Lousy Shifting - Where to Start? - Page 2 - PeachParts Mercedes-Benz Forum

Diseasel300 · #1 01-29-2017, 09:09 PM

The weather is improving and days getting longer and the time has come to tackle the problems of truly crappy shifting in my SDL. Spent days reading this forum, watching videos on Youtube, reading other forums and repair articles, visiting a certain "source" website, etc, and I've come to the conclusion that I have no idea where to even start with this car.

Car is an '86 300SDL with the 722.3 transmission. For all intents and purposes, let's assume all the hardware on the transmission is what it came from the factory with (the rest of the car has been that way), and that someone has taken the time to change fluid and filters at some point in the history of the car (no I don't have any records to prove it). ODO says 173K, but had the famous gear failure, so I suspect more like 225K at least.

What it's doing:

1-2 shift is late (~4200 RPM with light throttle) and VERY firm and abrupt *WHUMP*.
2-3 shift is late (same RPM as above) and acts lazy in engagement. Kind of like it makes the shift, then firmly engages. RPM doesn't flare and this sort of 2-stage shift happens regardless of load or acceleration style.
3-4 shift is late (>4000 RPM) and varies in firmness, but usually on the firm side.

When the accelerator pedal is hard down (floored, but not engaging the kickdown switch), the engine will accelerate to ~4600 RPM (governed - more on that in a minute) and have a delay before shifting gears. It does this for all upshifts in a WOT condition or when the pedal is more than 1/2 way down.

Downshifts are simply ridiculous. The transmission will not downshift under load unless the kickdown switch is pressed and held, then will not shift up again until the kickdown switch is released (the SL doesn't do this, if the KD switch is engaged, it'll shift up anyway if the RPM is high enough). The quality of downshift is extremely abrupt (it almost feels like someone's ran into the car). On occasion, the downshift occurs, then IMMEDIATELY re-upshifts.

I suspect I have several issues going on here. I have fluid and filter change on the list, but want to hold off on it until I know if there's something I need to be servicing inside the transmission while the pan is off.

The fluid is bright red and doesn't have a burnt smell. When stone-cold in the morning, shifting into reverse takes less than 1 second, with a firm "whump" on engagement. Same when shifting from R to D. Once the transmission has warmed up (5 minutes driving or so), the shifts into R and D are nearly instantaneous. This leads me to suspect that the mechanical health of the transmission is probably OK.

My car has the injection pump from a 350SDL in it. As a result, the redline RPM is lower than stock. I have a feeling this has a lot to do with the ridiculously high shiftpoints, basically at the rev limiter of the IP (~4700 RPM).

I assume the bowden cable needs adjustment for one thing. Would turning it in help lower the shift points to a more acceptable level? If I give the engine very little throttle (like resting my foot on the pedal to take off), it seems to shift nice and early, which is how I'd expect it to do with light throttle. The 2-3 shift is still crappy and slow though.

Additionally, I suspect that the modulator may not be working. I've played with the VCV on the side of the injection pump and no position made any difference. I even unplugged the vacuum line to the transmission and it made no difference.

I've read a lot about piston and spring replacements in the valve body as well. Are the symptoms above indicative of a piston with worn spring?

Issues with the "flying saucer"? Switchover valve? VCV?

It's important to note that the transmission doesn't slip and I've never experienced a "flare" in RPM during a gear change. 2-3 is a bit weird, but it doesn't ever "flare". I also have no leaks or drips. The pan gasket has some "sweat" around it, but nothing on the floor under the car (surprisingly the engine doesn't drip oil either).

Help!