03 Columbia AC problem

I work on an 03 Freightliner Columbia for a wood hauler. The AC wasn't working and my gauges showed no freon and drawing a vacuum on the low side, minimal or no high side pressure. With one can of 134a and an ounce of dye I found some dye around the front seal of the compressor. We have been adding freon to this truck every summer to get through the summer. After replacing the receiver dryer and compressor and charging, I get 35 pounds on the low side and 212 on the high side with 80 degree ambient temp. I noticed however without the fan turned on the high side pressures will creep up toward 300 psi. The driver claims without the fan on the ac won't work. With the fan on, everthing seems to work fine. Shouldn't the fan also be controlled by some part of the AC system to keep air moving over the condensor. I'm at my limit here and need some help. Thanks.

 

Thanks FYDA. I never let the pressure get that high before I turned on the fan. The driver however claims it isn't cycling, either the fan is on and the AC will work or if the fan is not on, it won't. Since the fan does not seem to be cycling on I suspect there is a problem with a thermal switch. Any idea where to locate it on this unit? It could just be a wire.

 

There is a switch on both sides of the receiver dryer. I'll start there to check for a problem. Thanks.

 

Thanks, I got it repaired and the driver's are ice cold. The forum was very helpful.