Stumped on dreaded auto trans shifting issue
 

Hey all,

New to the forums, first mercedes owner... Hi ya!

Picked up an 81 300D real cheap with some issues. I'm trying to work on the shifting. As soon as I picked it up it simply didn't shift from 2nd-3rd unless you floored it and built up RPM. I drive it around town this way as it's the only way to stay ahead of traffic... anyways

I did some research, messed with the vacuums.

I found out that it does this late shifting when the whole vacuum system is unplugged from the brake booster (so without the use of any vacuum into the modulator).

If I plug in the modulator directly to the brake booster output (bypassing even the VCV), the opposite shifting happens (lots of vacuum to modulator)... this time it shifts super early and gets slippy. Long story short... it seems that messing with the amount of vacuum does in some way affect the "when" of my shifting issue, from one opposite to the other.. from shifting late to shifting early and staying... so now...

I've isolated all the vacuum controls and routed the CVC valve and the modulator directly to the brake booster. When I plug in the VCV into the equation, it seems to work on the gauge.. and the vacuum seems to bleed when the throttle is activated (see video).. all good.. should shift better, right? Wrong. Now it shifts late again, as if it were all unplugged - which means too little vacuum and/or at the wrong throttle/ratio, right?

CLICK HERE FOR A LINK TO THE VIDEO - this shows the vacuum reading going to the VCV valve with the car running. If this happens and it's shifting late... i would need more vacuum to make it shift earlier? Lighten the throttle/bleed ratio from the CVC?

Or perhaps the modulator? When I mityvac the modulator from above, it seems to hold vacuum but bleed consistently... from 15-5 in some 15 seconds.... Am I looking to have to take a look at the modulator? I've also heard about adjusting the modulator... to change the amount of vacuum it holds? maybe that?

I guess in the morning I'll mess around some more before taking a closeup look at the modulator...

youre ready for this. the final word on old merc vacuum http://www.peachparts.com/shopforum/diesel-discussion/158216-its-critical-how-you-set-your-transmissions-vacuum-system-your-diesel-mbz.html

First thing I would check is that the modulator holds vacuum.

You have posted two threads with the same question. Your first thread has received responses already.

So please choose which thread you want us to read. We will respond there.

For now I suggest you do nothing until you read the other responses.