Ok,i have a 1988 gmc s15 4x4 high sierra i bought it cheap body needed work but frame was good and had the v8 in it with 4.3 wiring harness,we put new box,new cab etc,but the problem is the cab wasnt a 4x4 cab and the dash harness from fuse to computer was for 2.8 truck runs great but tranny wont shift anyideas what might be the cause i am thinking the 2.8 wiring but dont know im horrible with wiring.

 

Sounds llike you have transmission issues.
In 1988 the only thing the ecm did was control the TCC and not shifts. You can buy pressure kits to engage the TCC when in 3/4 so no ecm is needed.

 

ya we checked it all out today with all our books we found out that the tranny is a 91 same as the 305 engine the guy i bought it from aparently doesn't know **** lol,still having problems with shifting thoughmwe checked it while driving with a probe and when we power up the probe we made it shift we are thinking maybe some wires or something got damaged when we flipped off the old cab,or we hope thats all it is,if not i am going to just put a 700r4 non electric tranny in and be done with the problem