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A/C Evaporator freezing up #5187579 08/15/19 06:36 AM
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nwjones18 Offline OP
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I replaced the a/c system on my 88 F-150 in the fall, and ever since I did that I've been having problems with my evaporator freezing up, and the other day I noticed the suction line was frozen pretty much all the way back to the compressor which obviously isn't good. I drew a vacuum on the system prior to charging, and the pressures looked pretty good. Could this be caused by the freon being low, but not low enough to kick off the compressor? I was charging out of 12oz cans which always feels like a bit of a crapshoot for getting the system filled just right.


2016 Ram 1500 5.7 Hemi 30k QSUD 5w-20 Mopar filter
2008 Honda CRV 125K QSUD 5w-20 Fram ultra
2007 Mazda 3 2.3 130k 5w-30 syn-blend Denso filter
1988 F-150. 4.9 I6 251k. 10w-40 QSHM Motorcraft FL1A

Re: A/C Evaporator freezing up [Re: nwjones18] #5187587 08/15/19 06:48 AM
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billt460 Offline
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I had that happen once on my truck, (Ford F-150), and it was low on Freon. They filled it and all was well.

Re: A/C Evaporator freezing up [Re: nwjones18] #5187607 08/15/19 07:34 AM
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Chris142 Online Content
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Low pressure cycling switch on the accumulator isn't letting the compressor cycle off. Easy fix.


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Re: A/C Evaporator freezing up [Re: nwjones18] #5187608 08/15/19 07:36 AM
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WyrTwister Offline
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2 common causes are reduced air flow or low on refrigerant , or possibly both .

Not sure , but I suppose the sensor that monitors evaporator temperature could be faulty ?


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Re: A/C Evaporator freezing up [Re: nwjones18] #5187617 08/15/19 07:46 AM
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mk378 Offline
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There is no temperature sensor. As @Chris142 said, it's controlled by pressure. The switch is supposed to shut off the compressor at about 25 psi, at which pressure the boiling point of R-12 is still above the freezing point of water. That means that the evaporator can't get too cold no matter how much refrigerant is in it. (Being undercharged will cause inadequate cooling, not freezing).

What refrigerant did you use? Also did you remove the air first?

First check would be to unplug the switch and make sure the compressor stops. If it doesn't theres an electrical problem or someone's well-intentioned modificaton.

Last edited by mk378; 08/15/19 07:47 AM.
Re: A/C Evaporator freezing up [Re: Chris142] #5187621 08/15/19 07:51 AM
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nwjones18 Offline OP
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Originally Posted by Chris142
Low pressure cycling switch on the accumulator isn't letting the compressor cycle off. Easy fix.


I had that thought when I was initially having the problem, but it will cycle the compressor when it gets down around 22psi (whatever the correct unit is) which I think is correct, but by then my evaporator is already freezing.


2016 Ram 1500 5.7 Hemi 30k QSUD 5w-20 Mopar filter
2008 Honda CRV 125K QSUD 5w-20 Fram ultra
2007 Mazda 3 2.3 130k 5w-30 syn-blend Denso filter
1988 F-150. 4.9 I6 251k. 10w-40 QSHM Motorcraft FL1A

Re: A/C Evaporator freezing up [Re: mk378] #5187629 08/15/19 07:54 AM
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nwjones18 Offline OP
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Originally Posted by mk378
There is no temperature sensor. As @Chris142 said, it's controlled by pressure. The switch is supposed to shut off the compressor at about 25 psi, at which pressure the boiling point of R-12 is still above the freezing point of water. That means that the evaporator can't get too cold no matter how much refrigerant is in it. (Being undercharged will cause inadequate cooling, not freezing).

What refrigerant did you use? Also did you remove the air first?

First check would be to unplug the switch and make sure the compressor stops. If it doesn't theres an electrical problem or someone's well-intentioned modificaton.


Should have mentioned that in the first post. Truck has been converted to R-134a and the pressure switch will kick off the compressor at 22-24 psi. Yeah I had read low refirgerant could cause it, but it is hard for me to believe a system that is literally blowing freezing air is low on freon, but I'm no expert. I could adjust the pressure switch to kick off the compressor at a little higher pressure.


2016 Ram 1500 5.7 Hemi 30k QSUD 5w-20 Mopar filter
2008 Honda CRV 125K QSUD 5w-20 Fram ultra
2007 Mazda 3 2.3 130k 5w-30 syn-blend Denso filter
1988 F-150. 4.9 I6 251k. 10w-40 QSHM Motorcraft FL1A

Re: A/C Evaporator freezing up [Re: nwjones18] #5187631 08/15/19 07:57 AM
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mk378 Offline
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If there is air in the system, the partial pressure of the refrigerant will be lower than 22 psi (which is the total pressure of air + refrigerant), and it can freeze. Not using a vacuum pump before charging will lead to problems including this one.

Re: A/C Evaporator freezing up [Re: nwjones18] #5187634 08/15/19 08:05 AM
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nwjones18 Offline OP
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According to this chart maybe I should set it closer to 30psi to keep it a few degrees above freezing?

https://forum.aircondition.com/forum/reference-and-resources/111-r-134a-vs-r12-temp-pressure-chart


2016 Ram 1500 5.7 Hemi 30k QSUD 5w-20 Mopar filter
2008 Honda CRV 125K QSUD 5w-20 Fram ultra
2007 Mazda 3 2.3 130k 5w-30 syn-blend Denso filter
1988 F-150. 4.9 I6 251k. 10w-40 QSHM Motorcraft FL1A

Re: A/C Evaporator freezing up [Re: nwjones18] #5187636 08/15/19 08:13 AM
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Chris142 Online Content
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What's the humidity like there?


02 Wrangler Napa high milage 10w40
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Re: A/C Evaporator freezing up [Re: nwjones18] #5187646 08/15/19 08:25 AM
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WyrTwister Offline
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At least 30 psi . If it is pretty HOT in the cabin of the vehicle , I may run it to 35 psi . As the interior temperature of the car comes down , the suction pressure will probably come down . The ambient temperature and volume of the air passing through the condenser coil affects all of this too .

Remember , temperatures and pressures . That is the fundamentals of A/C & refrigeration .

Seems to me , the A/C systems in the newer vehicles contain much less refrigerant than the old systems . And are much more sensitive to being a few ounces high or low . Much more touchy .

Also , remember air flow , Must have adequate air flow . Stopped up cabin filter and / or evaporator ( or condenser ) coil , will negatively impact system performance .


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Re: A/C Evaporator freezing up [Re: Chris142] #5187659 08/15/19 08:42 AM
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nwjones18 Offline OP
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Originally Posted by Chris142
What's the humidity like there?


Pretty much 90-100% humidity and 90-100 F lately. Feels like a rain forest.. The few times I did kick on the system when it was cooler it would freeze up quicker. It also will freeze up faster on lower fan speeds (not shocking) The whole system including condensor and evaporator was replaced when the compressor was, so I know there isn't a bunch of trash or anything stuck to the evaporator. If you run the fan on high and its blazing hot outside it won't freeze up, but if its cooler, or the fan is on low or one notch up it'll freeze.


2016 Ram 1500 5.7 Hemi 30k QSUD 5w-20 Mopar filter
2008 Honda CRV 125K QSUD 5w-20 Fram ultra
2007 Mazda 3 2.3 130k 5w-30 syn-blend Denso filter
1988 F-150. 4.9 I6 251k. 10w-40 QSHM Motorcraft FL1A

Re: A/C Evaporator freezing up [Re: nwjones18] #5187726 08/15/19 09:50 AM
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Freezing can also happen when the system is overcharged


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Re: A/C Evaporator freezing up [Re: nwjones18] #5187774 08/15/19 10:37 AM
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mk378 Offline
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Again I ask did you remove the air before charging?

Charge by weight not low side pressure. On a conversion try to use the same amount the manufacturer specified for R-12, unless the high side goes too high (and you know the fan clutch is good), then it is OK to use slightly less. Never use more that will be overcharged.

Last edited by mk378; 08/15/19 10:38 AM.
Re: A/C Evaporator freezing up [Re: nwjones18] #5187808 08/15/19 11:20 AM
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JMJNet Offline
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If you retrofit with R134, this refrigerant does not like to be overcharged.

It has to be just right.

If you fill it the same amount as R12, it may be too much.


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