1994 Jeep Cherokee changing the pcu and the procedure to do

My 94 grand cherokee had a problem with the fuel pump not getting the signal to pump. I already changed the fuel pump, preasure regulator, relay, filter. Nothing changed so I put it in the shop where they diagnosed it as a bad pcu not giving the fuel pump the proper ground. So we exchanged the pcu for one with the same exact number ( from a wrecking yard) and it seemed to fix the ground problem because the jeep started rigiht away, but then it just shut down. Is ther something that needs to be done for the pcu to accept the signal from my jeep? Am I missing or forgetting to do something. Would I have the same problem if I had bought a brand new pcu? Signed desperately going broke fast!