What did you do to confirm the problem is from control unit?

There are so many things that could cause a high idle. I spent a whole 3 months working on my 1991 560 SEL, including a trip to dealer, but could not ID the root cause.

Finally, I just plug teh Idle control valve with a cork which has a 5 mm hole. This is well discussed on this forum. That took care of high idle proble for good. Plus, it costs me $0.