What controls the compressor clutch to engage?
Sometimes when I turn the air on nothing and other times its fine.
I have driven say 5 miles with the a/c on putting out hot air and
then all of a sudden it starts to blow cold air.
When it is blowing hot air the compressor clutch is not engaging.

 

On your '90 only the pressure switch and the cycling switch control the compressor clutch.

 

it sounds like your low on freeon (dont know if I spelled that right)

 

Well it definitely isn't free....

But I agree, sounds like your freon is low. I believe you only have the single switch, right off the expansion valve, looks like a cone with a 2 wire plug on it, that's your pressure/cycling switch.

 

If you AC is cold and runs fine otherwise and then will turn off randomly, check for loose connections at the compressor, Low Pressure cutout switch mounted on the H-valve, assuming you have an adequate charge.

My A/C did that a few years ago back in August of 2006, it would freeze you out of the cab and then it would suddenly stop and blow hot air, sometimes if I hit a bump it would start working gain, I traced my problem to the an intermittent relay that was inside the Cycling Switch module.

This happened while we were on a short road trip with the family but I was back on down the road within about 30 minuets with my freezing A/C after I put together a quick relay circuit in a Home Depot parking lot.

Here is what I did, but this is only to replace the cycling switch, which you must first determine to be your problem.

https://www.dieseltruckresource.com/...h-t112028.html

Jim

 

When U say nothing comes out, Do you mean that NO air come out or it is just warm/not very cool air?
If nothing comes out, that could be a bad blower fan, relay, switch, etc.
If the cool air comes and goes, that could be low freon, possibly a bad AC clutch. or a faulty relay.

 

When I say nothing ,I mean that no cold air comes out, just hot air.
The system has a good charged of R12 Freon and the sight glass on the
drier goes from foam to clear when blowing cold air.