I'm not following your logic, but I get the feeling you're looking in the wrong direction. You haven't mentioned anything about spark. Too many people get hung up on the first thing they find missing, which for some reason seems to be the fuel pump, then they waste their time and money in that system. You also have to check for spark. The hardest thing to check for quickly is injector pulses so we don't look at that.
If you've already gone through multiple computers, at some point it should dawn on you that isn't the cause of the problem. The best place to start is by checking if the automatic shutdown, (ASD) relay is turning on. Look for the wire that is the same color at every injector and at the ignition coil. On cars that is dark green / orange. I'm not sure if it's that color on the trucks. Probe that with a test light. You can also probe either of the smaller terminals on the back of the alternator if that is easier. Digital voltmeters usually don't respond fast enough for this.
You should see the test light light up for one second when you turn on the ignition switch. If it does, the ASD circuit is working and the computer has control of it. You should also hear the fuel pump hum for that one second. What is important is that relay must turn on again during engine rotation, (cranking or running). If it does not, suspect a failed crankshaft position sensor or camshaft position sensor.
If the ASD relay never turns on, suspect a problem in one of the 12 volt feed circuits to the Engine Computer. That can also cause a loss of communication with the scanner. Typically there are four 12 volt feeds, but that fourth one shows up from the ASD relay to verify it turned on when the computer requested it to. There should be one feed wire that has 12 volts all the time, and one that has 12 volts only when the ignition switch is on. There's a third one that is used mainly for the voltage regulator. I can't remember where it comes from, but I think the engine would still run if that one is missing.
If you need help with the wire colors or terminal numbers, one of us will dig up a wiring diagram and find them for you.
Tuesday, December 27th, 2016 AT 3:40 PM