On my '91 740 non-Turbo the idle is set to 750 or so. When I turn on the A/C the idle drops about 300 rpm and does not return to 750 so it idles low and shakes the dickens out of my exhaust system. Have not been able to find anything about this issue in the 700/900 FAQ. Any ideas what may be causing the idle to not maintain 750 with the A/C on?

Ths IAC should be open when you're idleing allowing air to bypass the closed throttle plate. If it was bad I would think it would also affect the idle when the ac wasn't being used. Something has to happen to tell the ECU to inject more gas when the ac goes on to prevent the compressor from draging down the engine speed. There's a relay somewhere that's involved with both activating the compressor clutch and also sending the ECU a wakeup call. There may also be some switch on the heater controls but if the compressor is going on I would think the switch is ok. There's a relay on the heat/ac unit behind the glove box but I don't know if that's the one. Maybe someone else has the location? Could also just be a compressor thats seezing up but you should get some idea of this by observing the belt when the clutch ingages. If the belt looks like its really straining and loosing it's grip the compressor may be hard to spin.

When the AC is on the idle should be at 1000 RPM. I'd look at the Idle Air Control valve first. That is what adjusts the idle.
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Paul NW Indiana '89 740 Turbo 120,000

with the A/C turned off and the motor (for obvious reasons)
you should be able to turn the centre of the pulley on the
compressor (the part which has the electric clutch on it)
with not too much effort required. as for the idle control
valve, hell I dont know!, but if its playing up, you may be
able to get away with winding up the throtle linkage for
a while until it is fixed.

Hope this proves useful.

I'm no expert and can't guarantee my thinking is accurate, but when I think of the physical belt and wheel system that connects the A/C to the engine, could it possibly be that the wheel the belt wraps around is stuck somewhat or is tighter than it should be? I don't know if a tight wheel would be enough to slow the engine down that much, but I think one should be able to physically turn the wheel with a bare hand (without the belt connected obviously). Someone might want to help me out on this one... ;)

Just a first thought that came to mind...
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-jp ('83 245 DL, 210k, 4+OD, no A/C, piss yellow color - but hey, it's got a computer in it)