Harsh downshift from 4-3 W126 420SEL
 

What could be the reason behind a harsh downshift between 4th and 3rd on my 1990 420SEL w/217k miles or so? All the other gears shift fine up and down.

It is possible to do a smooth downshift if you are barely touching the throttle and manually shift from 4-3 with the lever. If you're doing 60+mph and you plant your foot to the boards, it jerks the whole car like a shotgun has gone off! Kickdown from 3 to 2 is perfectly smooth at any speed, it's just 4 to 3...

Any ideas?

Cheers, Martin

Its very common.....usually a vacuum control issue. My car has done it for 70k+....though it will be getting a new trans soon, we'll see if that one does it too...

As it's a common issue, what can be done about it? Has anyone gone through the procedure of fixing it, and if so, what did they do? Are there any prime candidates for faulty components etc.?

Cheers, Martin

Tired seals inside the transmission are the most likely cause of this problem. I had my 380's transmission rebuilt at 232,000, primarily because of harsh (manual) 4-3 downshifts. It helped, though I must say the problem is not entirely solved.

Another area to look at is the integrity of your driveshaft's flexible couplings and the center bearing support. Replacing those items on my SEC improved the 4-3 downshift dramatically - no more harsh clunking. I've almost finished the same job on the 380 and it'll be interesting to see if that helps.