Hello, I swapped the transmission from a '95 Deville into an STS of the same year. Yard said they are compatible. After the swap the computer is throwing a code, "Box Ratio Unknown". Sorry I am home now and can't remember the code number. Anyway this code when thrown disables traction control and also shuts down the trans pressure control solenoid. The STS shifts and drives perfectly accept that it really slams into gear from park to drive and reverse. Likely from the pressure solenoid being disabled. Okay so I assume that the two transmissions have diff gear ratios. My question for you all is whether or not there is some fix for this issue. Maybe the dealership can reprogram the computer to accept the new ratio? I thought about just swapping computers but this of course is not an option between these two models. Maybe there is something I can do under the trans side cover? Swap valve bodies? I dunno. I am really not interested in pulliing this transmission out again. Any thoughts?

If I am not mistaken, the differential that your STS came with was geared at 3.71, and the Deville was 3.11. Obviously, having your computer flashed would be the easiest fix. Whether or not that's an option, I'm not sure. If not, you will have to replace the differential. I'm not a mechanic, but I think it may be possible to do so without pulling the transmission from the car. The differential is located in the extension housing - at the "output" end of the transmission.