A/C not cycling

Most AC systems have a temperature probe that monitors the air temperature flowing out of the Evap. Core (what would be freezing up).

They are usally set to turn the AC Compressor off a few degrees above 32 F so that the humidity in the air wont freeze on the outside of the evap core. Once the temperture rises to another set value it will allow the compressor to engage again.

If the Evap core freezes over it is unable to transfer the heat out of the AC system (its effective area is greatly reduced). This will cause the pressure in the system to rise and rise until you hit the high pressure switch, turning the compressor off. During this rise time the AC will work like crap, although it may only take seconds for it to rise high enough.

I'd imagine the humidity was also somewhat high on the days it freezes over?

So depending on how the Temp. probe is wired on this vehicle, you may have a shorted wire, a loose wire, or faulty probe. But I would say definatly look into this area.

If it is a clutch set up, make sure your clutch is engaged when this happens....You will hear the ac clutch engage and disengage during different rpms...

Mike

The clutch is waht turns the compressor on and off as I was refering to in my last post. If the Evap core is freezing over the compressor is most definately working.

Most likely the temp sensor is bad causing the evap to freeze over causeing the pressure to go high causing the clutch to not lock up/ no AC.

If the evap. coil (section of the system that cools the inside of the car) freezes over, the systems ability to absorbe heat is reduced and the pressures FALL causing the low pressure switch to open (system will not run or will cycle). The condenser coil up by the radiator, releases the heat into the air (not enough air over the condensor coil or over charged system will cause the high pressure switch to open causing the system not to run or cycle). If it is a R-134 (refrigerant) system, I would recommend having a qualified shop change out the refrigerant to "hotshot"- made by ICOR. It is a much more efficient gas and does not suffer near as great a cooling loss at high ambient temps. Just my 2 cents. Sorry for the tech. babble I've been in the HVACR trade for 19 years.

valley82 said:

If the evap. coil (section of the system that cools the inside of the car) freezes over, the systems ability to absorbe heat is reduced and the pressures FALL causing the low pressure switch to open (system will not run or will cycle). The condenser coil up by the radiator, releases the heat into the air (not enough air over the condensor coil or over charged system will cause the high pressure switch to open causing the system not to run or cycle). If it is a R-134 (refrigerant) system, I would recommend having a qualified shop change out the refrigerant to "hotshot"- made by ICOR. It is a much more efficient gas and does not suffer near as great a cooling loss at high ambient temps. Just my 2 cents. Sorry for the tech. babble I've been in the HVACR trade for 19 years.

Click to expand...

my bad

So Low pressure, not High, but shouldn't he check into his temp switch?

Yes, also if the refrigerant charge is low (a leak in the system) less of the evaporator would be cooling the air going across it so the air temp would be warmer, but the part of the coil doing the cooling would be colder than normal (could and will cause the evap. coil to ice over). To trouble shoot the system you really need a gauge set. If the clutch is engaged (compressor is running) and the system is not cooling, 1. compressor bad (not pumping the refrigerant) 2. system is low or out of gas and the low pressure switch is not cycling the clutch (compressor) 3. evap. coil is iced over typically from "2." of the low pressure switch is toast.
Again, sorry for the TECH. babble, hope this helps.

Starting around 1990 the auto manufacturers switched from R-12 to R-134 refrigerant. alot of the systems would not work for s@#$ when the ambient temps. get above 100 f.. chevy trucks were terrible in the summer on road trips to the colorado river. Soon after the manufacturers increased the size of the systems which corrected the problem.

hey 1969mach1351, your A/C shouldn’t cycle on and off on hot days (as you described before your problem) Now what i am thinking is your a/c system was low to begin with then on a hot day it was too much to deal with. Since its a 91 it probably means that you have r-12 unless it has already been swapped over to r-134a, If your have r-12 then go to a parts store like napa and get a can of freeze 12, its supposedly compatible to r-12. You don’t really need gauges just fill it up until your a/c pump is on constantly and the two tubes coming out of the evaporator core are the same temp, this will get you close enough.