put police trans in car

The donor may have been bad and if there are electronics then you may be over your head , , I would have the original rebuilt with performance up grades , There ain't no cheap way out

A live data scan should reveal the problem, but this sounds like a bad speed sensor at the transmission.

Check your electrical connections as well. Maybe one got missed and just needs to be plugged in. Or maybe one isn't plugged in all the way. Hope that helps! Jim

Some of those transmissions have different pin locations coming off the transmissions. Compare the old connector to the new one and see if all the pins are the same location