About 500 miles ago my tranny started to shift like an old muscle car with a shift kit. Shift timing is as it always was but it is crisp to the point of being annoying. There is no flaring or other discernable bad habit to note.

This happened all of a sudden so I'm thinking it's a vacuum issue. The hose from the intake manifold to the vacuum modulator on the tranny was replaced to account for a possible failure there but nothing changed. I moved on to adjusting the modulator to soften shifts but the little key turns as if it is acting on nothing. I can turn it in either direction any number of turns and it has zero effect on shift quality. On top of that, the key will no longer settle back into the crowned recesses before replacing the rubber cap.

I'm thinking the diaphragm has expired allowing something to occupy the last little bit space that the key nestles into. Any thoughts on this issue? Can the modulator easily be replaced?

The car has 256k miles and has always run like a sewing machine. I literally grew up with it as it was my mothers car so I know it like the back of my hand. This will be the first work the car has required outside of scheduled maintenance in its entire life. I feel as if it may finally be broken in

Sam

Hi Sam ,what i think might of happened is when they replaced the modulator that they broke the plastc shift mechanism/finger or what you call it that is inside the tranny.Once the modulator is removed the plastic extension/finger usally comes out to,and if someone did not pay attention could of broke it or not reinstalled it.To remove the modulator it is a bit tight but it is duable. Harald

Sounds like you've made some very good observations. I'd agree that it sounds as if the modulator has given up the ghost. Just keep in mind, if you plan on making the repair yourself, that the transmission on these cars needs to be lowered, in order to replace the modulator.

__________________
2000 c280 Sport (looks like an upgrade, feels like a downgrade)
1991 190e 2.3 (sold)

If it is the diaphragm you would be burning transmission fluid and puffing white smoke out of the tail pipe. Do you have vacuum at the modulator hose, at least 17 inches. If so then it could be the modulator or a stuck pressure regulator valve. If you what to change the modulator do not buy one base on the color code of yours, it can be the wrong one. Order one by the 722.xxxxx number stamped on the passenger side pan rail just bellow the BI, front piston. Once installed it has to be adjusted, the right way is with a pressure gauge, but you can use the in the field method. Turn the T handle CW to raise the pressure if you have any slippage or flair up on hard acceleration.

You may have to remove the transmission cross member and drive shift to access he modulator. Remove the two Allen bolts from the modulator retainer and then carefully pry it from the case.

I just watched my mechanic change this part in my 190d 2.2 this past weekend. He put a vacuum gauge on it first to see if it holds vacuum. It didn't. Then he raised the car, and then removed the rear transmission mount and unbolted the flex disk. He may have removed the shift rod; couldn't tell. Then he pried it down, and put a 2x4 sideways between the car and transmission to hold it in place. This provided a lot of room in which to remove the part.

He replaced it with one of the same color.

John Cacavas
Los Angeles.

There is no smoke and the fluid level is static so based on C32AMG's note, it's probably not be the diaphragm itself. I have not actually measured vacuum at the modulator hose but I can see now that I should have done that first....before replacing the hose. That may have been $17 wasted.
I will test vacuum tonight and report back with the results.

The issue of not being able to fully insert the key into the modulator still bugs me, though. The modulator has never been touched, except for the quality adjustment key, so I'm assuming that no outside force has damaged the plastic shift mechanism inside the trans. Any more thoughts on this, specifically?

Thanks,

Sam

I've tested the modulator and it holds vacuum as long as one could want and there is no fluid getting past the diaphagm.

I also took her on a long drive and made a point to encounter as many driving situations as possible. Looking back on it, I feel lucky not to be sporting a citation from the local enforcement I found a way to make it shift perfectly smoothly every time. I know the car well enough to predict every shift within a half second or so. If, just before an upshift, I back off the throttle and effectively slam the modulator with a heavy dose of vacuum, it's perfect. Downshifting on decel is equally smooth. Again, a high vacuum situation.

So, it seems that low vacuum, when the throttle is opened any appreciable amount, directly coincides with the stiff shifting. The only thing my pea brain can latch onto based on this is the modulator.......even though it holds vacuum just fine. Are there any other failure modes? I probably should just replace the thing but I really hate to just pitch a part at the problem even if it is only $50.

I know there are some great Benz tranny tuners out there. Any and all ideas/ suggestions are appreciated and will be acted upon.

Sam