Date registered: Sep 2004
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RE: headlight low beam not working 87 300sdl
This is my first try at helping somebody else, here, but I've spent a fair number of hours chasing electrical problems in the W126 chassis.
I assume that you've considered and dismissed the headlight control relay as your problem in the check-up. I believe you Benz uses the same one as my 420SEL. If so, it's that long one (126 542 01 32) in the upper left quadrant of the fuse-relay box under the hood. It has 34 separate connecting pins and, if it's good - but not properly installed (not "seated" well), it MAY be responsible for headlight malfunctions. Make sure all of the pins are making good contact in their sockets - there a 3 separate sockets under mine.
The 126 chassis electrical, in general, is not voltage controlled (not 12V or "+"). Instead, its switches and relays apply ground ("-") to control your headlights.
Of course, I'm certain that you attempted the replacement with a known-good bulb and that it is the proper one for your car. I also assume that you've checked and perhaps cleaned the connection contacts from the chassis wiring harness at the headlight connectors.
Other potential problems include any bare, "chafed", or broken wires & connectors you may find while looking at the control relay or other chassis wiring.
Good luck with the "fix"; if you decide to throw money at the problem and the control relay is you prime suspect, I often see them for sale on eBay for around $25. If you want a new one, the "going" rate "on the web" is seems to be about $60 to $70 - sometimes with "free" shipping! -- Ol' Doc Haynes[:)]