The easiest way to diagnose this problem. Key on engine off, pull the plug from the idle control valve, if you feel it switching, means you have power. If you do not feel it switching when you connect/disconnect the terminals, you have 1 no power to it, or 2 seized. To check power get a test light and check the yellow and brown, I think thats the wire for power. If you have power great, you have a bad valve. They are not cheap. If no power, go after the Overvoltage relay. It has a fuse at the top, and Benz has issued new relays that superseded the old ones. Thats probably going to help you out, if not, post back.
GVB