I've always called it the Idle Air Control Valve (IAC) but Mercedes calls it the Idle Speed Adjuster. The rubber diaphragm in my unit had failed and was causing a high-idle situation. It could keep a good idle when cold, but as the engine reached operating temp, the idle would creep up to 1000rpm.
I spent quite some time narrowing down the fault to the IAC. I tested the Idle Control Module, the Over Voltage Protection relay, the CIS coolant temp sensor, Throttle Position Switch and all the vacuum connections. I just hated the idea of buying a new IAC on a guess. I eventually found the fault to be the IAC with a little test I came up with. It's not very scientific, but it works.
Let the engine warm up (allowing the idle to go up to 1000rpm). If your idle is much higher than this it could still be a problem with your IAC but this test won't help. Once in the high-idle situation, quickly switch off the engine, disconnect the valve and spray a little silicone lubricant into it and reinstall it before the engine temp drops. If the idle immediately goes back to a more normal 650rpm, then you need a new IAC.
The dealer quoted me something like $600 for a replacement. I bought an OEM replacement made by VDO. Although about $300 cheaper, I was still a little annoyed that a VDO unit was only $200 in the U.S. Thanks Canada.