It's on the left of this picture. It delays the upshift when the engine is cold to get the catalytic converter warmed up faster. It's on the left fender well on the E320, I think. Not sure on yours. It has vacuum lines attached to it.
Look what else is on this diagram- the elusive Cold Start Valve and Relay that I was trying to find for another thread. Strange it's on the transmission diagram. I never thought to look for it here.