I bought this car used over a year ago and this problem has plauged me for the entire time. I took it to a Benz Mechanic and he couldn't figure out what the problem was. I swapped that device that sits under the air cleaner with one that I know works and still no change. If I disconnect the power from that device the idle returns to what we be normal i.e. about 700RPM with the power connected it idles anywhere from 1500 to 2000RPM. If I disconnect the power from it and reconnect it the idle will return to normal, but idle fast again if the car is turned off and restarted.

I recently had to replace the overload protection relay because my ABS light was on, and my car was hard to start and would idle very roughly and eventually sort itself out. In this state the idle once the car got warmed up was always normal even with the device under the air cleaner connected. When I replaced the overload protection relay the fast idle problem re-appeared. Anyone have any ideas?