I had an odd problem on my '90 740 where all of the dash warning lights would flash off and on when the A/C was turned on. I also was having this strange somewhat of a squealing noise coming from the pulleys on the engine. My regular mechanic initially thought the the noise was my A/C compressor (at this time I wasn't having the warning light problem) I decided to go ahead and have the compressor changed even though the A/C was working great and hadn't given me any problems. After the A/C change the squeeling noise went away for about two weeks. Then it came back but was only audible if you reved the engine up...and then only for a brief time right at start of the rev. I took it back to my mechanic and he had someone get into the car and rev the engine while we watched and lo and behold we saw the problem. It was the Harmonic Balancer. It was moving in and out each time the engine was reved and putting the belts out of alignment which made them squeal. He replaced the balancer and no more noise and no more warning lights. Problem Solved. I found out later that its evidently a common problem on 740's but I couldn't find any description of my type of problem on brickboard. So I hope this will help any of you out there with similiar problems.

It's described in the FAQ. Common problem, solved by preventive replacement of the balancer.
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See the 700/900 FAQ at the drop-down menu above right.

IM Extremly suprised your mechanic didnt check for that with all the warning light's on, those warning lights either mean your alt's going or not spinning fast enough, go get your money back for the A/C compressor! Nice to hear its resolved either way

I may have not been a clear as I needed to be in my original message. The warning lights didn't come on until about 2 weeks after the A/C install. And the installation of the A/C did stop the continuous squealing. I hope that clarifies. The combination of the "chirp" on rev and the warning light "flash" is what prompted me to take the car back in and hence the discovery of the balancer problem.