1985 f 150 4wd- duel tanks- when i fill gage reads full for about 10 miles then drops to 0 and then slowly comes back up to 1/4 and in the next 10 miles goes to 0 and stays there. both tanks do it. mech said gage good and they couldn't find the prob. could i have a bad ground somewhere or maybe the switch that switches tanks [seems to work okay as i can switch tanks with no prob. chasing this for awhile, but i need to know how much fuel i have as i occasonally take pretty long trips in this ol girl. thanks jim

 

Quote:

Originally Posted by jimbo64

... when i fill gage reads full for about 10 miles then drops to 0 and then slowly comes back up to 1/4 and in the next 10 miles goes to 0 and stays there. both tanks do it. mech said gage good and they couldn't find the prob.

Did this mechanic open up and take a look at the sending unit? I really wish I
had taken a picture of mine... the inside is basically a rheostat - variable
resistor - that consists of a small, brass contact that glides up & down against
a piece of material that has a REALLY fine wire wrapped around it along its
length. That wire makes up the resistor; the more resistance in the circuit, the
less power to the gauge there is. This is how the needle moves back & forth.

That wire wears out and breaks; mine had unraveled and was floating around
*****-nilly inside of its container, hitting the brass contact *****-nilly, causing
the gauge to be erratic as all Hell. I'll bet you have a similar situation....

 

ctubutis, good description of the workings of the sending unit.

My single 16 gallon tank in a 1984 acts very much like described by jimbo64. When I fill up, the gauge will read empty until 5-8 miles have been driven and then the gauge will indicate almost full and at approximately 25-35 miles driven will have gone to empty.

The original owner was retired when I purchased the truck and for several years, the truck was allowed to sit for extended periods. I have a theory and would like to see what others think... Since the gauge does consistently work under the previously described conditions and goes full range, the float must be okay and the external electrical connections as well. What I think might have happened is the tank was almost full when parked and since he lives about 5-10 miles from the nearest gas station that is where the brass contact was touching the resistor wire. The gasoline in the tank "coated" everything with "varnish" except where it could not "coat" the resistor wire under the brass contact. So, when the brass contact touches the resistor wire at any other point, the electrical connection cannot be made and the gauge falls to empty. The resistor wire cannot be broken or else the gauge would never work (correct?). Oh, by the way, I have tried using several bottles of Techron with no improvement. What baffles me is that one would think the theoretical varnish would wear off as time goes by and it has not in 15,000+ miles of driving.

Just a theory and someday, when I paint the truck and remove the bed, I will remove the sending unit and see. In the meantime, I am thankful for the trip odometer.

 

Quote:

Originally Posted by 1986F150six

ctubutis, good description of the workings of the sending unit.

Thanks; I really wish now I had taken pix when I had the opportunity, they
would help a lot. I might still have some parts in my garage someplace but it's
just toooooo dang cold out there right now....

Quote:

Originally Posted by 1986F150six

Since the gauge does consistently work under the previously described conditions and goes full range, the float must be okay and the external electrical connections as well.

To test the gauge & wiring: Disconnect the wire from the sending unit and
ground it, the gauge should swing fully in one direction; un-ground it and it
should swing fully in the other direction. Of course, the key needs to be in the
ACC or RUN position.

Quote:

Originally Posted by 1986F150six

The resistor wire cannot be broken or else the gauge would never work (correct?).

Sometimes, it depends (again, I wish I had pix). It'll depend on where the wire
breaks and what half ends up touching the brass contact. Merely breaking that
wire is not enough, by itself, to render it totally inoperable, it just won't be
accurate.

Your theory sounds good but, like you say, I would expect the varnish to wear
away pretty quickly.