I need urgent help

I have a 1996 Blazer...the problem is it won't start. I already replaced the fuel pump and nothing changed. I scanned it and it didn't show any codes. If I add a little gas, it starts but only for a moment. I've checked for fuel pulses and they are there. I've checked the connectors, fuses, distributor power, and coil, and there is power. I hope you can help me...it's a 4.3 Vortec engine. If anyone has the computer diagram and could share it with me...thanks.

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Did you check the electrical part of the ignition switch? It might only be sending power to the starter motor, but cutting off the current when you release the key.

if all this has already been checked

of the computer