First off Hello to everyone! I'm new to this forum. I have a 1990 Blazer fullsize and my fuel pump won't shut off... it runs at about 8v with the ignition off. You can hear the pump run very quietly all the time and when you turn the ignition on the pumps revs for a couple seconds and goes back to running quietly. I've checked the fuel pump relay and its fine. I put a switch on it and that works for the time being, the truck runs great. Does anyone have an idea where I should look next? This is very frustrating!!!!

 

It looks like you have a problem with the fuel module. This module was put on the heavy duty models and would power the pump for 20 seconds and then shut off if the vehicle was not started.

If so equipped, the fuel module is located on the outside of the brake pedal mount and will have three wires going into it; power, ground, and feed out to the fuel pump.

If you do not have the fuel module, the relay may not be disconnecting properly or there could be a short in the wiring to the pump somewhere between your switch and the relay.