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.
A/C Evaporator freezing up
- Thread starter nwjones18
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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.
Low pressure cycling switch on the accumulator isn't letting the compressor cycle off. Easy fix.
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 ?
Not sure , but I suppose the sensor that monitors evaporator temperature could be faulty ?
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.
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.
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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.
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.
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.
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.
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.
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
https://forum.aircondition.com/forum/reference-and-resources/111-r-134a-vs-r12-temp-pressure-chart
What's the humidity like there?
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 .
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 .
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.
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.
Freezing can also happen when the system is overcharged
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.
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.
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Originally Posted by mk378
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.
You asked a few times but he states the answer in the first post. (emphasis mine)
Originally Posted by nwjones18
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.
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.
You asked a few times but he states the answer in the first post. (emphasis mine)
Originally Posted by nwjones18
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.
Pulling vacuum will boil water and turn it to vapor, but you still need to pull it for a certain amount of time to draw it all out of the system. So the question is, for how long was the vacuum pulled?
Reading OPs replies, it seems the system is freezing quite rapidly. Moisture in the system will cause this.
Incorrect charge amount, usually takes a long time to freeze the evap coil and the lines in the engine bay.
Reading OPs replies, it seems the system is freezing quite rapidly. Moisture in the system will cause this.
Incorrect charge amount, usually takes a long time to freeze the evap coil and the lines in the engine bay.
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I pulled a vac on it for 45min, then let it sit overnight then vacuumed for another 15min before charging out of principal. If I crank it up and start driving with the a/c on low it will generally freeze up in 10-15min if I'm on Max a/c. If I run it on outside air and high fan it may never freeze. Fair disclaimer my vacuum was one of the HF specials that uses an air compressor, but I think I got it down to -26-27mmhg. Its been a while since I did the charging.. But system capacity of R-12 was 52oz and I put 48oz of R-134 in it (if I recall correctly. I believe it was 4 cans) and pressures looked good. I'll hook the gauges to it again and confirm what pressure the compressor is cycling at. I like the theory that the pressure switch is set a little too low.
I let the truck run for around 45min this morning and took pressures under various a/c settings. Kept the windows down so it didn't turn into a total freezer inside the cab. Ambient temperature was 85F with around 95% humidity. Honestly I don't know how to interpret these readings they were done in order and I gave it time to stabilize before writing down the pressures. I was surprised by the large pressure difference between norm/max a/c.
1. Max a/c 3/4 fan 26/175
2. Max a/c low fan 23/195 (evaporator froze)
3. Norm a/c high fan 50/275 Evaporator thawed quickly
4. Norm a/c 1/2 fan 45/275
5. Max a/c 1/2 fan 33/235
1. Max a/c 3/4 fan 26/175
2. Max a/c low fan 23/195 (evaporator froze)
3. Norm a/c high fan 50/275 Evaporator thawed quickly
4. Norm a/c 1/2 fan 45/275
5. Max a/c 1/2 fan 33/235
When you replaced the system, did you replace the clutch cycling switch on the receiver dryer with the R-134 version? If not, they are NOT the same. The R-12 one cycles at a lower pressure. The R-134 version will have an orange plastic thread base. Judging by the pressures you posted, the system isn't cycling off at 35PSI. You want the clutch cycling switch from a 1995 or 1996 F-150 with factory air. It might plug in, or you might need a pigtail from a junker.
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