ReplyI would think that there would be a solenoid in the liquid line to prevent the flow of refrigerant when off.
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I have posted this question before on a automotive forum but got no replies. I was wondering if anyone knew what keeps the rear evaporator on a car from freezing when the rear blower is not on?
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I would think that there would be a solenoid in the liquid line to prevent the flow of refrigerant when off.
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Probably a solenoid
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My 1996 Chrysler Town and Country has 2 evaporators and they both have their own fans. I was thinking the same thing as Trouble, some kind of solenoid that controls flow.
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There is typically a low pressure sensor in series with the compressor that shuts the compressor off if the suction pressure becomes too low. That is what keeps the coil(s) from freezing most of the time. Under certain conditions they still will freeze no matter what but you won't know it except for the loss of air flow.
The auto industry calls it a subcool switch or something like that.
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So if I do not want the cooling in the rear of my van to be on then the front has to suffer because it is going to cause the compressor to shut off on low pressure?Originally Posted by DeltaT
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No matter how the system is configured, one evap or two or three, most all auto air conditioning systems have a low pressure switch in series with the auto compressor to protect the compressor. When the coil(s) get too cold the suction pressure gets too low and the low pressure switch starts to cycle the compressor on and off.
This is happening no matter what and most people don't even realize it.
The other control, if you have it, is the supply air thermostat sensing the discharge temp coming out of the A/C coil box. That will also cycle the compressor on and off.
So, as one example, if someone has the blower on low speed on one or both coils the discharge air sensor thermostat will, again, start to cycle the compressor on and off to protect the system again.
The only real way to know about your particular car is go exploring for components and/or start experinemting with how the thermostats and controls work. It should be very easy to do by just starting one coil first and turning the fan to low to see what is cycling the compressor. It will cycle even though you may not be aware of that.
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I would think that a TXV on each coil would do the trick also.
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Yes, They both have a TXV. That was my original thought but I was not sure... Now that I think about it, if there was a solenoid that cut off the liquid line wouldn't the system then be overcharged for the one evaporator that is being used?
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Can't remember what controls it, but some vehicles have variable displacement compressors.
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On most cars, except some GMs, the compressors cycle constantly even with 1 evap. Check it out, pop the hood with the A/C on, car cool and watch it go on & off constantly. If really warm it won't but my new Fusion, even if hot out, once the car has cooled down it runs about 50% of the time if that.
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Most new vehicles have an orifice that feeds both evaps. With the orifice system, the compressor cycles off and on with the low pressure switch. If you don't run rear blower the front evap should work just fine. If you've got a TXV on each evap it should work even better but I haven't seen a TXV on a auto for quite a few years!
If you can't fix it with a hammer, you've got an electrical problem.
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the rear evaps have TXV's and a solenoid. The front evaps have orfice tubes, TXV's, Variable displacement compressors with control valves.
Country Bumpkin, what year and model do you have? I have info for up to 2001 and can look it up for you.
Last edited by Gunslinger; 03-30-2011 at 08:22 PM. Reason: typo
Doug
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The rear evap have a solenoid to stop the refrigerant flow...
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I have a 1996 Chrysler Town And Country. It is the 3.8.
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Looks like it has an TXV for each evap. This is a complex system. You can buy the Mitchell CD on just this vehicle for about 30 bucks; then you'll have info on the whole vehicle
Doug
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I have the Dodge version of the same vehicle.
Both evaps have TXVs and there is a liquid receiver to hold extra refrigerant near the radiator to compensate for varying conditions. There is no solinoid, the TXV just shuts almost completely when there is no load.
There is an evaporator temperature sensor clipped to the front evap to cycle the compressor off if the evap gets near freezing. There is no low pressure cut out in these vehicles.
There is a high side pressure transducer located near the alternator that tells the computer the high side pressure. When you first turn the system on the computer checks the high side pressure to make sure that it is high enough to indicate there is liquid in the system. If the high side is too low the computer assumes loss of charge and wont start the compressor. If it is in an acceptable range the compressor is started. As the high side rises, the fans are turned on at a low speed. If the pressure continues to increase, the fans go to high.
once the system is running, the computer cycles the compressor based on the front evaporator temperature.
remember, since evaporator temperature is based on suction pressure, both evaporators will run the same temperature regardless of differences in airflow.
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