Unplugged Idle Control Valve 1990 300 SEL
 

All this started with with a hard to start car with ABS light on. I had a blown OVP fuse in my 1990 300SEL, I replaced the fuse and the car idles 1600 rpm and is easy to start, when i remove the good fuse, it idles normal but is hard to start. I Know the OPV will have a lot to do with hard starting, here's my question. What i have found is that when i disconnect the idle control valve, the car idles at 600 rpm and runs normal, with it connected it idles high. Is there a problem with driving with the ICV unplugged or will it affect the cold start value? I drove 10 miles today with it unplugged and all seems fine. Another questions is how many volts should be at the ICV, i am showing 2.16 volts. I have also cleaned the ICV and when i put power to it i can hear it operating, 12 volts only, with it connected i cant hear it operating with 2.16 volts. Not sure if all has to do with the OVP relay or not, I tried to find one at a salvage yard today but no luck.

Most likely you have a sensor input issue with the fuel system, causing the high idle.

Leaving it disconnected won't cause many problems.

Update, I replaced the OVP relay new from Mercedes, no change. I have also noticed the last few days in the mornings that the car doesn't want to change gears when cold and i can hear a loud sucking noise, almost sure it's the air intake. I plug the ICV back up and it shifts fine and noise goes away. After it warms up i just unplug ICV and all seems fine. Like you said, there shouldn't be an issue but i would like to pinpoint the issue. Thanks

Up-shift delay is normal in cool/cold weather & has nothing to do with how the car idles.

Testing the air-flow position sensor is a starting point, hope you good with volt/ohm meter.

Ended up being throttle micro switch, but ever once in a while right after starting up i will still hear that loud sucking noise and car doesn't want to change gears. i have also noticed sometimes after i let off the gas coming to a stop the sucking noise comes back and will go away after i accelerate again. After starting sometimes it will go away when i put it in a different gear of i might have restart the car.