Hi all,

I have a question about a truck i just bought (1990 F150). Im new to f150's but i have plenty of experience in early 90's EFI vehicles. My truck has dual tanks but neither of the level gauges work. Since both of them dont work i thought it could possibly be a electrical problem, so i bought a haynes manual to check out the schematics and see how the fuel tank switching mechanism works, and the sending units. Well the haynes manual is complete trash and was a waste of $20. Does anyone know where i can download a factory service manual? Also please enlighten me on how the switching of tanks (along with switching of gauges) work on my truck.

Thanks.

 

With the '90 and up vehicles, each tank has a complete sending unit contained within each tank, which have individual pumps and fuel level sending floats. The switch on the dash simply supplies power to whichever sender is selected. Bad grounds or a bad fuel gauge could cause your problem, though corrosion is more likely causing issues with a vehicle this old. Autozone has some good online service documents you should check out.

 

Its not obvious howto find anything about the sending units wiring on the autozone thing. So youre saying when i flip the switch to the front tank it sends power to the sending unit/pump in the front tank, then when i flip it to the back tank it switch power to the back the same way? Cause both tanks work, the gauge just doesnt work on either tank. Is the wiring for the pump independant of the wiring for the sending unit? I need a good diagram.

 

could be the float is sunk, like 6 bucks at ford house. am i reading correctly that the pumps work just not the gauge? easy fix, but i think it is time consuming on a regular truck box..

 

yah both pumps work but neither gauge do. i kinda doubt both floats got stuck or leaked into. is it easier to take the bed off than it is to drop the tanks? i read that somewhere.

 

It Could Both Be Out, The Float Is Just A Brass Cylinder That Has Been Soldered At The Factory Thus Creating A Small Pin Hole On The Solder Joint As They Get Older. And to replace It Just Clips On The Sending Unit In The Tank, But To Answer Your ? On The Bed I Do Not Have A Regular Bed So I Do Not Really Know But I Would Probably Let the Gas Get Low On One Tank And Drop It, And Replace Just One At A Time And See If That Fixes It. Also If The Tanks Show Full It Could Be Electrical (bad Ground Or Something , But If It Reads Empty I Would Suspect The Floats. My Floats Sank And I Replaced Them And Now I Can See My Gauge Go Down A Lot Faster(lol).

 

I have a similar problem. For a while my guage would read almost a 1/4 tank when both tanks were full (it worked right off and on), now the guage reads way past full on both tanks. The guage is pegged out and is in line with the speedo at 0 mph. Both sending units can't be bad at the same time.

I could use help too on figureing out the problem.

imloggedin, is your gauge not reading at all? Where is your guage stuck at?

 

Both my tanks pump fine and the rear guage works ok but the front tank reads empty all the time. Since I have a 19 gal tank up front, I just write down my mileage at fill up. With ten MPG, I switch to the rear after 180 miles. This system has been working for over a year and a half but it is getting old. I guess it is time to drop the front tank. Just waiting for the rain to stop here in Oregon. Should be around the end of June.

 

What does your fuel gauge read? It could be a bad gauge or a bad ground. Test the gauge and make sure you've got current going from the the gauge to the sending units.

And yes, it is completely possible that both floats have sunk. Fords are well documented for this problem. It could also be corrosion. Disconnect the wires, clean all contacts real well and see if that helps. It could be a sunk float in one and a corroded connection in the other. It really could be a number of things. Start with the easiest ones first and work your way up to replacing the floats.

 

yah the gauges show completely empty, no matter where the switch is. itd be great if i knew what wires did what. like had a real wiring diagram. i cant even find the switch and fuel pumps in the stupid haynes book. i need to know how its wired before i can test anything.

is it easier to take the bed off than drop the tanks? its a long bed.

 

With it reading empty it's either a busted gauge, a broken connection, or sunk floats. Again, I'd start by testing the fuel gauge to see if it's working right. Test connectivity between the gauge and the pumps. Check the wiring connections at the pump for corrosion and clean if necessary. If none of these work, I'd drop the tanks and replace the floats.

You'll have to drop the tanks. You can drop the tanks fairly easily with just one person and a floor jack. Siphon the gas; support it on the jack using a small piece of plywood to even the weight; disconnect the wiring, fuel lines and gas hoses; remove the tank straps; and lower it down. You may want to lift the truck on jack stands to have more room to work. Removing the locking ring to get the pump out can be a pain. If you damage it, they can be replaced inexpensively.

The link below is the wiring diagram for a 1990 F150 with the 5.0 or 5.8. The 4.9L is a bit different, but the fuel wires are about the same. Based on what I can decipher, and I'm no expert at wiring diagrams, but

1. a brown wire feeds from the relay/ECM to the inertia fuel shut-off
2. Red/Yellow wire feeds to the dash to the tank selector switch
3. Red wire feeds to the front tank pump. Brown/White feeds to the rear tank pump.
4. Both pumps have an orange wire that connect together to a black wire that is the ground.

That's where the diagram ends. There are two other wires connected to each pump. Both are smaller in size than the ones listed above. One is black. That's the ground. The other is blue/yellow and that I'm 99% positive that is the wire that connects to the fuel gauge from the sending unit. I will clarify that the front tank is blue/yellow. I didn't look at the rear tank. It may be another color but the same setup should exist. I'm thinking these feed back into the tank selector switch and on to the fuel gauge. I do not think they connect to the ECM.

Hope this helps you out. It should at least help you in diagnosing the problem.

Here's the link to the diagram. Scroll down to about halfway to find your model.

http://www1.autozone.com/az/cds/en_us/0900823d/80/0a/3a/41/0900823d800a3a41/repairInfoPages.htm

 

I should mention that the wire on my 1993 F-150 5.0L was blue/yellow. It may be a different color for your 1990. Still, the same set-up should be there regardless the year.