'85 380sl idle speed problem

[tullheinz]

'85 380sl, 75K miles, generally extremely reliable and runs very well.

During last couple of months idle speed had occasionally gone to 15k then returned to normal 6-8k. Now stays at 15k +
Cleaned idle control valve with WD 40. Get function ( can hear click and see rotation of internal motor) on bench by applying 12v dc. When installed the idle speed is still at 15k. Only seem to get 6+/- v dc from plug powering the unit. ( Also tried second control valve from local junk yard - assumed probability of two bad ones low?) Idle speed does not come down with either control unit. If I power the unit with full 12v dc, engine stops.
Assume problem either valve or electronic control. Am I correct ? Anybody with similar problem and saved by something else ?
And where is that electronic speed control box hidden ?
Help !
Tullheinz