This is the same problem I'm having. The advise I'm getting is that the servo is shot and probably also the amplifier or at least if replacing the servo replace the amplifier.
Some of what I've learned:
1) Vacuum in mine seems fine through the system except there does not appear to be a vacuum tank. Vacuum is supplied directly from the pump.
2) I can actually make a few jumpers across the vaccum activated switch after pulling out the dash controls (after plugging a few lines) and get the a/c and fan to come on and be activated by the temp dial. The a/c compressor never turns off, however.
3) I'm now figuring where to look to see if the servo gets a current. If it does then my guess is the servo is shot.
4) I've been looking for one and rebuilt they run about $500--steep. Plus the amp is another $100 or so. Performance has a solid state one for about $700. Not cheap but I've already replaced the compressor, dryer, expansion valve, temp switch, pressure switch. In for a penny, in for a pound?

Anyone know of a rebuilt one that is cheaper than $500????