Okay I've read around this forum and many others and I've kinda got the gist of the problem now but I want to be 100% sure before I proceed.

1989 F-150 5.0 302 Dual Tanks

Here's some history:

For a few years now my fuel gauge almost always reads as empty, sometimes out of no where it will go up and down a few times but that's about it. I always figured it's just the sending unit in the front tank and figured that there is a good chance that the rear tanks sending unit was probably fine. Ever since I've had the truck I've only used the front tank, but due to rust and some leaking fuel I've been wanting to use the rear tank, so I filled the rear tank up and switched tanks, no change in the fuel gauge.

Well, after I had the rear tank selected for a while I started to have some engine bogging problems that got suddenly worse to the point where my truck wouldn't start... Well, come to find out my tanks never actually were switched and it was still pulling from the bit of gas left in my front tank.

So, really my question is since I know my fuel tank selector valve is messed up can that cause my gauge to mess up? I don't think it does but I want to be 100% sure.

Also, I pulled my dash apart to check the gauge and do some testing at the tank switch, I figured I'd just pull the switch harness apart and check the connections and see if the gauge pegs... It did, so if I'm right does that mean that it's either wiring or the sending unit in BOTH the tanks?

Oh and I did some testing at the switch and I read I should ground Y/W wire and the gauge should peg full, well, it didn't, in fact it did the opposite, it went further past E... So I'm pretty confused here guys.

I'd have all this solved if I'd just take the bed off/drop the tanks but I really want to be sure it isn't something simpler.

 

Fuel sending unit...

 

Quick question, since I'm trying to tackle two problems here (tank selector and fuel gauge) with what little knowledge I have about the fuel delivery system.

Question: since my truck has a non-electric tank selector and runs only on the pressured line, what is more likely the problem, the tank selector valve or the low pressure fuel pump in the tank?

 

The 1990 Service manual shows:

On F-Series midship (standard) fuel tank, the fuel sender electrical connection can be disconnected and attached to a second test sender outside the tank. The float can then be manualy operated (up and down) while the fuel gauge operation is observed. Proper gauge function indicates a problem with the fuel sender. Improper operation indicates a problem with the gauge or wiring.

The switch assembly is a double throw that switches pump and sender.

How do you switch tanks now? Is it added on? or Factory switching?

 

The sending unit changes resistance to change fuel level indication.

Low ohms at full
High ohms at empty.

 

Just to be clear, I can test the ohms at the switch right? If so which wire do I test?

And yeah, it's a factory switch.

EDIT:
Also, just out of curiosity and well, cheapness I'm wondering since I have a full rear tank and my tank selector isn't working could I just switch the fuel lines over on the selector?

 

Just wondering if anyone had an answer to this?

 

I'm not an expert on these trucks, i just bought me one myself, BUT the changing of the fuel lines, if you change the line that goes from the tank to the engine you also need to change the line that is the fuel return from the injectors, filter and such, or else you probably will fill that in the other tank while it takes the fuel from the first tank.

 

Well a little update on the fuel gauge this time, yesterday my gauge started to jump around which reminds me of a loose wire or bad ground (correct me if I'm wrong, like I said before I'm not an expert) and not so much a SU problem.

Where can I find a schematic or where are the grounds for the gauge/SU at?