Vacuum switch on 1985 300D
 

I've been tracking a Climate Control issue with my 300D. During the check of vacuum lines I found a small black line leading behind the brake booster was not connected. Behind the booster there is a small switch with an electric two wire connector at the bottom. It has a small tube that sticks straight out from it and an odd looking little cap on the top. I used a piece of spare tubing as a sleeve and connected the loose black hose to it. I didn't see any change.
Question is, what the heck is that switch/valve and what does it do?

I believe that is the switchover valve that should have one plastic line going over to a fitting on the back of the intake manifold and the other going to the ALDA capsule on the back of the injection pump. Without it connected you don't get the increased fuel when the turbo kicks in. This line also gets plugged quite often close to the manifold.
Try posting over on the Diesel Discussion section for more help.