The A/C went out on my 300SD today. It had been working fine and was converted to R134 a few years back. I put some freon in it a month or two back, and checked the charge again today and the gauge said it was very overcharged. The compressor is also not turning on, and some prior owner has run a jumper wire to the pressure switch on the drier(not sure, the thing on the driver's side of the radiator). I have owned the car nearly 3 years and it hasnt caused me any problems, so like a fool I never fixed it.

I have asked a couple people and they said it sounds like a bad or clogged expansion valve. Any other opinions on this? Thanks for your help guys, I'm not looking forward to a 30 minute commute in Memphis heat tomorrow.

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1985 300SD
216,000 miles

If the compressor is not running, there is no gauge in the world that can give you a clue as to how much refrigerant is in the system.

With the pressure switch wired out (you need to fix this), you should have voltage at the clutch connection if the climate control demands AC. Check for voltage there first. If you have voltage but no clutch engagement, you could have a bad clutch or even a bad clutch gap adjustment. If you have no voltage, work back toward the climate control system.

That is what I was thinking but I have no actual experience with A/C at all. Any idea why any one would run a jumper from the pressure switch? The jumper runs from the switch wire to another wire in the bundle that the switch wire feeds into. I realize this is maintenance at it's worst, but what part going bad would cause someone to do this? Just a bad pressure switch?

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1985 300SD
216,000 miles

It could be that the wire to the compressor itself failed within the harness, and this was the fix. Or it could be that the switch was jumpered to start the compressor. There's no way to tell without checking for voltage and/or continuity.

If the pressure switch is bad, you should replace it. You should also replace it if it is original, since you're running 134a and require a binary switch.

It is a binary switch (2 prongs, right?) but I may replace it anyway. I guess I'll get out a test light or multimeter and check everything before I start tearing into the system. How sensitive are the switches to voltage fluctuations, would I be ok using a test light?

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1985 300SD
216,000 miles

Binary switch means that it incorporates both a low-pressure cutout to prevent the compressor from starting and a high-pressure cutout to shut down the compressor if the high-side pressure goes too high.

The original R12 switch is a low-pressure cutout only. All replacement switches are binary switches.

Voltage fluctuations should not cause a problem if your clutch gap is correct.

Great, thanks for your help. I'm going to check them now.

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1985 300SD
216,000 miles

I decided to check the fuse before I started just for the heck of it. It seemed good so I pulled the Klima relay and blew out the socket. I started the car back up and still no compressor until I hit the defrost button with the temp wheel on hot and then I heard it click. I switched back to the center button and for a while it would only blow out of the defrost vents, though it eventually went back to normal.

The good part is I have AC working properly, the bad part is I dont know why it went out in the first place. Any ideas on what particular parts of the climate control system to start looking at? As a side note, the last time I drove the car before this mess started I hit the auto button, whereas I normally just leave it on high

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1985 300SD
216,000 miles

I have no idea how the climate control works on that car, outside of the cryogenics which are pretty much the same on all of them. Of course, the electrics in my '96 system are very different from yours.

I've read about resoldering the board on that thing in your car, but I haven't done anything like it.