Does yours have ABS? If so, is the light on?
When the OVP fails the RPM does tend to go up. If it's anything like my 560SEL, the OVP in inside the firewall in the center. It will have a clear plastic cover where you can see a 10A fuse in it (sometimes a dual fuse).

will look like this

yes ABS light is on thanks will look for relay

On a 85 380se, the OVP relay is in the fusebox. Has a red fuse on top 10Amp

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John
1986 300sdl
1985 380se

When my 500SEL had a high idle, it turned out to be the idle control unit behind the instrument cluster. My ABS light wasn't on all of the time though.

How does the failure of this ABS system relay effect engine function to the point of a 1700RPM idle?

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RIP: 80 300SD
RIP: 79 450SEL
2002 E430 4matic (212,000km)
2002 ML500 'sport'

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It's actually the other way around. The OVP is an Over Voltage Relay, when it fails two things will happen, the car is hard to start and the idle would go crazy high. Once the idle shoots up the ABS warning light comes on to tell you that your ABS system will not be on when you need it. Now this is my opinion but I had the exact same situation and the problem went away for ever after I replaced the OVP (not just the fuse). Inside the OVP there is a Zener diode that's suppose to regulate the voltage that feeds the CIS system, so if it is shot then I have a feeling it sends a signal to the ICV (Idle Control Valve) to compensate which will most likely kick in the high idle. My 2 cents!