Ford Repair: 91 F150 fuel pumps, dual fuel tanks, inertia switch

Question
Hi Dennis,

I have a '91 F-150 with a 300 and dual fuel tanks.  Neither of the fuel pumps will run.  I have checked it as far as the transfer switch.  If I am correct Ford switches ground for electrical control.  I am reading ground to the transfer switch with everything connected and 7 volts with the switch unplugged.  I would think that it would read 12 volts open circuit but…….  Can you tell me if this is correct and perhaps a suggestion as to where to go from here?  

Many thanks,

Fred Schroeder


Answer
OK, Fred here is the message I sent you the first time. Apologies are not necessary for your previous comments. The system shows that I sent this April 10 at 3:29pm.




Here is a diagram of the EEC connector under the hood. With this you can keep power to the fuel pump circuit for testing. When you turn the key ON the pump will only run for a few seocnds if the engine doesn't start. By jumping this connector terminal it puts the fuel pump relay into the ON position and keeps it there. From here you can follow the circuit and hopefully find where the interruption is.

Well the diagram won't copy/paste! This is how to find the right terminal: Hold the connector with the wires away from you. There should be four wires on the bottom, two on the top row. The wire on the bottom row on the short side of the connector is the one you want. I think in the nineties there was only one wire on the bottom at either end. I don't remember the wire color, sorry. When you ground the correct terminal you should hear either a relay click, the fuel pump run, or both.

System Diagnosis
Note: Grounding the FP lead at the DLC will allow the pump to run continuously with the ignition switch on.

Fuel pump relays were a common failure. You might also check the inertia switch, that is either behind one of the kick panels, or on the firewall below the heater box. make sure the button on top of this switch is down and stays down. You can test it by checking for voltage at both terminals with the key on, and the connector grounded as I described above.

You are correct, you should have battery voltage at the pump selection switch. Make sure you are not checking the yellow/white wire, that is for the fuel guage input.
I think the fuel pump wires should be orange/black, pink/black, or yellow/black. I don't have the schematic here at home.

I wish I could provide you a diagram, but I don't have one to copy. On your truck I believe the selection switch changes the voltage to the correct pump. Otherwise you are correct that Ford uses ground side switching alot.

Goodluck, let me know if you get confused!