AC freezing up

Freon could be low causing it to run Constantly. Could have moisture or trash clogging the expansion valve. More than likely the high pressure side trinary switch is bad, causing it to run constantly and building too much pressure. You’re on the right track. You’ll see where the probe goes when you change the evaporator. No guarantee the probes your problem. My experience is when they’re bad, compressor won’t come on. If evaporators still good, and can’t find a new one. Good cleaning and flush can be done. Very hard to get completely flushed though. Not a closed loop. It’s a header/ fin style. Best replace it, and condenser if it’s corroded or maybe has debris in it. Definitely a new air dryer, and pull a vacuum. 7% oil to Freon ratio. Only way to figure the correct amount, is to flush all hoses, drain compressor. Account for 1 Oz. Remaining in compressor. I’d change at least the expansion valve on bunk, and the heater control valve, also. I think yours has air over electric for bunk and cab. If so change them out. It’s going to get expensive if you can’t get hoses off the expansion valves. Might want to look into a cost effective way to make your own. Hopefully the compressors already been upgraded, and hoses have fittings. If not, good time to upgrade. It’s a lot of work, but should give at least 3-4 yrs of trouble free service. Till something else goes bad. With A/C it’s always something, almost every year.

 

If you’re looking to just get it working for now. I’d try a new air dryer, and trinary switch. Might just get Lucky and get off cheap, for now. Won’t lose any Freon in the process. Dryer may pick up any debris and moisture if any. I’ve had luck getting an expansion valve to start working properly after turning the A/C on and off a bunch of times. Worth a shot.

 

The older 379 has a small condenser it isn’t enough area to dissipate the heat so the fan will run all the time except if it’s fairly cool out.
The dirt and mud on the fins from years of use really have nothing to do with it freezing....that just slows your airflow down. The freezing comes from years of crap that gets in the tubes inside and restricts flow. Flushing it won’t really help. A hard extended vacuum will help more than anything as it will boil the moisture out of it. Also remember these systems were designed for R12 and 134 just doesn’t work the same in them. If you can find one an updated evap core is the answer

 

Why

Sounds like a plan. I switched my 92 WS to 134A. Using the inter dynamics conversion kit w/pag oil. Replaced original compressor with new same R12 type, with fittings, since my hoses were new. It was cheaper anyways. Same compressor, as the new 134a type, without fittings. Never changed to vitron o rings, or silicone hoses, like everyone claimed at the time. Shops were charging $1000 in ‘98 to retrofit Trucks. Cost me $180, with new compressor. Worked great.

 

You just take the 4 screws out of front of a/c heater box and take it off and you will see the temp sensor, you can go to Napa and buy a new temp. switch that has a knob that you turn that you mount on dash that can control how cold you want it in your truck, in the desert you can turn it way down but in hi humid areas you can't run it on full cold with/out it ice up, or you can just buy one like you have that set at factory temp. setting and vac. out the heater a/c box with a shop vac. or try to blow it out with a air line, but its a easy fix on fine trucks like the one you have?