300SEL Electric Gremlin

OK, I have a 1988 300SEL with a strange electrical problem. Whenever I turn on the headlights my speedometer dies. All the other electrical systems work just fine, but the speedometer drops to zero. After about two minutes, it jumps back to normal, however it keeps droping down then jumps back up. If I turn the lights off, the speedometer works great. I am thinking is a relay somewhere, but I do not have the electrical grid and I am not sure where to start. All help will be greatly appreciated.

[mbshop]

check all fuses carfully first. but need to check the charging system and the chassis/engine grounds. seems like a ground problem.

george

Thanks George. All the white/blue/red fuses are fine. Am I missing something else? I have checked the charging system and it is great. Where can I start to check the grounding system?

[mbshop]

ok, well you go underneath the car and check the ground strap from the engine to the chassis. but something occured to me. if you have no other issues then i'm wondering if you dont have a problem such as a loose connection in the instrument panel. might take it out and check. kinda hard to tell you what to look for but just check the connections and also see if you got a burnt circuit on the printed board.

george

So I am still looking for a grounding problem? I already looked inside the speedometer and at the circuit board. Everything seems ok. The reason why I am thinking is a relay, is due to the fact that it only happens when I turn the lights on. But I will look again. Thanks.

[mbshop]

will check my info on tuesday. for sure nothing else dims or otherwise
acts up? i'm thinking of a voltage reg built in the inst panel but may
not apply. can't remember everything.

george

Thank you Sir,

Nothing else acts up, except when I turn on the lights. That is when the cruise control and speedometer quit working. After a few minutes the speedometer and the cruise come back. Where would I find the voltage regulator on the instrument panel?

[mbshop]

sorry, no regulator. speedo shop says you most likely have a bad ground.
the speedo housing is a ground. so take it all apart and look for burned
printed circuit or signs of burn/corrosion on the housing.
says speedo is looking for ground and is finding it somewere else.

george

[wbain5280]

Check all the brown wire ground connection for corrosion and make sure all the connections are tight.