Troublesome pagoda intermittent stop lamp problems - PeachParts Mercedes-Benz Forum

CarolinaMBZ · #1 01-06-2008, 11:10 AM

1970 280SL. I have been trying to solve an intermittent stop light short/failure for some time. Previously put a bent copper wire in the stop lamp switch plug at the pedal and got them to come on when they previously wouldn't. Bingo! or so I thought. Put in new switch yesterday and they worked fine, for a while. Stopped working after I drove it a while. In reflecting on it last night (with no further diagnosis yet), the pattern seems to be that the lamps work fine when cold but fail after the car warms up. Very perplexing. The reverse lamps (same fuse) also don't work but I haven't yet tried to get them to come on during a time when the brake lights are workings (will do next) or spent time diagnosing that yet. Despite this, the turn signals always work (same fuse too (supposedly anyway)). It appears that bypassing the switch to get the brake lamps to come on was apparently simply contemporaneous to them beginning to work again. The wiring diagram only shows connections at fuse, switch and plug to tail lamp assembly (previously checked but will recheck as part of continued diagnosis/binary search).

As fun as binary searches are to diagnose intemittent electrical faults, is there any guru out there that has found a common gremlin solution (like my favorite trick for W123 turn signal failure which simply involves turning the emergency flasher on and off a few times to "magically" fix it) that will save me a couple of steps?

All help very much appreciated.

Arthur Dalton · #2 01-06-2008, 11:55 AM

Give fuse #5 a spin in it's holder....Benz fuses will fool you b/c they loose contact on the ends with the holder, specially under load [ like brake lamp activation]..best way to check Benz fuses is with a test lamp on each side of the fuse and ground, making sure the test is done on the holder contact, not the fuse..a good fuse assemby will have power on BOTH sides..
If the fuse is dropping on that circuit, a tell will be to watch the fuel gauge drop at the same time...............[ according to my 113 schematics]

CarolinaMBZ · #3 01-06-2008, 04:36 PM

Will do. Thanks a bunch.

Mike D · #4 01-07-2008, 06:31 AM

Also check the ground connection. It should be a post or bolt on the rear firewall behind the seat.

CarolinaMBZ · #5 01-07-2008, 09:24 AM

Thanks. In the 5 minutes I had available after driving it home from work last night, I confirmed that there was juice at the stop lamp switch even though the lamps were still not on. A few minutes after shutdown the right (looking at rear) one came on and in taking the left rear bulb assembly holder panel off and squeezing in in the middle, I could make that one come on (it has been tempermental in the past but was rectified (or so I thought) by cleaning the left stop lamp socket terminals). I suspect that since the right bulbs wires run through the left that somehow this short is temparature-related from operation expanding the plastic or metal enough to short out with correction after the car cools off a bit. I still may be completely off base of course as this seems really strange and is not on the muffler side of the car. I"ll take the panel apart this weekend and give it a good contact cleaning and inspection to see what happens.

I will post the results of this and ultimate outcome for any future searchers out there who ever have this weird problem.

Thanks again for the help. I will also find and check that ground for sure.