Electrical system quit working 280SE

[LJBryan]

New to the group and need some help.

Bought a 1970 280SEL about 6 weeks ago. Good shape and everything was working - even blew cold air. Tons of fun to drive.

The headlight switch was a little loose in the dash and tough to turn. Never really drove at night but all the lights worked when I got her. Cruising around and back/forth to work no problem. Starts up immediately, engine hums, and smooth shifting auto trans.

First night out and pulled into the driveway. When I turned off the headlights, the car shut off and radio quit. The headlight switch was hot as hell. Now nothing. Can't get anything to work...

Thoughts? Where to start?

[wjsvb ✝︎]

My guess is an ignition ground is open (not making contact) under the hood or dash.  The circuit defaults through the switch heating it up nicely.  Check all grounds, starting with ignition.  Should not have to

[wjsvb ✝︎]

The laptop ate my reply....the event should not have toasted the switch, but test it before cruise night!  Jon

[m300cab]

this happened to me once, right after I sold the car, the guy accused me of sellling a car w an electrical problem
when I sold the car I told him , I had to charge the battery to get it started and he needed a new battery
well
he said he charged the battery, still completly dead
I said put in a different battery to see what happens
well, wouldn't you know, the big electrical problem was SOLVED !
maybe your battery is completley dead?

[LJBryan]

Thank you for the feedback. Checked the battery - all good. Changed all the fuses - there were different amp than listed in my repair manual. Volt meter on the fuse box and everything was dead...

Struggled with what to do. Have a mechanic locally but was going to have to tow the car there for him to look at it. I was seeing $$$ with the prospect of taking my car to him to both figure it out and fix it. Suspicious about that headlight switch since it all happened when I turned off the headlights. The switch was loose and hard to operated. Then I was faced with removing the dash to get the headlight switch out - not looking good. Ultimately realized that the air vent pops out of the dash and the headlight can be pulled through - took 5 minutes.

See pic for what I found. The switch had rotated arcing the main hot wire coming from the battery and completely fried the wires. The hot wire was just loose in the dash and no longer connecting to the ignition. I have since bypassed the switch with a simple electrical cap. Car cranked up on the first turn. Everything working except the headlights. They do come on when I pull the bright light switch. Bonus - all the interior lights, fan, and clock are now working since I had replaced all the fuses. Ordered a used switch off ebay that still has connectors and wires attached so I can hopefully reconnect/solder the wires back together. Will wrap that baby in electrical tape too so no more arcing.

[LJBryan]

And here is the car!

[Benz Dr.]

I'm pretty sure all of the power coming from the battery goes through the head light switch before branching off to other functions.

 I had a similar problem once with a 190SL that had a bad misfire. After looking at everything in the ignition system and finding nothing wrong, someone told me to check the head light switch after driving with the lights on and when I touched the back of the switch I nearly burned my hand. Once I changed the switch the misfire was gone and the lights were much brighter without the flickering I had been seeing.

Bottom line: although rare, your headlight switch can be a source of many problems if it has an internal short or high resistance.   

[wwheeler]

On my '60 W128 Ponton, for sure the power wires go to the headlight switch and the power is distributed from there as the Dr. says. Same switch on that car as my '68 W111 and that sure looks like yours. So on the Ponton era cars, the main power wires were cloth braided. As the braid disintegrated, so did the insulation. That of course leads to shorts and other very bad things. I am sure the wiring improved on your car. But maybe the wire coming into the switch is grounding out? Might want to try to trace that wire as a possibility.

Good luck.

[m300cab]

it's amazing that these wire harness last 50 years with little degradation.
but then new cars are having problems after 10 years, I know of a few C class cars
that have trouble with the signals to rear lights and turn signals!

they certainly don't make them like they used too.

[114015]

Hey Bryan,

Good you found the problem.
Kick out the old light switch as far as you can.

The problem the Benz-Dr. (Dan) described is absolutely correct.
The main current towards the dash comes from the battery, via the starter, voltage regulator and then to clamp 30 on the light swich. From there it divides to the ignition lock switch and fuse 1 on the fuse box.
That means all the current behind the dash passes the light switch.
A deteriorating light switch is therefore not as rare as you might assume - and it is hell dangerous. Ask me how I know.
Effects like that can burn off your whole car.

Good you found the error before more severe things happened.

If I were you, try to find a new switch, not a used (40 years old) one.

Please see attached.
Please check the number first with your MB dealer; number appears to have changed.
The Russian EPC is a mess .... Can't read that cyrillic stuff properly.

Good luck,

Achim

[wwheeler]

I have most of my dash apart on my W111 which is sort of similar to LJ bryan's car. One interesting thing is that there is a thick felt pad just below the headlight switch on the inner dash frame. The switch wires actually touch this part of the frame! The terminals are also not far away either. The pad is there I suppose in case you move the switch back with the battery connected or if possibly one of the wires rubs through it's insulation. Either way, bad situation. And guess which wire is the closest to the frame? Yes, the big red hot wire.

Main lesson is that if you do anything to the headlight switch, ALWAYS disconnect the battery first.