1991 Toyota Previa Idle speed with Air con on.

Air Conditioning problem
1991 Toyota Previa 4 cyl Two Wheel Drive Automatic

My Previa idles at about 750 rpm, and smoothly, when I turn on the Air con it slows and shakes noticeably, So far no one has been able to 'fix' this. My understanding is that the 'ECM' should automatically increase engine speed to compensate for the increased load. However I dont know what else the ECM is supposed to do (retard spark/advance spark or maybe something else?)In order to find the problem I connected my vacuum gauge and at idle (no A/C) 20 to 21, but when I turn on A/C vacuum drops to a steady 16 and starts shaking.
I dont think the A/C is in any way connected to the inlet system so how can the vacuum drop?If I manually increase the idle speed up above 800 it runs smooth again but vacuum stays around 16/17. I am thinking that the ECM is doing something to the timing?If it does do you know what it should be doing so I can check if it is working correctly.
Hope you can help
Thanks
Graham