1987 W124 headlight troubles

[dieselmania]

Trying to trouble shoot a no high, no low and no dash high beam indicator. Parking and fog lights all work. Switch appears to be working although I don't have a pinout, testing with a meter shows it energizes different pins on every setting. No power ever reaches the four fuses that supply the power for each headlight and each beam, and the dash indicator (fuses 13,14,15,16). Power does reach each fuses for the parking and fog lights when the switch is turned respectively.
My Mercedes factory service manuals are not much help, the pinouts and headlight wiring diagrams are nowhere to be found in either of the two chassis manuals, unless I am missing something. I looked for an hour.
My conclusion at this point (partly a guess due to no diagram) is that everything must pass through the combination switch after it leaves the rotary dash switch, and the problem must be in the combination switch.
Anyone with experience on this, or having a diagram, is welcome to chime in.

Thanks, Brian

[t walgamuth]

Sounds plausible.

[Alec300SD]

https://www.startekinfo.com/StarTek/outside/12264/Program/ETM/ETM.pdf
Dieselmania, does this link work for you?
Pages 61 and 62 of the 219 page pdf (FSM pages 124 and 124/1) are the circuit diagrams for Headlanps/Fog lamps.

[optimusprime]

Would look for the lighting relays to start with ..

[optimusprime]

If you have checked the multi switch for the lights. Then the next step would be ignition switch .With ignition activated the lights come on with the ignition switch .No activation no lights

[optimusprime]

Just check F6 fuse

[optimusprime]

info of diagram w124 lighting .

[Diseasel300]

There is no relay in the headlamp circuit on the 124. At least not in the '87. The diagram is shown on drawing/page 124 in the ETM section of the FSM. It is linked above.

Turn the ignition switch to run and turn the headlamp switch to "head". Check Fuse 6. If you have no power, the headlamp switch is likely bad.

If you do have power, then the parking lights should be on. Check Fuses 13 and 14 for power. If no power, the problem is the headlamp switch or the combination switch.

I'd be suspecting the headlamp switch over the combination switch. It would be unusual for the combination switch to fail in a way that none of the modes worked.

[dieselmania]

Originally Posted by Diseasel300

There is no relay in the headlamp circuit on the 124. At least not in the '87. The diagram is shown on drawing/page 124 in the ETM section of the FSM. It is linked above.

Turn the ignition switch to run and turn the headlamp switch to "head". Check Fuse 6. If you have no power, the headlamp switch is likely bad.

If you do have power, then the parking lights should be on. Check Fuses 13 and 14 for power. If no power, the problem is the headlamp switch or the combination switch.

I'd be suspecting the headlamp switch over the combination switch. It would be unusual for the combination switch to fail in a way that none of the modes worked.

Did as you said and with key to "run" and rotary switch to "headlights" I have power at fuse 6, I have parking lights and I have power to fuses 13 & 14. But still no headlights. Am I mistaken in thinking the headlights should work whether the key is on or off? With power at 13 & 14 I wonder if both lamps are bad?

[dieselmania]

Originally Posted by Alec300SD

https://www.startekinfo.com/StarTek/outside/12264/Program/ETM/ETM.pdf
Dieselmania, does this link work for you?
Pages 61 and 62 of the 219 page pdf (FSM pages 124 and 124/1) are the circuit diagrams for Headlanps/Fog lamps.

Thanks Alec, I actually did find that link later last night and have been mulling over the diagram

[Diseasel300]

Since you have power making it through to Fuse 13 and 14, you know that the combination switch is at least partially working.

Check for power at Pin 3 (yel/blk) on your headlamp plug. If you have power there, it is plausible you have lamps out, though it would be unusual to have all your headlamps fail!

Try repeating the above test, but put the combination switch on high beam. Check for power at fuse 15 and fuse 16.

[dieselmania]

No power to fuses 15 & 16 with ign on and headlights on with combo sw on hi. Switching to low have power on fuses 13 & 14 but no power to any pin on headlight connector except for fog light terminal when pulling out on rotary switch (fog lights work). Pulled and tested lamps: OK. Power is not reaching headlamp connector after leaving fuse box. Pulled fuse box, everything appears clean and dry, looks factory new under there and connectors are still factory taped.

If I recall the combo switch had been giving us trouble on high beam, lights would just go out when switching to high. Car seldom gets driven at night so it was something I just forgot about. Also the combo switch does not operate easily or smoothly. But if there is power at 13 & 14, that comes after the combo switch correct?

I think the combo switch must be bad on the high beam side and possibly an open connection on the wiring between the fuses and the headlight connector. Just going to be a pain to chase, pretty packed in there.

edit: power comes into the fuses from the bottom, out on top?

[Diseasel300]

If your fuses are good, it makes no difference which end the power comes into. You should read 12V on both ends of the fuses. If you have the original aluminum fuses, replace them with brass ones. The aluminum ones can develop cracks that make you THINK they're good, but don't pass any current.

Fuses 13 and 14 are your low beam fuses. 15 and 16 are your high beam fuses. Power is controlled by the combination switch. If you get no power to 15 and 16 when pushing the switch forward, your combination switch is probably junk.

[dieselmania]

Originally Posted by Diseasel300

If your fuses are good, it makes no difference which end the power comes into..

It does when you are trying to follow the current flow. I traced the power leaving the fuse to the relay compartment behind the fuse box, where, according to the schematic, it passes thru the lamp monitoring module before leaving for the headlights. This is when I had my "aha!" moment. Power leaving the fuse to the module but not arriving at the headlamps eh? I reached over and wiggled the module and that's when the lights came on!

Removed the module and pulled the cover, all looked good, no burned spots or loose solder joints. All the pins and sockets were clean. I wire brushed the pins with a stainless brush and cleaned everything with electrical contact cleaner and compressed air. Put it all together and now have low beams. I know the high beams will not work till I replace the combination switch.

Not sure if the module will hold, will have to try it on a bumpy road as I am not 100% sure it is the connection. May have to order one of those as well.

Thanks for everyone's help and I will update with any progress on the high beams.

[Diseasel300]

If you're tracing current flow through a fuse, you should ALWAYS check both ends of the fuse anyway as good practice. It's all too common to have a fuse with a cracked element cause hours or days of frustration only to have the problem resolve when the fuse is finally replaced, especially on these cars that are famous for having issues with fuses.

I'm slightly intrigued by your warning module discovery. I could see it having an effect on one of the inputs, but to wiggle it and have all FOUR having been a problem? Keep in mind, each filament has it's own connection to the warning module! It might be a good idea to pull the plug up out of the fuse box and check to make sure you don't have chaffed wiring or something else flaky going on down below.

If you don't find anything out of the ordinary, it would be a good idea to re-solder the pins on the lamp warning module. They're known for developing cracks in the soldering and causing intermittent connections.