Car with rear A/C

[CountryBumpkin]

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?

[trouble time]

I would think that there would be a solenoid in the liquid line to prevent the flow of refrigerant when off.

[ZeroTolerance]

Probably a solenoid

[CountryBumpkin]

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.

[DeltaT]

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.

[CountryBumpkin]

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.

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?

[DeltaT]

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.

[doc havoc]

I would think that a TXV on each coil would do the trick also.

[CountryBumpkin]

I would think that a TXV on each coil would do the trick also.

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?

[midhvac]

Can't remember what controls it, but some vehicles have variable displacement compressors.

[ZeroTolerance]

GOOGLE!

http://www.fordscorpio.co.uk/aircon.htm

[BaldLoonie]

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.

[Wendo]

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?

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!