Transmission thump or bump accelerating out of a turn
 

Hello,

I have a 93 400e with 76,000+ miles and sometimes when slowing for a turn then quickly accelerating the transmission thumps or bumps. It will also do this when slowing in a straight line to below 10 mph and then accelerating again before actually coming to a complete stop. I replaced the fluid and filter roughly 4000 miles ago hoping to cure this. The ATF was drained from the pan and from the torque converter. All other times the transmission shifts properly. The "firmness" of the shifts are consistent during the 1-2, 2-3, and 3-4 shift. From P to R, the engagment is smooth, no delay. From R to D, no delay but engagement bumps a little bit.

Any insight would be appreciated.

Thanks,
Nolan

This is a common problem. Mine does the same thing but at about 20 mph. I don't recall the exact cause but if you do search you may uncover additional info.

In my case, I just try to avoid hard acceleration when coasting at that speed. Mine's been doing this for 30,000 miles.

Did a search on "harsh downshift" and here's one of the threads:

"The complaint: , quoting
"Harsh 3-1 or 2-1 dowhshift when driving slowly at 12-15mph, and abrupt 80% + throttle application."

The Remedy:
"According to ALLDATA is to install a modified Plate-type check valve with throttle (13 in the illustration) , P/N 124 270 02 89, install into shift valve housing of valve body. (the modified/improved plate valve is red in color.)""

Thanks for the information. Would you consider this a DIY type job? I don't want to attempt it if springs or other small pieces will easily be lost.

Other than this modification, are there any other ways to finesse the transmission externally? We're these cars like this since new, or was it just a poorly designed check valve that just fails?

Thanks again

Nolan

Good questions --

Mine started doing this at about 100,000 miles -- or at least that's when I noticed it. If it had done it while under warranty, I would have had the dealer fix it.

Don't know for sure about the ease of doing this. On other cars I've worked on, taking off the valve body wasn't too hard, though you have to be very careful to keep everything exceptionally clean.

Check your front (and maybe rear also) flex discs. This sounds exactly like what was happening to my car as the front disc started to get worn (About 70K miles)

Not a bad thought to consider flex discs, though they usually last longer than 70 k miles. Easy enough to differentiate. If it's the transmission, as I suspect, it's due to a rough downshift. Watch the tach as this occurs. Are you downshifting from 3-->2 when the thunk happens?

The thunk happened on the 3-2 downshift. I've looked at the flex discs and they are intact with no cracks.

I played with the modulator increasing the pressure by turning it clockwise 2 full turns. It seems to have helped it. I also made sure the modulator is holding vacuum. I've driven it a while intentionally trying to reproduce the "thunk" and it has only happened once under full throttle coming out of a turn. Driving normally has not caused it to happen. The shifts are slightly firmer, but not harsh.

I'll continue to observe and report back.