This is an interesting discussion : as for the idling speed control of 603/602 engines, I've just checked my CD. On all models it is electronically controled : the speed is ajusted by an actuator on the back of the IP that receives a signal from the EDS control unit, who in turn receive a signal from a speed sensor on the starter ring gear. But there is also an "ajusting screw for idle speed" on the back of the IP. That would be the one that Jassper talked about.

As for the electronic "idle speed" ajusting device refered by Peter, the CD talks about a "resistance trimming plug for idling speed" that was fitted on model year 1986, 1987 and 1989 (W124, W126 and W201). All diagrams from 1990 and up related to US models don't show that device and I didn't find it on my car.

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DR
1992 300D 2.5T 351 000 km