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The idle control valve is electronically controlled and must be energized to reduce the idle speed. If the control module is acting up or the OVP relay has failed, the default strategy is to leave the ICV open and default to a high idle.
Check out your control module for the ICV, you probably have broken solder joints, it's a very common issue. It will cause erratic operation and make you pull your hair out trying to nail it down!
The control circuit of the ICV is transistorized in nature, you must measure voltage with it connected to the ICV, you can't accurately measure the circuit voltage just by probing the plug. You can test the ICV itself by applying 12V directly to the terminals (make sure you have the plug disconnected). The engine RPM should noticeably reduce and possibly stall.
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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)
Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)
Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]
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