91 Isuzu PU V6 Fuel Pump

I was working on my friends 91 Isuzu pick-up with a v6 in it. The fuel pump quit working and I thought it was a bad pump, but hot wiring the pump and the pump runs and the truck will continue to stay running. We lost the
12v going back to the pump. The fuse and relays appear to be okay. The check engine light is on when the key is on and there are no codes stored. Is there something with the check engine light and the power to the pump.
Manual was terrible as we were using the Trooper schematic. Thanks, Dave

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