1999 Other Mitsubishi Models NO ELECTRIC OVERDRIVE SHIFT

VIN #JW6AAF1H3YL002649 / TRANSMISSION #8I0369.
This is a 1999 Mitsubishi Fuso cabover truck with a diesel engine & 3-spd automatic trans with an electronic (push-button) overdrive gear. The trans wasn't shifting right and the speedometer wasn't working so I replaced both the cable gear as well as the internal shaft gear (both were bad) and the speedometer works fine. The electronic overdrive button is located on the shifter handle and the dash light indicates whether it is in on or off position; the light is working. The transmission shifts normally from low to second to high gear but then won't shift into OD; truck will run around 50 mph but engine rpm is 3300 and dash gauge shows redline at 3500 rpm. Have a thought that the speed sensor might be bad but don't have any diagnostics to check nor do I know what a good sensor should ohm out at. Need suggestions or diagnostic procedure to find out what the problem is.