A/C trouble 1978 450 SL
 

I have problems with my a/c, it blows hot all the time.
Even with the a/c and fan off, it blows very hot air.
I took it to my mechanic, he said the clutch and compressor seemed ok, but that he needed some tool that he didn't have to diagnose the problem.
He said that he could bypass the heater core to stop the hot air in the cabin, but that wouldn't fix the problem.
Is this hard to do, could it be done by me easily?
Also, what is the name of the tool my mechanic needs?
I have a friend who has died, but i'm sure his wife would still have all of his stuff, but I need to know what it is to ask her.

Thanks,
Don Solomon
1978 450 SL