Does anyone know whether, on a California emissions equipped '85 300D turbo diesel (w/ KLIMA relay), there is some sort of idle control system that is supposed to kick up the idle when the a/c compressor is engaged?

Since the beginning of this summer, when the a/c compressor kicks on, my idle drops from around 750 RPM to somewhere in the 500-550 RPM range if the car is stationary. On a few occasion, the car has died while at idle or when returning to idle (e.g., this morning a jogger made an unexpected move as I was about to make a right turn-I began to accelerate and then braked suddenly when I realized he was going to run into my path, and the car died).

Without the a/c on, the car idles well (for an '85 diesel with 300,000+ miles) and at ~750 RPM, as stated above. But the minute the compressor kicks on the idle drops to around 500 RPM and the car begins to shake. If I "keep my foot in it" a bit, it will idle fine and, of course, the shaking stops. The behavior is the same hot or cold.

The a/c compressor is around 3 years old, and the a/c is properly charged. It is an R4 compressor running R-134A. Last summer, the low idle issue didn't exist. I know there is a microswitch on the valve cover that cuts out the a/c when you accelerate rapidly...that switch has been disconnected for 5+ years and so shouldn't be an issue.

Last weekend I was doing some work to my wife's car and she needed to go to the store and asked if she could use my car. To put it mildly, she did not enjoy trying to maintain idle with her right foot while using her left foot to brake.

Thanks for any ideas anyone may have.

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Christopher
'06 Mercedes E350 station wagon (silver/black)
'85 Mercedes 300D (black pearl/palomino)
'85 Mercedes 300SD (smoke silver/burgundy)
'79 Cadillac Sedan DeVille

'05 Toyota Camry (because always running is nice)

'85 Mercedes 300D sold back to orig. owner 8-1-06
'84 Volvo 264GL Diesel, owned 2000-2013