When I got my truck the the cycling switch was bypassed and the compressor ran continuous. It would freeze in about 30-45 min of driving. On 100° day it would cool 40-50° on a 80° day it would get down to 15-25°. With the cycling switch installed intially it cooled to 40° and I'd be happy with that, but then it creeped up to 50° and stayed there. It was only about 11 am and I'd say about 80°. Without the cycle switch it would have been blowing much cooler. I've only had the switch in for a day. So I'll see what happens today. I was thinking about pulling the probe out some and see if that helps.

 

If I pull the temp probe half way out and turn the A/C on low I can get it to go sub freezing, except right now, leaky hose!

 

I pulled it out about half maybe more and got it to drop to 40-41 when it was 91. I wonder if the device has degraded over time. Be nice to know what the temps were when these trucks were new?

 

These trucks were designed for R12, the condenser and evaporators could be a little larger to keep the pressures down with R134a, but you still should be able to do better than 40. What is your high side pressure looking like with the truck running, warm and cycling?

 

I'm pretty sure it still has R12 in it. It use to blow better then 40 with the cycle switch bypassed. I'm not an AC guy, but my friend is and has gauges at home, or I could take it to a shop. Do you think the compressor is weak? The system is probably original.

 

 

It'll get down to 20 if it's 80 degrees out and I bypass the cycle switch, but it does take awhile to get there. Maybe the compressor is weak? If I replace the compressor, what other components should I replace?

 

Check your pressures. I think you probably just need to add a can of R12.

 

Okay I will, thanks!

 

Are the condenser fins all bent up and filled with crud? That can cause loss of cooling. I was getting 38 degrees from the vent on 134-a, on a very humid, rainy day in Mexico. IIRC the most you can get out of these ac systems is around 30-35 degrees below ambient temps...Mark

 

When the compressor cycles off I'm assuming temps rise to 40-50 maybe?

 

It would probably go to ambient temp, if it doesn't cycle back on. I shouldn't cycle off at all, if the cycling switch is working and adjusted right and all other affecting conditions, like engine temps are relatively normal. In AZ, kind of a hot place, the ac never cycles off, on these old systems anyway. The new compressors on the newer vehicle are constantly running and cycling on and off. I like simplicity...Mark

 

Now I'm confused. Isn't that the reason for the cycle switch is to cycle the compressor and keep it from freezing. Mine freezes after 30-45 minutes when I bypass the cycle switch in 100 degree temps.

 

I don't believe that they cycle on and off while they are above the set temp of the cutout switch. I could be wrong, but mine will run continuously and not cycle at all on a hot day (most summer days here). The new ones will, I think, but these shouldn't. I believe that freezing can be cause by a dirty and clogged evaporator coil, inside the car. It is a common problem especially if you live within 50 feet of a pine tree. the needles get in through the cowling in front of the windshield. Ask me how I know...Mark Edit: Check this thread:

https://www.dieseltruckresource.com/...t-t298318.html

 

Depends on a lot of things like ambient temps and how much charge is in the system.