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Old 08-05-2002, 10:08 AM
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Join Date: Dec 2001
Location: Long Island, New York
Posts: 4
Question w124 A/C compressor short cycles

I did some work on my a/c system on my 1987 300D turbo.
Replaced a/c hose, Klima relay, control unit & rec/drier. I recharged the system with r134. The only problem I'm having now is that the compressor keeps cycling on and off. We have the proper amount of 134 in the system, so I'm not sure if it could be the pressure switch or something else. I'd like to be sure before I evacuate the system again. And sometimes the blower fan dosen't come on until I turn off the ignition and restart the car! Any thoughts are appreciated.

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Old 08-05-2002, 02:04 PM
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Join Date: Dec 2001
Posts: 565
Many MBs have a speed sensor on the back of the compressor. You might use the search function here for it's description/purpose. This sensor will deactivate the compressor if it detects a mis-match. Cycling the ignition is required to reactivate the compressor. Your situation sounds a little like the speed sesnor is at work.

Short cyling can be caused by numerous things to include too much refrigerant or too little refrigerant. Did you test the high/low side with gauges? I believe it's been said here that you need approx. 80% of the original R-12 load when using R-134A.

How much oil was used? I wonder if an excess amt. of oil could short cycle the compressor?

I'm sure you'll receive more answers than mine soon.
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Old 08-05-2002, 06:25 PM
Posts: n/a
Excess oil will have little to do with short cycle.

You are most likely low on refrigerant. 80% of R12 volume is a good start, but you need to charge by trial and error, but using gauges to see your progress.

The 124's are some of the worst cars there are for conversion, so you will probably have to experiment a good bit, if you get too much 134 in, the high side will sometimes start to "run away."

The speed sensor will not cause short cycling, it will simply kill the compressor and it will not engage again until you kill the engine then restart.

Good luck,
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