Hey guys,

I've had my 190E since last October and the AC has never really worked properly since I got it. Today I tried to mess around with it a bit to see exactly why it's not blowing cold, well, at all! The compressor is new and so far I've been told that it's full of freon and that my evaporator might be bad. Thing is, after today I don't really know if that is the case.

I started her up, pressed the AC button and felt/saw the compressor kick in as well as the fans. Air started blowing sort of cold out of all the vents pretty instantly, but I wasn't moving so I didn't expect much. I turned it off, and took it for a short drive. I tried turning the air back on again and nothing, just blew outside air. Tried turning it on a few more times and still nothing, and it drove the same, so I came home. I came to a stop and turned the AC on again to see if the compressor was coming on at all, and it wasn't. No jump in RPM, nothing.

So I shut her down all the way, took the key out etc. and the fired her up again. Put the AC on a viola! The compressor kicks on! It starts blowing somewhat cold, so I take the chance and start driving around my complex. I came to a stop and turned it off and saw that it did, then turned it back on again. It looked like it tried to come on, then didn't.

I don't know what the problem is, but it looks and sounds like something is causing the compressor to shut off and then it resets itself when I shut the car off. Who knows, it might be my evaporator, but I'm not sure. The air isn't freezing by any means, and it's at its lowest setting, so I'm not sure if that is a sign of something or not either.

Any ideas? She's a 1988 2.3 8-valve if that helps at all. Thanks!

Does your car use a compressor speed sensor? That will kill the compressor until you restart the car. An old or loose belt could trigger it. So could a dirty sensor. Of course, so could a compressor that is about to eat its guts.

You may be low on refrigerant, causing not-cold-enough air. If it really has a bad evaporator, that probably means leaking. I don't know how you can say that a car is full of refrigerant, but the AC doesn't work because of a bad evaporator.

I agree with MATT. Those are the three most likely causes.

I am a DIY, and not a professional, but I have experienced the slipping fan belt and a bad sensor...

Fan belt slippage is, in my opinion, the most common cause. Check the tension and quality of the belt. If it slips, the engagement (rotation) sensor in the compressor will sense it and "lock out" the compressor until you reset it by turning the key off and on. (Yes, sometimes it will reset after a period of time, but may slip right away and kick right back out.) Mercedes belt tensioners are quite troublesome.

My compressor engagement sensor went bad recently on my 92 SL. The "business end" of the sensor is inside the compressor, so if it is somehow dirty, it is not really anything you can take care of without draining the freon and removing the sensor. In my case, the wire from the sensor on the REAR of the compressor that runs to the 3-prong connector near the clutch had become brittle and the insulation had cracked and was shorting out the signal from the sensor. (I replaced the entire compressor.) My suggestion is to inspect that wire and make sure you have all 3 conductors (two from the sensor, and one from the clutch coil) connected to the harness at that new compressor. This style of compressor is used on many other cars without the sensor. Surely the new compressor is the right type with the sensor! On my sensor, a good reading between the two sensor wires was about 600 Ohms.

The low pressure thing is possible, but I don't know that it will lock the compressor out until you restart the car... That sensor is a pressure switch on the receiver/drier. (Closed is working, open shows low pressure.)

Leakage is about the only thing that goes bad about an evaporator. If the freon leaks out enough to disengage the compressor, it won't reset and start working when you restart the car.

Good Luck!

__________________
02 S500
92 500SL
92 400E (Sold)
87 300E (Sold)
83 300D Turbo (Sold)
75 300D (Sold)
74 240D (Sold after 20 years)

This car sat most of it's life, the thing was barley driven, I'll bet you it even has the original belt on it! I'll have to check that out, although I've never had the belt slip (at least I've never heard it slip). As for the sensor, I have no idea, but I'll check it out when I get a chance and make sure that it isn't corroded or anything.

I don't know if it matters, but when I do turn the AC on the compressor will stay on as long as it wants until I turn it off. Then it wont come back on. The compressor was replaced in January of 2007, and it was brand new. I had it tested in April because I thought it only needed a recharge, and the guy said it was full of freon and that my evaporator and dryer were bad, thats about it. He was the same guy who put in the compressor.

Thanks for the help guys! At least I can eliminate the evaporator as the culprit for now!

Until YOU turn it off?

This does not sound like the belt, or slippage sensor.

Will the compressor cycle in and out as needed, but just not come back on if YOU turn it off?

I believe on your car it is the Klima relay that kicks the A/C compressor out for slippage, etc. I have read on this site that they also sometimes fail.

__________________
02 S500
92 500SL
92 400E (Sold)
87 300E (Sold)
83 300D Turbo (Sold)
75 300D (Sold)
74 240D (Sold after 20 years)

Yeah as soon as I turn it off once, it wont come back on again. I'll take it for a longer drive today to see if the compressor does cycle on and off, but yeah it comes on the first time then after I turn it off it wont come back on again if I try. Maybe I'll make a video haha.