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I purchased my 92 300SL a couple of months ago and have been slowly getting it sorted. The a/c system had been working well til the other day, when the heater wouldn't shut off. As I have been contemplating replacing the stereo I had pulled it out, and the HVAC control unit, and then put them back. It was after this that I noticed the problem. I tried using the diagnostics info I found here, but that didn't get me far. The only clue came when the signals came back reading the top as closed, when it was off and the drop top was down. When I was playing with the radio earlier I had also taken off the light assy around the rear view mirror, as it had been cracked and pieces were starting to fall off, and had glued it all back together but not yet re-installed it. So, I hooked it back up and started the car up, and the heat problem went away. Seems there is a sensor of some sort in that light assy and when that circuit is open, you get all the heat you ever wanted, and then some. Put the sensor back in play, and everything works as advertised. Maybe this will help someone with their a/c system when it wants to heat things up.
Now if I can only get the top to operate....
Mark
 

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'03 SL500, '03 SL55, '97 SL320, (2) '91 300SL, (2) '91 500SL, '00 S500 -- all for sale
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Carlo$ said:
thanks for the info. pictures would have had been good.
I suggest buying a copy of the Mercedes WIS for about $10 from eBay, or better still, get the R129 Tech DVD from MBUSA at a cost of $75. Then you will have pictures of nearly every electrical component in your car and much, much more.

The sensor in the overhead control panel is the in-car temperature sensor.

Troubleshooting a non-working top on a '95 and earlier and particularly a '93 and earlier R129 is very easy. I would begin by reading codes as explained in many threads on this forum...
 
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