740 GLE 1988 AC compressor clutch always on.

My 1988 740 GLE non-turbo can't seem to disengage the AC compressor
from working when I put the fan switch on the "O" but still gets you
cool air through the vents if you have the air steering going for any
of the three AC options on the AC control panel. But once I put it on
the any of the others controls the 4th (from the steering wheel in US
models) it stops getting me cold air but I am not sure if the
compressor is engaged or not.

In a friend's car he changed the AC control unit inside the car from
the Junkyard and it worked correctly for some time but it came back
again to not work correctly.

What is exactly wrong is it the AC control unit inside the car?
Is there some circuitry in the unit that needs to be fixed or is it
some vacuum switches that get busted?

During the winter time and good summer weather, I just disengage the
AC but pulling the electric plug close to the accumulator.

Thanks guys.


--Hameed.