Vacuum Line Under Dash Not Plugged into Anything!

Common concern on 1980 to 1985 Mercedes Diesel 240D, 300D, 300CD, 300TD

Over the years we have received many emails that go something like this: " I have one single black vacuum tube coming from the engine compartment, through the firewall about 10 inches just above the steering column. It is connected under the hood to a "Y" connector. However, inside the vehicle it is connected to nothing. I took a picture but do not know how to send it to you. What is it suppose to attach to? Thanks! John"

Play Video

Problem & Solution

Common among these chassis:


Most owners are very surprised to learn that this black hard plastic vacuum line does not connect to anything!  (see picture above). It is actually a "VENT" line for the VCV (vacuum control valve) that sits on top of the back of the diesel engine injection pump. The VCV valve controls the shift quality of your automatic transmission and it is designed to "bleed off" excessive vacuum to keep the transmission from "slipping" during rapid acceleration. 

If it were just vented in the engine compartment the surrounding air turbulance created during high speed driving could create "back pressure" to the valve - thus preventing the valve from bleeding off the correct amount of air. So the engineers decided it would be best to vent it inside the passenger compartment of the car. That decision has proven to be a good one as venting the VCV to static air allows more accurate adjustment of the VCV with corresponding smoother and more consistant transmission shifts.

If you would like to learn more about your VCV valve and how to adjust it please refer to the related products to the right: