Injectors not firing.

[threegdieselauto]

First time on a forum so bare with me. We have 1991 Gerorgie Boy we're working on. Has the Ford 53 chassis with the Ford 460 motor in it. Just replaced a fuel pump but motorhome will still not start. Checked and there is fuel all the way up to the fuel rail. Pulled injector clips off and checked for power with noid light. Noid light did not light up. So proceeded to check voltage. On power side of injectors have 12v. On ground side I'm showing 8v. At first I was thinking there might be an open or short on the ground side since there is voltage on the ground side. So I test for an open or high resistance between the powertrain control module and the injectors ground side. There was neither high resistance or an open. Upon further investigation I've read the injectors are ground side switched through the PCM. Tried out another PCM from a van that had a 460 in it also. Still no luck. Kept looking for more information and seen something about a PIP signal from the distributor also playing a part in the injector operation. So grabbed another distributor and hooked up noid lights again with yet again no luck. Also did check for high resistance at the injectors themselves. The resistance was within spec.

So my main question is what gives a signal to the PCM to fire the injectors? I've heard of a possible fault TFI/ICM but not very familiar with the ford fuel systems. Any input would be greatly appreciated as I've been searching for weeks for any information on this situation.