Transmission interchange
 

Will the transmission from a 1989 300SEL fit in a 1989 190e 2.6?

190e has 722.409 300sel has 722.351

I understand that there will be differences in shift points etc, but can it work?

The two transmissions are completely different. The 722.4 is a smaller version of the 722.3

I installed a 722.3 in place of the normal 722.409 in my W124 260e.

You can get the 722.3 in, however there a few physical differences. - Length of transmission , torque converter, lockout switch, vacuum regulator, end yoke diameter, etc etc. - besides being much heavier.

I removed the 722.3 and reinstalled another 722.4 after a couple of months.
The shift points are different as well as the 2 and 3 gear ratios. The diff is different for the 722.3 compared to the 722.4. Most unsatisfactory drive result.

It's not worth screwing around with.

If the rest of your 190e 2.6 is in good shape get your 722.409 rebuilt. If your car is just so so than find another good 722.409 and install that.