I have a 1981 Merz 300SD (I call ol trusty) my wife was driiving it today, and when she turned on the air we had a castrophic failure. A rancid odor began to come out from the A/C vents, so she immediately shut off the A/C and continued to drive the car home. She said something was running under the hood when she parked it. By the time I got home the battery was completely dead. I hooked up a battery charger to the battery and let it charge up, when I tried to hook up the battery cable it sparked alot and now the headlights are on constantly and I cannot shut them off even with the headlight switch. I even tried using the high beam switch on the column turn signal lever. I have looked at the headlight switch and examined the wiring underneath the dash. I cannot find any burned wires anywhere under there. The motor will crank over if I get the battery charged up due to it being directly wired to a switch when about 2 years ago the ignition switch went out. This kinda acts like a problem with the ignition switch because the circuits do not shut off when the key turned off. There are several things continually running under the hood when the key is off. I would greatly appreciated any help tracking this down. I love my mercedes and want to get it back on the road again. thanx guys. Oldsdroptop

ha I saw this guy on craigslist



Its probably your ignition switch since you mentioned it failed once and you did a temporary fix. Or a relay problem. The vacuum can be a failed pump or leak. On an 81 sd the door locks are independent of the engine vacuum system so that rules that out. Either you have a leak somewhere or your ignition switch is not supplying vacuum either.

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1981 300SD 512k OM603

The headlights still stay on when I remove the #2 and the #4 fuses which according to the diagram are the 2 fuses which control the high beam headlights on the left and right. I pulled the fuses and the headlights still stay on all the time. So needles to say I'm totally stumped how these lights are staying on when the fuses to them are pulled. I'm still leaning towards the ignition switch unless I hear from someone else that it might be some other electrical component. The heat/air seems to be working the fan comes on and works correctly. I am using a 100amp racing cutoff switch right now as I need to still drive this car (because it burns WVO and saves me $100 a week. There are also a couple other circuits that are staying on when the key is turned off. thanx guys. Oldsdroptop

the headlights sound like their grounding somewhere. Did you try pulling the leads off the back of the headlight switch to see if you can get them to go off that way? Then you can start narrowing down.

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1981 300SD 512k OM603