Hi there. I have an International 4000 swather with a cab but my A/C isn't working. When I turn the cooling selector know on it will kick in the compressor and then it will cut out right away. This will continue as long as the cooling knob is on. I do not get any cooling inside the cab. Am I low on refigerant, or is there something else I could check out, i.e. plugged a/c radiator (cooler) in the cab?