Tractor aircon fault

[dieseldoctor]

Hi folks, i am wondering if anyone can shed any light on this fault.
I have an aircon system that won't cycle the compressor, the symptoms are

High pressure creeps up too high,cutting out on hp safety switch
cooling air warm
condensor clean and flowing air, but pipes are about 55/50 degrees Centigrade
evaporator clean has a good flow of air into cab
changed all filters and cleaned dirt off evaporator and i can see through the fins ok
changed expansion valve
changed thermostat while i was in there
changed gas
changed reciever/dryer
swapped compressor for one off identical tractor that aircon works fine on
flushed all pipes with cleaning fluid and ofn

any idea's on what to do next
system cycles fine if you remove fan from evaporator therefore reducing airflow over matrix.

[GM Tech]

And why do you think compressor must cycle????? Under high load it should run steady-- anything above 80 degf ambient, I would expect continuous run.. A compressor cycles to avoid freeze-up of evaporator, this only occurs when demand is low (low ambient or little or no air flow through evap) and compressor has more capacity than is demanded by evaporator.

If all heat transfer components are clean, then you probably have too much refrigerant in the system.