85 300D shifting problems Can see loose vacuum line

[Silber Adler]

This just started suddenly Won't shift till 4500 rpm.
Inspection showed loose vacuum line coming from vacuum modulator on fuel pump.

Can't seem to find out where it fits.


It looks like it goes through a port into the passenger compartment. I can slide it through a fitting in the rubber grommet (along with several other lines) but can't locate any rubber line there for it to plug into.

It goes to the transmission, of that I am sure.

Where does this line go?

[toomany MBZ]

Yes, it does terminate inside the cabin.

It is used as a vacuum bleed down. The VCV uses less vacuum as the rpm increases. Clean air inside.

Usually the shift points are controlled by the Bowden cable. It is adjustable. On top of the cam cover is an accordion covered cable. There is a white nut at the top, adjust that. The other end connects to the transmission.

EDIT: Appears as if it's time for a brake fluid refresh.

[Silber Adler]

The point at hand, the vacuum line goes into a rubber tube? Or a module?
The bowden cable and the vacuum control didn't both fail, so the cable is fine, right?

[Zacharias]

Originally Posted by Silber Adler

The point at hand, the vacuum line goes into a rubber tube? Or a module?
The bowden cable and the vacuum control didn't both fail, so the cable is fine, right?

That vacuum line is an open vent, it doesn't connect to anything. On many cars it's completely missing by this time (as on both of mine). The original intent was to run it to the passenger compartment to avoid any crud being sucked up from the engine compartment into the VCV.

A Bowden cable failure usually produces the opposite of what you describe (car wants to shift through all the gears way too early).

Assuming you have carefully checked all the vacuum connectors for wear (splitting, looseness) and other loose lines, I suggest you get a vacuum gauge and check your vacuum pump output:
Mercedes Vacuum Troubleshooting

If that is okay then try this:
Transmission flaring or clunking? VCV DIY - Mercedes-Benz Forum

The line to your transmission would be running off one of the splits coming from the top of the VCV. I have an '85 as well and I am pretty sure the way your vacuum is plumbed bears little resemblance to mine. You may want to look online for a vacuum schematic and check on that.

[toomany MBZ]

This may help, post #6 third pic is the second attachment.

Vacuum lines rebuild 1985 300d

[Silber Adler]

It was the switch under the fuel pedal. Everything had normal vacuum and I could see the vacuum bleed down as I adjusted the throttle linkage.

I read here that someone had these symptoms with a stuck switch under the fuel pedal. I disconnected it and all is right again.

I probably can't down-shift by applying full fuel but never do that anyway.

Thanks to all that wrote me about this.

[Zacharias]

Congratulations. You are the only person in living memory who has actually been able to confirm a stuck kickdown switch causing those symptoms.

It has been often cited as a possible cause, but no one seemed to ever have a thread where it was the "eureka" cure to that problem.

Okay, Cooljay, you were right on this one, I admit it.

[Silber Adler]

We were on the road in our trusty 86 vdub diesel. My wife was reading your posts and did some research and found the post about the stuck kickdown. When I got back I did the diagnostics and got normal normal. So I thought why not. Just unplug the switch.

[Silber Adler]

Here is the thread that shows the correct answer, Cho nails it.
shift switch