each lamp has to go in and out of the relay and through a sensing shunt inside.

Had a intermittent marker light failure on my 124 that was caused by this relay. Drove me nuts until I figured it out.

Do you have the schematics for a 124?

[Phillytwotank]

Thanks for the replies. JB, I do not have a great resource for the wiring diagram since w-124zone moved. I’ll have to try to find the new host again. As far as the status of my “lighting module”, I did have a constantly out running light issue that cleared up with a salvage yard replacement. Running light came on but I think that’s when the one fog light issue started. I took apart the original one and checked for continuity between the pins and the board. Found a few suspects and re soldered a few of the joints. Don’t know how to check anything there beyond that small part of the circuit. So, the replacement has a fog light problem and the original has a side marker light problem. I may try to find a third one unless I can Lear more about how they work and attempt another repair.

[Phillytwotank]

I hadn’t mentioned yet that I just replaced my headlight assemblies with some euro depot units. I was waiting for the proper 6 hole plugs to arrive so I could get them wired up and loose the dinky pigtail adapters. I am certain the new lamp housings are not at fault for the right fog in-op. It was out before the new stuff. I verified again that there is no power to the pin in the plug that goes for the fog. Left side works. I don’t really want to throw out $50 for another used light module but at this point I suspect it’s that. I’m guessing the other end of wire from the right side plug is in the relay box, in the plugs underneath the lighting module?
Can anybody confirm or dispel my assumptions on that?
Anybody have a known good spare lighting module they want to part with?
Anybody have a schematic or a resource for one that could help me??

[jay_bob]

See if this helps, I made this running down the issue with my wagon when I lost a marker light. I reverse engineered the schematics to figure out what each pin did.

[vstech]

I do have several w124’s to play with… gimme a call and we can test the one in my barn.