No power to AC compressor

[75taguy]

Took my 75 to a local shop last week to have the car tuned and the AC serviced (a new R134 adapter since one was leaking). On the way home, I felt like the AC was working and then thought it quit. Next time I drive it, same thing. I thought it was working and then it wasn't. Today I got to looking at it and I didn't have any power to the compressor. Then all of the sudden I did and the compressor kicked in and everything started cooling just like it was supposed to. When it was working, I went in the car and started messing with the control panel and the only time the compressor would disengage was when I turned it off. I went in the house to look through my service manuals to see if it was supposed to be on when not on AC. I go back out and try stuff again and I don't have power to the compressor again. All fuses seem to be fine and the problem seems to be intermittent. I guess I have a couple questions.

#1 Are the vacuum lines just for opening and closing doors for ventilation or could I have a vacuum issue that could be causing the problem?

#2 Where is the power supposed to come from to activate the clutch on the compressor? Is it all in the control panel or is there a whole list of things it could be? Is my control panel potentially the problem since the compressor wasn't kicking out when I did have power or was it functioning just like it's supposed to?

I don't want to contact the shop yet and blame them for issues they didn't cause, but maybe they got something wrong when tuning the car, that's why I ask about the vacuum lines to the control panel.

I've tried looking at the wiring schematic and to be honest, I get a little confused by it when it gets to the ambient switches and everything else. Thanks in advance for any help

[Wallington]

There's only power direct from switch when on that position. There's a pressure switch inline which will drop out when the gasket leaks. It shouldn't be engaging the clutch with ignition,  unless switch was on. So, where has the wiring or harness been altered, There's a live feed somewhere that shouldn't be. Do you have wiring diagram to look at, GM version or othe. I see you had one. Perhaps it's a symptom of a faulty combination switch on top of the hater control. The orange crab looking thing. Even if the issue was already there, the AC shop may still be good for fault finding, assuming none of the symptoms were present when done. A compressor that engages when AC off is something they should have noticed.

[75taguy]

Thank you for the response. Looks like my next step is to pull the heater control and see what I've all got going on there.