Cadillac Repair: 2000 seville fuel pump, fuel pump relay, blown fuse

Question
I just swapped the relay for another so the relay is good, the fuse is good, the fuel pump is definately not running, and the fuel pump is definately only getting 5 volts (and the battery is fully charged).  My question at this point is, is it supposed to get 5 volts and the pump is bad?  or is it supposed to get 12 volts which would mean that there's a bad ground or some other bad link in the system?  I'm thinking it's the latter... and if it is, any ideas on where to look for the problem?


2000 Seville STS - I ran the gas tank bone dry and when I re-filled it, it wouldn't start.  Fuel pump isn't pumping.  Found a blown fuse for the fuel pump and replaced it.  Fuel pump relay clicks on and seems to be working.  Not sure which connector is for the fuel pump but I'm guessing it's the one with 3 wires.  Only getting 5 volts at the pump when I turn the key on and/or crank the engine over.  Shouldn't this be 12 volts?  Are there sensors / relays I should check?  Should I check the computer for a problem code and how might I do that without a GM tester?

Thanks,
David

Answer
It should get battery voltage at the gray wire. when you removed and replaced the fuel tank, did you hook up the single black wire that comes from the center of the gas tank to a suitable ground or did you just break or cut the wire off. It needs to be grounded to the frame.

Fuel pump test:

1. Get yourself a helper.

2. remove the gas cap.

3. have one person listen in the gas tank at the gas cap opening.

4. have the other person turn the ignition switch to the ON position ONLY so as the dash lights come on. Do NOT attempt to start the engine.

5. As soon as the ignition key is turned to the ON position, The person at the gas tank should hear the fuel pump run for TWO SECONDS and then it should STOP.

6. Does it ?.

7. If it does NOT run for the 2 seconds the fault is either the fuel pump or fuel pump relay.

As soon as you turn ON the ignition switch to ON position, Just before you turn the key to the start position when the engine starts to crank over, A 2 second battery voltage signal is sent from the computer to the fuel pump relay and then from the relay back to the fuel pump in the fuel tank. This runs the fuel pump for ONLY 2 seconds of which pressurizes the fuel system in preparation to start the engine.

Once the engine starts, The computer BYPASSES the fuel pump relay and  sends constant contionous battery voltage from the computer to the oil pressure switch of which is now CLOSED because the oil pressure is UP because the engine is running. Therefore the battery voltage flows thru the oil pressure switch back to the fuel pump to run the fuel pump constantly until you turn OFF the engine.
Therefore if your POSITIVE you are hearing that 2 second pump run just when you turn the ignition switch to the ON position ONLY, just before you turn the ignition key to the start position, I would have to say  you have a weak fuel pump upon initial start up.
If you are NOT hearing that 2 second fuel pump run, Then I would say that the fuel pump relay or the fuel pump is defective.