Western star ac turns engine fan when hot even while driving

Front of radiator may be full of debris. You can rinse it with water from the backside. Regular garden hose. High pressure might bend the fins. Afterwards, turn fan on, and rinse from the front. That will pull water through the fins. That might help for a while, if so the proper way to clean it, is to remove the ATA charge cooler, a/c condenser, leave a/c lines hooked up, set it aside, and clean the front of radiator. Get it all cleaned and make sure the fins aren’t plugged so air can get through. I usually take my front grill off, then with the hood open, stand in front of the radiator. Makes it easy. Depending on how hard it is to remove the grill on your Truck. Be careful not to damage the ATA or condenser. They sell coil cleaner at big box stores. Brake clean works, and a soft brush, or a brass wire brush. might need a small screw driver it fins are plugged up bad. A zip tie usually works good. Jamming it through each fin. It can be an all day long job. You can also buy a cheap fin comb. Helps straighten them out. Clean it till you see daylight through it.