My compressor wont engage until 3500 ish but stays on. It does make nasty noises when it gets hot out. When i first put it on after being rebuilt, it would turn on at idle but is getting worse and worse. I bypassed my ETR switch so i only have to floor it once on my drive to get cold air. The R4 is truely a POS. I plan to do a sanden conversion before the real heat comes and it craps out. I don't think you can do anything about your problem with out removing the compressor. mine is not an electrical issue and im sure yours isnt either. It is most likely a failing electromagnetic clutch coil or the "air gap" has somehow gotten too big.

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83 300CD- sanden, dual p/f condensers, 160a alternator, ect
91 300TD- 722.6, #22 head, 3.5L IP, w140 manifolds, ect