I have worked on quite a few chevys and can tell you what I've found to work with this type of problem. When you replaced the starter, did you tug on the fusable links that connect to the solenoid to see if one of them was burned and loosing power? There are two or three short pieces of fusable link attached to wires that power the fuse block and other electrical circuits at the solenoid / battery cable connection that will stretch when you pull on them if they are bad. If they are, then you won't have power for the fuel pump, relays and other circuits to make the truck start.
If one is bad, cut the wire behind the link connection and put a **** splice and piece of wire on it with a ring terminal,and connect it to the solenoid. Turn on the ignition and listen for the fuel pump to buzz. The fuel pump is controled by the ECM and makes the pump turn on for 3 seconds to prime the system with fuel before starting the engine. One of the wires on the solenoid with a fusable link does this. If this works, your done.
If the truck won't crank when you try to start it, check the "S" terminal for power with a test light while someone turns the key. The "S" terminal on the solenoid comes from the ignition switch, to the neutral/backup / clutch switch to the starter. You'll have to check every point in between these for power to locate the problem. One bad connection or part will keep the starter from getting power to the "S" terminal. You can use a screw driver to test if the starter is working by touching the two terminals together.
While your under the truck, look at all the wires in the harness for any chafing or rubing, I once found a wire that rubbed through and was causing the problem.
Hope this helps, Craig
Confused as to why you are replacing things that crank the engine over, like the starter, and things that are just for fuel, like the pump?
Can't tell if it is cranking over or just not firing?
But if it is not cranking, then consider the ignition switch, neutral safety switch, engine ground strap, and battery terminals.
If you think it is not getting fuel, then try some starting fluid, to make sure it does spark at least.
If it does have spark and cranks, then you should put a gauge on the fuel rail. If there is no pressure when you try to start, then check voltage at the pump with a test light.
If none, then follow back to relay.
If none there, then try jumping at the relay, just for testing.
If that works, then it could be safety cutout system.
Some vehicles use an inertial switch, some use oil pressure switch, some use alternator feedback.
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