Ok, I was driving to work and my truck just shut off. My truck is a 91 4.9 f150. When I tried to start it the pump wasn't turning on. So i figured it went because the relay is still clicking. I was already down a tank.

So I dropped the driveshaft and tank today, and got a new pump threw it on and it still didn't work. So I went and got the voltmeter out and put it on the harness' ground and hot, and it is only reading about 7.9 volts. I am assuming that the pump needs 12 to run.

What else could cause this? Could it be a bad ground somewhere? Could it also be the sending unit?

 

OK, so if your smart enough to use a DVOM why didn't you use it BEFORE you wasted your time and money on a pump that doesn't need replacing? This engine is injected. Did you do a fuel pressure test at the fuel rail? Pull the DVOM back out and start doing voltage drop tests from the BATT all the way to the fuel pump connector. You can work either from the pump to the BATT or from the BATT to the pump. Check the relay and fuse closely. If you know how to solder you can make jumpers to elevate the fuel pump relay above the fuse panel for testing purposes. Get 4 male and 4 female terminals just like the ones on the relay base and socket. Solder or crimp some wires between a male and female. Plug the males into the relay socket and plug the relay into the females. Now start testing the relay coil and control circuits. Don't just test to see if they have voltage, but check for voltage drop too. You have to work point to point from the BATT to the pump connector doing voltage drop tests. You need a schematic for this. You can use Alldata or Mitchell or Helm for this info. Voltage drops show up under LOAD, not during static coditions. Keep this in mind as you do your tests. You may need an assistant to help you do these tests so you can test under LOAD. Test under LOAD here means the fuel pump MUST be running during testing. The fuel pump only runs for 2 seconds so you have to have your DVOM already hooked up when your assistant turns the key to the "ON" position.

 

First off do not use a DVOM while working on the fuel system circuits as they will not tell you what is there. Your 7.9 volts is more than likely zero volts. Always use a test light with a bulb in it to load the circuit. When the bulb lights you have power and when it does not you do not have power.
A DVOM is great for working with sensors and the computer but not in the fuel and ignition systems circuits.

That all said you need to ground pin #6 of your EEC self-test connector over on the drivers fender. Then turn on the key and the fuel pump should run or you should have a steady power to the fuel system wiring.
Start at the Inertia Fuel Shutoff switch and see if you have power there. If you have power in and not out push the button down on top of it and see if the pump starts running. If no go jump it out and see if they run now, if they do replace the Inertia Fuel Shutoff switch.
If you have power out of the Inertia Fuel Shutoff switch but not at the pump check the fuel tank selector switch if you have two tanks and the selector jumper if you have one tank. I know you said you had two but this is also for others that have a problem long these lines.

If you did not have power into the Inertia Fuel Shutoff switch did the fuel pump and PCM power relays click when the key was turned on? If not replace the relay that did not click. If they both did click then replace the fuel pump relay.











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Sounds like the grounds at the neg batttery terminal. Check engine light dim?