Hi,
I tried swapping into my 1998 S420 the rear differential from a similiar year S320. The gear change was from 2.82 to 3.45. For several minutes the car was great!!! No more heavy load feel on the throttle, the car now glided up to speed. But I could tell something was not quite right. It did not give me full acceleration power, and was slow to upshift. I knew that a computer was not happy. On the highway previously 70mph was 2100 rpm and the car always felt to be reving too low and weighed down. With the 3.45 gears the car was at a little over 2500rpm @ 70mph cruising effortlessly over inclines, light on the throttle. Note; even the vacuum controlled MPG gauge was now reading a near constant 25mpg. 4mpg higher than previous. Soon after the engine light came on, and all hell broke loose. The car down shifted to second I believe, reved way up, I had to get home with my flasher lights on,going slow.
I put the car back to stock, which includes, differential, axle shafts, rear half of drive shaft. The car is now OK, back to feeling like I'm towing something heavy.
I did a little reaserch trying to find out if I could somehow re install the S320 gears. I have found out that first, the transmission computer understands thru wheel sensors the axle ratio. So when I changed that, it set something off.

I was thinking if I could install the transmission computer from the S320 to solve the problem?
Please help, I need to know what needs to be done, if it can be done.
Is it only the transmission computer or also the engine computer that needs to be altered, or some other sensor to be changed. Also what kind of $$$ am I looking at if reprogramming is called for.

I'd love to kept this black on black 56kmi S420, but if gear change is too costly, I'm going to sell it.