A/c blowing cold then warm

For starters, is your hot water off?

 

Check the water valves, if so equipped & shut them off. Make sure the belt is properly tightened. You could just be low on Freon. Your truck has a temperature door that controls the hot & cold air into the truck. It is controlled by air pressure. If it's not closed completely, or comes open just a little, you will have the problem you have now. I had the same problem on my 379 Peterbilt, but I have an electric switch valve that controls the temperature door. It's was going bad & was opening the door at random intervals. New switch valve & problem solved.

 

Also the freon being low could cause the expansion valve to freeze up, then it has to thaw out. It converts the liquid to gas. I've found with blend doors they are typically stuck In the hot or cold position, but these other suggestions are good possibilities.

 

Replace the temp control on top of a/c heater box under R side of dash, it will have a 2 wire plug in an torx screws holding it to your box

 

First thing you need to do is have the refrigerant checked to make sure it has the proper amount. After recharging you need to make sure the low and high side pressures are at the proper levels. After you do that and it is still cycling on and off to often you can then start troubleshooting other things.