2004 Freightliner Columbia mid-roof w/sleeper, huge A/C problems

Hello all,

I am an instructor at a truck driving school which employs a know-nothing hack for a "mechanic" and we need as much outside help as we can get with an air conditioning problem. As stated in the title, it's an '04 Columbia with a sleeper.

Let me start with a run-down of parts the hack has thrown at it to absolutely no avail:
-- New compressor
-- New liquid line
-- New receiver/drier
-- New blower motor
I'm not certain any of these except the blower motor were actually necessary. For my own part, I've checked as many fuses and relays as I can get at, and wiggled/checked any relevant wires and harnesses, and have been unable to definitively diagnose anything electrical through these efforts.

Here's the current behavior and symptoms. You can turn the system on, and for perhaps 15-20 seconds, modestly cool air will actually come blowing out of the vents. Kiss that goodbye after those 20 seconds, though -- the airflow doesn't slowly warm up a little bit like you would expect of a working system, it instead goes VERY quickly and abruptly warm. Leaving it to its own devices for 15 minutes makes no difference in system behavior -- short periods of cool air followed by a quick turn to 3-5 minutes of warm air and no action at the compressor.

We need all the help we can get before this hack throws another $1000 worth of irrelevant parts at it. Does this sound like a huge freon leak the mechanic has been too dumb to find? I've gotten a suggestion of the evaporator temperature sensor? Any help is appreciated.

Thanks,

Larry