Hi Folks,

My climate control was working just fine yesterday, but today I have no air conditioning. When I set the climate control to warm temperatures I get heat, and when it's on cold temperatures I only get outside air blowing in with no air conditioning. When the vents are set to windshield defrost (the button on the far left) it only blows hot air regardless of the temperature setting of the climate control (is this normal? I hadn't noticed this until now.) I checked the Poly V belt and it's fine; it seems that the A/C compressor just doesn't engage. The fan(s) that usually turn on when the A/C is running don't come on at all. Also, a few days earlier the windshield washer pump stopped working (haven't had time to look into that problem yet). I wonder if that could have something to do with the subsequent A/C failure.

Any ideas as to what may have gone wrong? I've search this forum but was unsuccessful in finding info on this problem. I would appreciate any help I can get before I start poking around under the hood tonight.

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1993 300E 2.8

try jumpering the low pressure ac switch and see if that engages the compressor. if switch is bad or freon pressure too low, switch prevents compressor from engaging.

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2005 Mercedes C230k sport sedan, 6 speed
1987 Porsche 924S - 968-engined track car

Only jump the switch with engine off and listen for the click, but that is not the first diagnostic step. You are best served by first monitoring the low AND high side pressures and post results here.

Good luck,

Ok, I finally got around to taking the car to my indy to have the high side & low side pressures checked. He said that since the compressor isn't running he can't accurately measure the high/low pressures. He hooked up his guages and said the static pressure was about 120 (psi?), and that this indicated that the system was sufficiently charged. He noticed that when the ACC is set on cold and the car is started, the compressor clutch engages for about 1 second and then disengages immediately. Based on the measured pressure and the observation that the compressor engages for a second on startup he said that either the A/C speed sensor ($175 part ) is bad or the compressor is bad ($600-700 + labor ) and it's causing the speed sensor to shut off the compressor.

What's the best way to test the speed sensor?

Thanks again for your help!

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1993 300E 2.8

Oh, I forgot to mention that he recommended against bypassing the switch and getting the compressor going before first testing the speed sensor. He said that if the compressor is bad and something is broken inside which is jamming it, starting the compressor could spread metal particles throughout the rest of the system.

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1993 300E 2.8

Yes, your tech is being justifiably cautious. What I would do is turn the compressor by hand and see if it is free. If the compressor is internally shot, however, the debris has most likely already been strewn throughout the system requiring entire system flush.

Good luck,

I turned the compressor by hand and it seems to be okay. It turns with a bit of resistance, which I think is normal since it's "pumping" the refrigerant through the system, but nothing is jammed up.

Is there a safe way to manually engage the compressor clutch so I can get the A/C running and make sure the mechanical parts are okay and working? That way I'll be sure the problem is electrical and I can start testing the various sensors/switches.

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1993 300E 2.8

There is MB 6 step procedure to check the compressor clutch engagement. You can go to www.alldatadiy.com and get the document.

In general, it consists of a series of tests, which elimintate componets like the AC Push Button Unit, Compressor speed sensor etc.

To answer your question, you can remove the AC compressor control relay (also refered to as the Klima relay) and bridge pins __ & __ (you will have to use the search function of the forum as I cannot recall, I believe it is pins 5 & 7).

The AC worked fine in my 94 E320 if the pins were bridged, otherwise, the compressor clutch cutout soon after engagement. It passed all the tests expect the speed sensor resistance value. In any case, it turned out the 3 wire electrical connector on the compressor was bad. Replacing the connector fixed the problem.

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Zafar
94 E320 58000 Miles