Intermittent headlight problem (and its not the bulb or fuse)
 

Hi, I have a 1987 300TD wagon and the right headlight and the left corner running light are not working. If you switch out the headlight bulbs for each other, the problem stays in the same place, indicating it is not a bulb issue. I put a jumper wire across the fuse (14, I think, but I forget the number), and that didn't change anything. So the fuse is not the problem.

The brights work on both side of the car, so electricity is getting to the bright filaments, but just not the dip headlight filaments.

If the entire circuit is not getting electricity, where would I look to try to find the problem? I checked the voltages at each fuse relative to the block, and there was not voltage at that fuse (again, 14, I think). Also, why would a corner light and a headlight not work at the same time. That seems odd, since I think they are supposed to be on separate circuits.

This problem is intermittent. It seems to always occur at night or in the rain, and then when I try to solve the problem during the day time, suddenly both headlights are working again. I have been tricked by this 3 times now. Arrgh.

Any thoughts? My experience with the w124 so far is that is it rare to have a unique problem, so I am hoping someone reading this can give me a few suggestions of where to start. Thanks and have a happy new year.

Unfotunately it can be any where along the circuit. You are going to have to trace the circuit back from the light and see if you can discover what is going on.

When I had a similar issue on a old model US Car I ran a seperate wire from the headlight (in your case the low beam) that always worked to the head light that did not work even though the headlight was fully functional.

More recently on my Chevy Astro Van the low beam on the Driverside was always dimmer then the passanger side and swapping the Headlights did nothing.

I had something bust inside of the Head Light Switch. After I replaced the Switch the drivers side Headlight shined as it was supposed to.

On other lights I have had issues with the light sockets or the connectors.

Check the grounds
 

It takes two good wires to make a bulb light, and stay lit.
Finding them may be a different matter.

It also may be the headlight switch.

External Lamp Monitor module (device N7) in the relay portion of the fuse box

It’s the big long relay.

These have been known to develop intermittent connections internally. I had this problem on my 124 as well.

Fortunately every 124 and 1986-up 126 uses this relay, so they should be easy to find in a junk yard.

The problem is often the headlight socket itself. What I would do is test for power at the socket using a multimeter. Inspection may show that the contacts are corroded or the socket partially melted.

High beams and low beams are separately powered, so the fact that you have high beams only tells you that the ground is ok.

Awesome guys. Thanks so much for the leads. I will probably try to tackle it this weekend, so it is nice to have a few suggestions of things to check. I will post again if I figure out specifically what was the problem.

The fixture.