I was having intermittent problems with the IAC on my 84 500SEL a few years ago and it turned out to be the connector. My car would idle too fast. I could move the cable going to the controller and it would idle correctly. I VERY CAREFULLY removed the cover on the connector and removed each wire, one at a time, and closed each up just a little to make the connection tighter. I sprayed the contacts and the controller pins with contact cleaner and have not had a problem since. I also did the 2 wires at the valve just in case.

Bad solder connections inside the controller can also cause a lot of problems.

Paul

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84 500 SEL (307,xxx miles)