Fuel from rear tank only
 

Hello, I have an 84 F150 with dual tanks and I believe am only drawing fuel from the rear tank at this time. Both tanks and gauges worked fine when I purchased the truck last winter. A few weeks ago I filled up the tanks and set the switch to the front tank. I drove about a hundred miles and the gauge didn't go down at all. I stopped at a gas station and I flipped the switch to the rear tank and the fuel gauge showed a drop in fuel level. This lead me to believe that fuel was only coming from the rear tank regardless of the switches position. The front tank was still full and the rear tank had gone down (I checked this by filling up at the station). Should I check the fuel pumps or the fuel tank selector valve first?

I don't know what you have, but if your truck is original, you only have one fuel pump. You have a solenoid valve mounted on the frame rail about midway on the driver's side that switches the lines from the front tank to the rear. That's where I would start troubleshooting.

I believe everything is original. I will check out the valve then. What is the best way to troubleshoot the valve.

Put a testlight on the wire and see if you are getting voltage or not when the switch is switched from front to rear. If the voltage switches, then I would suspect the valve itself.

Also... check the fuse that controls the tank switch.

Looks like the gauges are working for now. I will check the tank levels when I fill up next time. The real cold temps we have here in WI have a habit of making things run haywire sometimes.

Conceptually the same, yes - test for voltage at various locations when the switch is set to various positions, see if the solenoid-operated valve is being told to open or close or not.

But the wiring changed in 1987 (or so) and the switching valves changed in, um, I think it was by the early 1990s. So, the specific tests we do here may not at all tell the truth on something as modern as a 1994.

Do an Advanced Search, search only the 1987-1996 forum, look for posts authored by subford, enter in your favorite search criteria, sort the results based on relevancy, and see what ya can find in their archives... might even have this listed in their Sticky, I don't know for certain....

On your truck the solenoid has nothing to do with the gauge, the fuel gauge is switched by the dash switch. The solenoid is what switches the fuel lines. The dash switch has two jobs, switching the sending units for the gauge, and sending a signal to the solenoid to tell it to switch the fluid lines.

Your diesel is a whole different ballgame. You do not have a solenoid, your valve is actually a little motor that mechanically moves a spool in the valve, and that switches the fuel supply lines AND the return lines AND it also has a set of contacts inside that switch the sending units. Lots of wires and 6 fuel lines going to it. And the original valve is known to go bad, and is very expensive and hard to find if you want the original one. But Autozone carries the valve at a better price, but it doesn't fit like the original one, but it functions the same. The fittings are different and it doesn't quite go in unless you do some modifications. But it does work the same.

Frankin, I should have been a little more clear. What I meant is that the gauge is steadily going down for the front tank now, so the level in the front tank is falling and the valve is working properly. Hopefully it stays this way.