a/c has a strange problem

It sounds like it has stopped cooling, most likely because of low refrigerant. Check your fuses. You can buy a can of 134 with a charging hose on it at a truck stop. Connect it to the low pressure hose. If it's low it draw from the can, compressor should kick in.

 

Do not over fill the system

 

some of the trucks have valves under the hood that you can turn off the heat going to the heater core. Mack trucks use Volvo engines and the Mack's that I have driven have a valve on the line going to the heater core and the line returning from the heater core. If one of them is turned off then no heat will be going inside of the cab and overpowering your A/C.

 

Use a ink pin or small screw driver and push the schrader valve down on the low side line next to the dryer which is near the firewall and if nothing comes out of the shrader valve except hot air there's no freon in the system and most likely a serious leak if some freon comes out just partial charge it with a can of freon with a hose attached. If it has a gauge on it get it to about 30 lbs and in the blue on the gauge.

 

Mine use to do that.it would freeze up from small leak I would turn it off and wait about 10 minutes and then would have cold air again

 

If its low on Freon the compressor won't run. You have to jump the binery-trinery switch to power up the compressor to suck in the Freon.