At highway speeds and rpm (>2000) the ACC on my '87 745Ti quits cooling. Fan is still blowing, but it is lukewarm air. If I am at lees than 2000 rpm or if I put the car in neutral @ speed so the engine drops to idle, the air cools like a &*?#! Frigidaire (pardon my French). Any ideas. I just tightened the belt on the compressor about a week ago. I also noticed yesterday at idle the compressor pulley seems to hang up for a second and stops.

Thanks

low refrigerant. At idle or start enough pressure on the low pressure side to enable compressor to run, but as suction builds up (low side drops) with compressor speed, low pressure switch cuts out compressor.

Confirm with pressure gauges...you should have at least 30psig at low side at 2000 engine rpm in most situations. If it drops to 10-15psig the low pressure switch will cutout the compressor.

"...I also noticed yesterday at idle the compressor pulley seems to hang up for a second and stops..."

It sounds to me like the belt may be shot. With the car in neutral and A/C on, run the RPM's up to >2000 and watch the pulley to see if it quits turning. Also, the bushings in the compessor mounts may be shot (common occurance) allowing the belt to loosen up.

could be you're slightly low on freon, and the evap. is freezing up. when it gets a little low, it actually cools more efficently, and if its humid, the coils will freeze. also at highway sppeds, the condenser is more efficient too, making it worse. in the old expansion valve days, there was a capilery tube at the evaporator that sensed temp, & would adjust the freon flow.Another clue is 'fog' from the vents in humid weather.

Have you checked the vacuum doors? If the motors or the vacuum lines fail, your cooling will diminish. At high idle vacuum, you may have just enough vacuum to run the doors properly; over 2k rpm, it might diminish.

I'll check. This car has Auto climate control and the aux. vacuum pump does run quite a bit when the air is on.