Ok guys, I have a 4.9L 1994 F150 with dual tanks, the front tank works perfectly and the rear tank pump just recently went out. So i thought, eh, just get a new pump, because I miss my big tank! Well that's not the case. Oh keep in mind the front fuel gauge does not work, but BEFORE the rear pump went out the fuel gauge worked perfectly.

So I installed the new fuel pump/sending unit thinking everything would go back to normal, and all i hear when the key is turned ON, is..... CLICK and nothing. But wait, there was a change! My tank had 5 gallons in it, but NOW my fuel gauge read way past the F when it used to work perfectly before.

So i put the fuel selector switch back to front, and sure enough I heard the front pump kick on. And the gauge dropped back to the E as usual. That's neither here nor there, but just some insight.

So far I have replaced the rear fuel pump/sending unit, fuel supply relay, and then the 20 AMP Maxi fuse in the power distribution box. Oh and I have already checked and rechecked the inertia switch.

I have heard maybe the selector switch on the dash could be bad, but then wouldn't that mean it wouldn't activate the front also? Anyone have any support on that?

Are there any other ideas?

 

So the new rear pump is not working? are you sure of the wiring/grounding

 

I am sure the wiring is correct, but I am going to take the bed off and double check everything.

 

My front fuel pump recently went out. I hope after I install it, things are back to normal and its not something else. I have a new Motorcraft one on order. I didn't want to get one of those cheaper ones that requires soldering and have heard they those pumps are not very good. What brand did you get?

 

If one of the pumps run (front in this case) then the fuel supply relay, the 20 AMP Maxi fuse in the power distribution box and the inertia switch are all OK


You have an open circuit to the rear tank.
It could be the selector switch on the dash, image of one doing the same thing below.
The fuel gauge circuit is separate from the fuel pump and the only thing they have in common is the ground on the radiator support, some plugs and the selector switch on the dash.



Wiring diagram, 1994 is the same:


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x2 on Subfords advice.
If the selector switch is OK check the connector that plugs into the fuel tank for damage. My old 94 had a burnt terminal on the connector.
regards
rikard

 

Thank you for all your support! It paid off! Sorry I was quite busy until last night and... I solved the problem, well one of them. So i checked the switch, and it was good. And the connection terminal was ok also. I checked everything twice, but still no power to the rear tank. So i figured it had to be something in the power dist. box. Well when I took out the relay and put it back in, and tried to see if the pump would go, it did. And has run strong since last night. But the fuel gauge still says way past F when I flip it to rear.

Oh and it is a Motorcraft pump TexasGuy001.

So maybe subford, is there anything else that could be causing the fuel guage to act up?

 

The relay had nothing to do with your problem if the other tank pump was running OK. They use the same contact in the relay and wiring.
Only and open circuit in the gauge circuit between the switch, sending unit and ground would cause the gauge to pig. Of course a bad sending unit would do it also.

The Fuel Gauge Sender's resistance controls the magnetic Fuel Gauge's pointer position. The Sender has a resistance of 145 ohms when the fuel tank is full and 22.5 ohms when the fuel tank is empty.