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  #1  
Old 07-16-2010, 09:10 PM
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Compressor always on when A/C is on?

I have a 96 C280. Does the compressor ever cycle when the A/C is on? The reason I ask is although the automatic climate control will lower the fan when the desired temp is apparently reached, the temp of the air blowing out never drops (which I would assume would occur if the compressor cycled off and on). As it is, at the lowest fan speed it can still get colder than desired. I know other cars of different manufacturers will start to cycle the compressor as the desired temp has been reached. Thanks for any input.

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Old 07-16-2010, 11:02 PM
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Only if switched off by the high or low pressure switch, I believe.
Have you done any testing of what the interior temp is, up near the dome light (where the interior temp sensor is located)? Could also be a fault with the evaporator temp sensor I suppose. Shoul dbe blending air to get proper temps. What do you have the temp set at, or just always cold? Would be interesting to watch for a change in discharge temp if you try setting the temp, lets say at the lowest temp (60?) and then try it at say 75 degrees.

OH alos just caught what you said about "fan in the lowest speed", I would also try doing some testing in the "AUTO" mode and not adjusting the fan speed at all, let it run in auto and see if the fan slows by itself. There should be some tutorials on the site or on the web somewhere that shows how to get in to diagnostic mode and check some of the input readings.
Gilly
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Old 07-17-2010, 02:02 AM
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Thanks for the response. Yes, I would set the temp to say 76 and it would get to the point where it is definitely cooler than 76 and the fan speed would slow down but the temp of the air would continue blowing very cold air and I would not hear the compressor kick off. Because the fans slow down when set at automatic temp, I'm guessing the control unit knows the temperature. Since it is obviously engaged, I would try to listen for the compressor, change in revs, or any indication the compressor is disengaging, but I can't sense any at all.
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  #4  
Old 07-17-2010, 06:15 AM
LarryBible
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This is not a pressur cycling system like a chevrolet. Under most conditions temperature is regulated by the automatic climate control by regulating air flow across the heater core or flow of coolant through the heater core.
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Old 07-17-2010, 02:36 PM
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generally,with automatic climate control,the fan may shut down,but it keeps the compressor constitently engaged,this is so you will have a burst of ice cold air.it does make the fan go up and down in speed,but the compressor has to remane constantly engaged,if you dont like this you should run it manually and remove it from the auto selection.it will save you some gas,but it willtake longer to kool,being thats the compressor has to restart to cool the air.
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Old 07-18-2010, 01:19 AM
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Just so I understand this, is it abnormal for the compressor to cycle on and off periodically, or are you guys referring to the blower fan than blows cool air into the cockpit? I noticed my compressor cycles on and off every minute or so, but seems to cool fine, even on 107 degree days. Is this abnormal for the compressor to cycle?
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Old 07-18-2010, 08:54 AM
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I believe (note when I say "I believe" it means I am not 100%), that the only thing that will turn the compressor off when the system should be operating, is the high pressure and low pressure switches. It's possible under such hot conditions that the high pressure limit is being reached and the compressor is switched off until the pressure lowers. The system would still be cooling. You could be slightly overcharged, but at 100 plus ambient, this might be normal. The switches are there for a reason!
Another contributing factor to high pressures would be an inoperative condensor fan (or suction fan, whichever the car being discussed has). The fan running for the AC system is low speed. On older cars the electric fan actually exists primarily for the AC system. High speed fan settings are for the engine, when the engine is getting too hot the fans come on high speed. Again this varies by car model, and definitely different if the car has ONLY the electric fan (called a suction fan).
Gilly
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  #8  
Old 07-18-2010, 07:21 PM
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I will have to check the pressures. The nozzle on my gauges that fits over the high-side schrader valve broke last year when I dropped it, so all I have been able to check is the low side. It could be the high-pressure. The low and high speed fans are working correctly. Thanks for the input.
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  #9  
Old 07-19-2010, 08:22 AM
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Starting with the W202 production in 1994, the compressor has variable capacity & "rarely" ever cycles.

The older W124 chassis doesn't have that design.

Using variable capacity compressors that run 99.99% of the time means that the SHOCK of the compressor clutch engaging at 3000 RPM is eliminated & compressor life is doubled!
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  #10  
Old 07-21-2010, 07:14 PM
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Does this mean that the system is supposed to be in "Normal" (i.e. not in Economy with the compressor off) most of time? Or at least, during the summer? I tend to toggle the EC switch on and off, thinking that I'm saving gas, but maybe I'm burning out the compressor clutch instead?
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  #11  
Old 07-21-2010, 09:27 PM
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Thanks, MBDoc. I was hoping that cycling was normal--at least for my E420.

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