1991 300 E (M103.983) has suddenly started idling at approx. 1500-1800 rpm at startup, regardless of engine coolant temperature. Disconnecting the power harness to the idle speed valve results in a drop of RPM to approx. 750 in park; reconnect the harness and the engine idles fine under all conditions. With harness left disconnected, engine idle speed falls below specs when drive &/or A/C is engaged.

Installed a new idle speed valve, with same results: high idle speed at start-up and then drop to normal when harness is pulled and reinstalled. Fuse on overvoltage protection relay is intact. Any ideas as to where to look next? TIA.