I'm not for sure on a 1688 but would guess it's close(but harder to get at compared 2388 with the gear shift being in differnt locations). So, I would guess there would be a little more work to unscrew the cover around the "H" pattern or perhaps it could even be below the cab on 1688. Anyway, on a 2388 it's below where the lever goes down below the plastic cover in the cab. When the lever is in the 3rd gear postion, there is switch or sensor that can tell it's in 3rd gear. I don't really remember what it looks like off hand, it been a few years since I have messed with one. Just have to disconnect to get 2 speed to work. Watch long hills as hydro oil light may come on. You might get 25 going down hill or if you have larger tires but with 30.5-32s the best we could cruise at was about 23 mph. It does make a big difference, although 2 hrs roading a combine is still a long ways to go.
