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Old 12-26-2011, 10:54 PM
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Join Date: Jul 2011
Posts: 385
Troubleshooting rear defroster - W115

I yanked the rear seat today and, with my handy multimeter and lovely assistant, tried to investigate why my rear defroster won't work.

The resistance as measured from one terminal to the other is 3 ohms.

There's no clicking sound from the relay when I pull the knob. However, the light inside the knob illuminates and I discovered that I have -12V at the terminals behind the seat. I'm assuming the negative is from me confusing the polarity. Let go of the switch - no voltage, no lamp.

My rudimentary knowledge of the system makes me think that the relay and everything downstream is working, however there's no permanent latch voltage to keep the relay shut.

Is there a timer or something that I should be looking at? Where is it located? Or am I headed in the wrong direction?


1974 240D "Boldie" 170K.- New timing chain/freshly rebuilt IP/replaced valve seals/injectors/upgraded stereo/new Bilsteins with Yokohamas/fresh paint and rocker panels plus lots of welds.
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Old 12-27-2011, 12:10 PM
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Join Date: May 2008
Location: southern CA
Posts: 408
I have the same issue with my 115, interested to hear what you learn.

1972 280SE 4.5
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