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TomO · #12 07-01-2004, 11:29 AM

I did some circuit testing and would appreciate help with my diagnosis. (Thanks to Jim who sent me links to benzworld where I found the correct wiring diagram (on a Russian website)--and it turns out I had printed the wiring diagram from the CD ROM last year but couldn't understand it then and forgot about it--duh). Indeed, the window wiring for the 94 E320 is totally different from older cars. I hooked up a voltmeter to the green/viol and black/viol wires that go into the door from the so-called "convenience control unit" located under the rear seat. These two wires are connected to the motor's green and black wires at a plastic block on the door. The way I figure it, the convenience control contains relays which reverse polarity of voltage to the motor depending on signals from the window switches. (And I had already determined that all my switches are OK by swapping them).

For my test I disconnected the motor wires and attached a multimeter to the green/viol and black/viol wires. When I hit the window switch I get 12.4 V, pos or neg, depending on whether up or down. Conclusion: the relay is getting the message to power window up and is trying to tell motor to go up. Then I connected light bulbs of various wattages to test the circuit under load. I started with an old H1 or H4 headlamp (about 0.5 ohm) and got full illumination and 12.0 V down but no illumination and 0.05V up. Conclusion: somehow the relay or the following circuit can't deliver the amps.

Then I tried the 5W (3 ohm) bulb out of the door ashtray and got a bright light and 12.34 V with the swich held down. With the window switch held in the up position the voltage started at 0.3 V and gradually increased to 1.1 V, while the bulb glowed very dim at first and gradually became less dim.

Then I hooked 4 tiny dash lights in series (23 ohms total) and got 12.37 V with switch down. With the switch up voltage gradually increased from 4.5 to 6 volts as the bulbs slowly got brighter. THEN, if I held the switch up for about 30 seconds the bulbs suddenly became much brighter and the voltage jumped to 12 V.

So I'm still guessing I have either a bad relay or a bad solder connection in the convenince control unit. I need help with the diagnosis at this point.

I read that the control unit is very expensive and it doesn't look very accessible. If so, I'm thinking a kluge (crude fix) might be the answer. I could wire in a DPDT switch between the door and motor wires and put the switch in the door ashtray. Throwing the switch would let the down window switch raise the window. (My wife's only criticism is she uses the ashtrays for litter bags.)

Any comments?