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Old 11-13-2013, 05:27 PM
Registered User
Join Date: Apr 2005
Posts: 70
Half dead electrics w140 300SD

I have a '92 300SD. I'm on the road and part of the electrical system cut out this morning. The heater motor, headlights, radio, directionals, taillights, instrument cluster (except battery & ABS light) all don't work. Windows, brakelights, starter, dine lights & lights in doors do work. You can't shut it off except manually with the lever on the fuel pump.

I took it to a repair shop, which I think is probably a good one, and he thinks it is the ignition switch. That makes sense as the shutoff is not electrical, I think, and so it must be something to do with the ignition switch. Is there more than one feed to the fuze box that runs through the ignition switch? I don't have any of my support materials with me, so I'm open to any help.

It's driveable and I want to get home, so I don't want to have it fixed here, if I don't have to.


Thanks, ARW

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Old 11-13-2013, 06:24 PM
TheDon's Avatar
Ghost of Diesels Past
Join Date: Oct 2005
Posts: 13,268
Seems to be normal W140 happenings.

Did you check the fuses to make sure none were blown?

Not shutting off by the key usually means a vacuum leak somewhere in that circuit. Thankfully the doors are not involved in the vacuum circuit. Might want to whip out the ole mity vac and start testing the vac shut off solenoid on the injection pump.
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Old 11-16-2013, 09:11 PM
Registered User
Join Date: Apr 2005
Posts: 70
W140, '92 300SD half dead electrics

I got it home driving in the daytime. The indie thought it was the ignition switch, I suppose because it does not shut off. However, the shutoff is an electric switch controlling the vacuum and mounted on the intake manifold. I could not figure out bad electrics and the usual vacuum switch on the ignition switch, but this explains it.

I'm trying to find the wiring digram in my WIA system, but it doesn't seem to be there. I'll look elsewhere on Monday, but it seems that ll the failures must be related as they all happened at the same time. i intend to trace the headlight circuit as it should go through just the fuses and the rotary switch. and should be the simplest to test.

Accepting suggestions.

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