I recently bought an 82 300D. I have been having varying shifting problems. I would have the usual 1-2 hard shift then depending on how much I depressed the accelerator pedal I would have either a smooth shift into 3rd or a large flare of 1100 rpm.

So I learned to trouble shoot the vacuum system. I started by replacing the switchover valves on top of the valve covers and ensuring that the little vacuum hoses were routed correctly and in good non leaking condition. Mine were in good condition but routed incorrectly. I learned to ref the diagram next to the radiator.

Then I checked the condition of all of the vacuum lines under the under the hood.

Next was the vacuum modulator on the transmission. It was in good shape. I learned that this determines the quality of the shift in the different gears.

I also looked at the kickdown cable. Mine was recently replaced and noted that this determines when the car will shift gears or downshift under hard acceleration.

So now was the vacuum valve on top of the injection pump. After a lot of researching and talking to other mechanics I determined the reason of the varying shifts was due to the amount of vacuum being presented to the transmission. This comes from the vacuum valve on top of the injector.

There are two connections on this valve. The main source of vacuum comes into the top of this valve. As you depress the accelerator this valve dumps vacuum to the line coming out of the bottom of this valve of which one line runs to the switch over valves on top of the vavle cover. These valves do play a part in determining the quality of the shifts. The other line is really there to work with the egr valve.

Also of note, there is a black line that runs from the bottom of the vacuum valve into the fire wall. This line connects to nothing and is vent line. This is where the vacuum is released or really ambient atmospheric pressure is allowed into the system. When you are driving and if you have the lower panel removed from under the driving side of the dash you can hear the hissing as air is sucked in. Well mine was plugged.

I also read Steve Brotherton's aritcle on transmission adjustment by way of tuning the vacuum system. He mentions making adjustments to the system but doesnt go into detail on how to adjust the vacuum valve. There is linkage that is connected to the throttle linkage. Mine was fine. I figured that as long as the range of motion on the valve was the same as the throttle linkage that was good. But I decided to take the valve off the car and open it up. What I discovered was that you can adjust the tension on the main spring and this will determine how quickly the valve releases vacuum.

I connected a hand held vacuum gage to the top of the valve and adjusted for mid throttle to have about 2 in of vacuum. The way I did this was to keep actuating the handheld vacuum pump and hold the valve mid way open. It's not perfect but works. Mine was originally had 13 in of vacuum at mid throttle. This is way too much for the transmission. I believe the spring had lost tension over 20 years. You can ref Steve's article to get the specifics. Anyhow, I put the valve back together and reinstalled it.

The tranmission shifts flawlessly now. Learning the dynamics of the vacuum system and how it affects the transmission was a huge learning curve for me. I am also very proud of myself because there were quite a few european mechanics that could not tell me how the system worked. I also saved massive amounts of mechanic labor and the cost of parts that I did not need to replace, which would be the switch over valves, transmission vacuum modulator valve and the vacuum valve on top of the injector pump.

I hope this helps someone else out. You can email me at gwehrung@earthlink.net

 

I guess you've talked to the wrong mechanics (or you have been in the wrong workshops)?

 

bought a great book from "Mercedes Source". Goes into some great troubleshooting for vacuum leaks. Price is very reasonable.

mercedessource.com

wished I had it earlier with all the other problems I had.