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I have a 1984 300D that has some strange electrical gremlins I'm hoping someone might know about. I bought this car about 4 months ago and sold my 1983 300D. The 84 had a much better interior and everything, pretty much, worked properly.

Issue #1: When I would turn on the headlights the radio would lose power. I traced this to the radio dimmer wire, cut and capped it, problem solved. Now I notice that if I open the glove box the radio shuts off and the glove box light doesn't come on (probably a bulb but, strange).

Issue #2: No dash lights. I have done all of the prescribed fixes that worked on my 1983 and replaced all the lights and still nothing. I know it's not a grounding problem because I made the dual wire grounding strap and have it attached to both sides of the back and then grounded to the chassis with all the other brown wires. The PO ran a wire directly from the left front headlight to a small light mounted on the steering column that projects onto the gauges. While it works, it also produces an annoying amount of glare on the both the windshield and the gauge cluster. The only thing I can figure are some fried paths on the circuit board. That's my next step (possibly solder in some jumpers if necessary). Any suggestions?

Issue #3: I can't seem to get the floor heating vents to blow any air. The A/C isn't working which I don't care about since this is my winter car but, my feet get rather cold. All other items related to the HVAC work properly. I removed the CCU and rebuilt it (resoldered loose cold solder joints, cleaned, etc). Still no luck there.

Issue #4: Rear defroster doesn't seem to work very well. Given the age of the car I'm not that surprised but, was hoping to remedy that one as well. I noticed some threads relating to "frost fighter" so I may try that and see if that helps by replacing the connectors.

Thanks in advance for any advice and/or suggestions!

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John Wernecke
1969 300SEL 6.3 (sold)
1983 300D (sold)
1984 300D
1992 500E
 

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'84 300DT, '97 S320
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#3:even though you've re-soldered, perhaps some component is fried.

Here's my reasoning: I have a short (or open) in my wiring harness to the CCU. When I had the original good CCU pulled out several years ago, still attached to the harness (can't remember why I had it out), I noticed that with the ignition in 2, depending on how I held the CCU (don't remember what front button was pushed - doesn't matter here), I'd hear a click in it. I could never make it repeatable enough to know just where it was. And I didn't have time then to troubleshoot it. Eventually my floor vents quit working with the heat. I opened up the CCU and a connection on the bottom board had heated badly. I bought a new CCU (got about a $100 discount at the dealer 'cause of some deal we did with other stuff). The floor vents worked fine. I took it out a few days later, and I could repeat that click. Well, still with not enough time to research it, after a couple of weeks, yeah, the vents quit working, I looked at the CCU, and the same solder spot had overheated. I really need to find the open/short. But I've left it like that for a few years now. Obviously I can't put in another working unit until I fix that.

Point is, ensure the components inside are good in addition to the re-done solder spots.

#4: In the early '80s someone made some defroster metallic-type paint specifically marketed to put on those lines. I used it on my 220D. I tried looking for the worn spots with an ohmmeter, and that kind-of worked. What worked best was a strong flashlight and my eyes. Anyway the "paint" worked, although where I put it on, when I turned the window heater on, those spots heated up quicker. They created a fog-free circle at those spots (3 of 'em), which went up and below to the adjacent lines, in addition to making the whole line work. Which was ok. Those worked for a few years, then I had to redo them, and a couple of new spots. My '84 300DT has only 1 working line, and my Villager Van has 3 lines not working. I've not looked for that "paint" in several years, but I should.
 

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Discussion Starter #3
Thanks for the advice. I'll give that a try. Darn electrical issues...
 
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