Why do MB Transmission shift so hard?
 

Just wondering, I remember these cars when they were new, and even then, they didn't have a "smooth shift".

Just wondered.

Your memory fails you - they glide when new.

Shifting hard now because the vacuum line going to it is not sending the right messages at the right times. Fix your vacuum problem and it will shift smooth again.

Mine still glides, 29 years after new. I did update a few valve body springs along the way.

They were once smooth, though the newest 123 I've ever driven was more than 20 years old. Your hard shifting is vacuum related. Vacuum controls shift quatily on these old MBs. No vac hard shifts, too much vac causes flaring. Flaring is also a sign of worn internals. Start troubleshooting the vac system. You've either got a leak somewhere, or the vac control valve on the IP is acting up and needs to be replaced or adjusted.

They made a series of running production changes to try to fix the shift quality, as I recall. I've never driven a W123 that consistently shifted right; fixing shift quality problems in one area almost always makes it worse at other times; e.g. it either flares when hot or shifts with all the subtlety of a sledgehammer to the head when cold. My old '83 had a freshly rebuilt trans and still frequently managed to shift hard, flare, stack shifts, and hang shifts at various times all on the same outing. ;) The only time either of my W123s consistently shifted well was when the accelerator was pushed to the floor; then they would bang off nice, crisp, well-timed shifts.

My theory is their heart wasn't really in it...they're a European car company, they want to build stick shifts. Automatics were probably seen as a sop to lazy Americans.

Oddly enough, the cars I've driven with consistently good automatic transmissions have all been GM products.

I all agree as well, if its shifting harsh or slow, flaring between shifts....(or Both) you have minor issues that need addressing. Mine has 200K on it...and except for a K-1 Accumulator that needed replaced to cure a flaring issue 60K miles ago....it shifts pretty darn good. So they can and do shift well if they are set up right.

Between the modulator, vacuum regulator and Bowden cable you should get it consistent and right.

Maybe off topic, but the vacuum regulator is a diesel thing only. Right?

Yes...since Diesels are unthrottled and have no manifold vacuum. And replicating manifold vacuum is a good indicator of vehicle load. Heavy load, low vacuum...light load, high vacuum. Proportional to go-pedal position.

I agree with playadude. MBs shift more like Hondas than Mopars. Shifts are purposeful but perceptible. The 4-sp Mopar FWD AT can be mistaken for a CVT.

Sixto
87 300D

CVT eeeeewww! That's just gross. Unfortunately I think the world is coming to these, what now with 8 speed automatics... headed that direction.

Or with straight electric cars, 1 speed tranny. Just spin the motor faster / slower with so much torque to burn, who needs a multi-gear ratiod transmission?

I drove one like that once! It was almost surreal. Just pure, uninterrupted torque. Reminded me of an elevator, but sideways.

You drove a Wonkavator?

Sixto
87 300D

Man, I loved that book when I was a kid. :)

The one behind my diesel was probably the best automatic I've driven in a car. Shifts came at the right time or at least pretty close to it, and were appropriately firm. Then after 300,000 miles the front seal started leaking and I took that as an excuse to replace it with a stick. To me, the worst stick is still better than the best automatic.

That said I will admit a grudging respect and possibly even a little fondness for the 3500 and 4500 series Allison automatics I've driven in heavy trucks.