Thanks for your reply.
You say that the sensor is easy to test (since it is off or on). How is this done? I would like to verify all possible options at this point. The car runs well in its initial cold start up but after a short drive the idle RPM drops to just above 500 and the idle is eratic!
I cleaned up the throttle plate and Air intake housing - no results...
88 300 CE
82 300 SD