Tackometer dont work

tackometer dont work .i have converted my 6.5 deisel motor to a 5.7 gas engine.my transmission wont change gears,its a 1997 gmc dually 3500 pickup automatic transmission 4+4.left in the deisel computer and added the 5.7 gas computer and wiring harness .runs great everything works exept the tack and transmission wont change. i think they are related but i cant figer out how to hook up tack wire into the deisel harness.any body know how? any help would be gretly appreciated.im pulling my hair out on this one .thanks