84 300SD Transmission "problem"
 

Hi,
ever since I got my mercedes 8 months ago, it shifts really hard into 2nd and 3rd, but fine into 4th gear. I was under there just now. There's a wire coming out of the Modulator valve, that isn't connected to anything. What does that do? There's also another black wire running from one side of the transmission to the other, and it's not connected either, as in it broke. Another thing I noticed is that with the car off and the tranny in park. I can still move the driveshaft a good quarter inch. Is that normal?
Thanks guys

The "wire" on the modulator valve is a vacuum line. It should connect to a rubber vacuum tee on top of the light colored vacuum device on top of the injection pump. The other wire is for the kickdown switch, located on the driver's side floor under the top of the accelerator petal when it's pressed down.

Thanks Pete,
Do you know how difficult it is to replace one of these modulators? I think there's a fault with mine, and I have one from a junk car I'd like to put in and test. Thanks a lot

The modulator can be a little tricky to replace. If you don't pull it straight out, there is a plastic "pin" that can snap off. (Don't ask me how I know) Depending on where it breaks, you may have a major disassembly on your hands.(Fortunately, 90' needle nose retrieved mine.) If it is holding vacuum, I would check for vacuum leaks in the rest of the system first.

Changing all of the flexible connections ($10-15) cured the shifting problems I had.

Sounds like a good thing I didn't disassemble without reading on it first. I was actually going to go to the junkyard and pull one off a transmission I found there. I'll check all the vacuum lines first. It shifts fine into the last gear, that's what throws me off. I'll check all the little vacuum connections first. One was rather easy to pull off.
By the way, the car is an 84 300SD.

We're talking about the same car (84 300SD). After I went through the adventure of changing the modulator, I still had harsh shifting.As in yours it shifted into top gear just fine. After reading here on this forum that the problem was probably a vacuum leak, I ventured into that territory and found the malfunction.

I am not saying that the modulator in not the problem. Just trying to save you from a lot of the headaches I went through. Once you take care of the vacuum leaks it will probably take care of your shifting problem. If not you can go from there.

right, I would go after all the vacuum leaks first. Bypassing the 2 lines to the EGR helps. One hard, plastic line comes from the vacuum supply in front of the brake booster and goes to a short rubber tube, then a restrictor, then a tee. One leg of the tee goes to the vacuum control valve, and the other to the modulator. If you do decide to remove the modulator, clean the heck out of the area first, man does that spot collect sand and related crap! Have a new o-ring handy too, they are cheap