There seems to be a ton of threads about fuel pump issues but none of them describe my problem. Please share any helpful info or threads that can help me get to the bottom of this.

Vehicle: 1990 F250 4X4 with 5.0L and dual fuel tanks

Problem: My rear tank (gauge and pump) works fine. Up until a few weeks ago the front fuel pump worked fine but the gauge would not work. The gauge always showed empty when the front tank was selected. Upon disconnecting the front fuel pump wiring harness the fuel gauge swung all the way to full+. This led me to think that the fuel sender in the front tank must be bad. I dropped the tank, replaced the fuel pump and sender with a Spectra complete unit. After completion I had the same result (front fuel pump works, gauge still says empty). Then, about 2 weeks later, the fuel pump stops working entirely so the truck only runs on the rear tank. I assumed it was a bad part so I dropped the tank again. I tested the pump (old one and new one) with a multimeter and it seems like the sender and pump should be working fine. In other words ohms change when I move the sender float.

I am guessing it must be some king of wiring issue or something. I couldn't find any wiring issues.

Has anyone ever run into this before? I don't think it is the relay or switch because rear tank works great.

Any help is appreciated.

 

The fuel selector switch has two sets of contacts. One for the rear tank, the other for the front. It could very easily be the switch or the wiring between the two. BTW...there is no relay in the circuit(s).

A simple ohm test between the common output contacts and the front tank contacts (with front tank selected) will tell you if the switch is good or bad.


A diagram from Subford

 

“Upon disconnecting the front fuel pump wiring harness the fuel gauge swung all the way to full+”

I believe this is normal.
Did you disconnect the harness at the tank?

 

RLA2005, thank you - info and diagram extremely helpful. I will post what I find out.

VJsimone - yes, I disconnected the harness at the pump and then the gauge swung all the way full.

 

Well, I pulled the tank selector switch and ran the ohm test. It seemed funny to me as nothing was grounded on my problem tank side when selected. Changed the switch, fired it up and I've still got the same symptoms. Front fuel pump and sender don't work.

​​​​guess I'll start looking for a short in the wiring or something.

 

One down. On to the next step.

 

Did you make the pumps run as well ?

 

Sure did. Hooked them both up to a battery and both pumps ran fine.

New update. I removed the new fuel pump and sender unit to inspect it. While it was out I hooked it up to the truck and the f#@ing gauge works!

Reinstalled it in the tank and fuel gauge is magically working. Now I'm confused.

Tried to start the truck but that tank still isn't getting gas. Wonder if I have to fill up the tank to prime the pump? Hate to install the whole thing again and fill it up with gas to test that theory.

 

When you said "Empty" did you mean pegged past Empty or on "E" ?

 

It used to be pegged past empty. Now it is working great. This is all making me think I have a short in the fuel sending wire. That HAS to be the guage issue.

Can't figure out why the truck won't run off the front tank though. When I turn the key I hear the pp prime. also, there is no selector valve on this vehicle that I can tell so I don't think that is the problem.

 

What? The pump that doesn't pump, primes?? That is what it is suppose to do, 1 sec prime , until you start cranking the Engine.

 

That is correct. I've tried it several times and I can't get the truck to run on the front tank. There is about a half a tank of gas in it currently.

 

With the switch selected to the front tank, have you tried using the EEC-IV Test connector to run the pump continuously?

 

check the fuel out of the dual-tank reservoir on the frame. sounds like the pump is working but maybe dual-tank reservoir isn't switching over to supply fuel to the high-pressure pump on the frame?

 

This vehicle doesn't have a dual tank reservoir or dual tank switch valve. Basically both tubes go into a Y connection. I haven't seen anything that resembles a high pressure pump mounted on the frame.