OP stated that the ELR plug was disconnected. The ELR system would have nothing to do with a fast idle issue here.

If all of the linkages are adjusted properly, check out the base idle adjustment screw on the back of the IP. It is a locknut with a hex shaft sticking out just above the ELR solenoid on the back of the IP. If that locknut is loose, it's possible the shaft has vibrated into a new idle position. When I got the 3rd IP for my SDL, the PO had adjusted the base idle to 1100 RPM on the adjustment screw (no idea why!).

With the ELR plug disconnected, the idle should be pretty low - ~500 RPM or so. The ELR solenoid should bring it back up to the 650-700 RPM by adjusting (pull up and turn) the resistor wheel by the brake booster.

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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]