W126 fog lights dead - PeachParts Mercedes-Benz Forum

LandYaghtLover · #1 01-22-2012, 03:09 PM

On my 91 560 SEL I have no fog lights. Replaced fuse 10 which is dedicated to those only. Still nothing. Checked relay diagram, none seem to control them. At least nothing was labeled for them. Just took switch apart and cleaned it up. The contacts at the base worked fine. Tested open/closed with meter. So I am pretty sure the switch is not the issue. Wires look okay.

Cant figure this one out. Is there are relay used for these that I can not locate? Any other ideas? Search came up empty.

sixto · #2 01-22-2012, 03:54 PM

The bulb out warning relay, the longest relay in the fuse box, interrupts the switch signal to do its thing. The relay in my 87 SDL caused a bulb out warning when the car went over a bump. I asked a friend to reflow the solder joints and it was fine. The fuses are upstream of the relay. Did you check for power at the fuses when you engage the fog lights?

Sixto
87 300D

LandYaghtLover · #3 01-22-2012, 06:49 PM

No, I did not check fuse power. Good idea. But I LOVE re soldering, so I will do it anyway. And when on, I get no bulb-out indicator. Sounds like I should. So I bet you are right. Going to go grab it now and work on it. Would be nice to have them back working again!

LandYaghtLover · #4 01-22-2012, 08:08 PM

I re-soldered the light out module and the flasher/turn, wiper module. Figured why not. No change in fog. I did not check voltage at fuse yet, left meter inside when I was out. I also pulled the fuse board up and looked underneath. Everything looked good. No bad wires, etc.

Very frustrating. Thats two systems on the car that are dead without obvious reasons.

compu_85 · #5 01-22-2012, 08:28 PM

I'd suggest you check for power at the lamp failure module. You could also put +12v to the outputs and see if the fog lamps light.

I can look up what pins on the lamp failure module to test at if you need the help.

-J

sixto · #6 01-22-2012, 08:30 PM

That's as easy as holding the key at ON before starting, right? Otherwise remove a tail light bulb to check the system.

Sixto
87 300D

compu_85 · #7 01-22-2012, 08:37 PM

Key doesn't matter... turn the headlight switch on and turn on the fogs, and check for 12v where the power for the fogs comes into the monitor "relay".

-J

sixto · #8 01-22-2012, 08:40 PM

I mean doesn't a key ON self test check the relay?

Sixto
87 300D

compu_85 · #9 01-22-2012, 08:55 PM

Hm... no, not really. When I had cracked solder joints in mine the failure was on the "output" side, so it looked like a burnt out bulb to the monitor. But if the failure was on the input side it wouldn't throw the lamp out indicator.

-J

LandYaghtLover · #10 01-22-2012, 09:01 PM

Pin outs would be cool. Really no clue on this one. Could still be the relay of course. Just one day they stopped working. That was months ago. I thought it was the switch but tests prove its not. And like I said, no lamp out warning either.

edit: By the way, I did see what appeared to be a couple cracked solder joints. These were the solders for the pins. About three of them did not look right.

RBYCC · #11 01-23-2012, 10:19 AM

Have you checked for a poor ground connection?

As far as a voltage check start at the connector at the fog light and work back to the next connector or control device till you find where the voltage break is.

LandYaghtLover · #12 01-24-2012, 09:57 PM

Confused. Fuse 10, according to the diagram, if for fog lights. And thats it. With car off I ran a meter between the fuse and ground. No voltage. But with lights in any position, with fog on or off, I get voltage across there. Should I remove fuse? (just thought of that) Or is something not right since I get voltage at that fuse at all times. Yet no fog lights.

LandYaghtLover · #13 01-24-2012, 11:21 PM

After some research, it seems fuse 10 should go hot when switch is in park or headlight position. Along with fuse 11 which is for parking lamps among other things.

I will have to test the continuity between the switch pin for fog and the GY/GN wire heading to the lamp out module. I am guessing this will be fine.

If I had to bed, I would say I am full circle and back to the module being bad or I missed a cracked solder point.

I think I will work on this in a couple days. Maybe... lol

LandYaghtLover · #14 01-30-2012, 09:50 PM

EDIT:

Ok, just did a bunch of testing and after doing it and closing things up, I thought of the obvious test. I just ran back out and ran 12v positive from fuse 10 to the fog lights via the outputs of the lamp out module.

Bingo. I got light. That rules out wires from module forward. And jumping the module did not work. I am pretty sure I am back to the switch. I think the voltage output is so low that while I do get 12v with meter, there is not enough amperage to drive the lights. And when checking with lamps on, I get 0.03v. With lamps removed and switch on, I get 12v at each. With either lamp in place, back to 0.03v. So I think they are talking whatever power they can get. This would also explain why removing both grounds at lamps and supplying a new ground results in the same scenario. Ruling out a bad ground also.

LandYaghtLover · #15 02-27-2012, 08:03 PM

Well I had a SLO even late last week. At least the brights worked. So I ordered a new switch. NOT happy that is a URO brand. Has a one year warranty I am sure I will exercise several times. I had planned on dissecting the original switch for posting here. Instead it will go in the truck as a backup!

Anyway, fixed my fog light issue. Must have been poor contact. Enough for voltage, not enough for actually lighting. Glad I did not have a fire in the switch or something!