Fuel gauge says tank is full when it is full but says my 16 gallon tank is empty when there is still 10 gallons left in it. So I pulled a sender from the salvage yard.

I put an ohm meter on it and operated the float lever not really knowing exactly what to expect.

So what should I measure with the float at the top of its swing and at the bottom of its swing? I was seeing 104 at the top and a flaky 19 (might read 104 again) as I wiggled the float arm at the bottom of its travel.

I pulled the resistor-can apart and gently filed the wiper that contacts the wire wound resistor. Then I added a little wipe pressure by gently bending the arm slightly toward the resistor. Now I get steady readings but the bottom resistance is now 50 ohms.

I look at how the wiper gets its ground from the pivot and ask how does this thing ever work. I guess I'll try to polish that up next.

Again: So what should I measure with the float at the top of its swing and at the bottom of its swing?

 

0-90 ohms are the official readings. Make sure there is a little spring on the shaft, you may have lost it when you took it apart. This puts side pressure on the shaft so it gets a better ground and is more consistent. I have lost them before and have used springs out of a ink pen.

Since you bent the wiper arm, that is probably why you are only getting 50 ohms now. 104 was close enough to 90 to work ok.

 

Research on old Bronco forums said the resistor was wire wound and varied from 73 to 12 ohms (70 empty - 10 full).

Recall I pulled a sender from the salvage yard. It had the wire wound resistor described on the old broncos. After I cleaned up the pivot holes and resistor wire with brake cleaner (a lot like circuit board cleaner) and a brush, it did in fact vary from 70 to 10 ohms. Franklin, I did not find your post until I had finished the job so it went back together without that spring. I was very happy and set out to pull my tank.

I discovered my sender was not at all like the donor. It had a plastic housing and a circuit board with many traces for the wiper to contact. Each trace had a film resistor in series with it. The wiper had basically wiped most of them away. At least the ones that represented the lower levels of tank fuel. So the resistance at the lower levels was an open circuit and made the gauge read empty for every contact point that was gone.

I was also surprised to find the mounting flange was close to twice as big in diameter as the donor. My heart sank a little.

Here I was with my tank down so to speak. I decided to try to put the wire wound resistor from the donor on my worn out sender. So taking the donor, I drilled out the spot welds between the gas line and resistor backing plate and pulled the resistor-can with its float. Then I searched for a way to align and mount it where my original plastic housing had been. Running the float through its whole range of travel, I scribed a line from half travel to the pivot on the resistor-can and called half a tank. My goal was to make that scribe line match the same for the resistor I was replacing. I later discovered there was more to that decision as I will try to explain. Anyway, the orientation of the float in the donor was straight toward the front of the truck while the one in my tank which had a much shorter float arm was aimed straight at the fender (quarter turn relative to direction of donor). I think the gas sloshing for and aft on brake and accelerate with little gas in the tank was responsible for the damage to the circuit board via the wiper tension. I found later that the float arm was too long for this new direction. It turned out that I had to mount the removable lid of the resistor-can to my backing plate which made the donor backing plate the new lid. The pivot axle that came out of the resistor-can was positioned in the larger hole used by the worn-out resistor pivot. I used three spot welds at the edge of my senders backing plate and the old-lid of the donor resistor-can. That turned out very well.

As I mentioned the float arm was too long. When I put the sender in the tank I could hear the float contacting the tank wall. I had to shorten the float arm by bending half of the excess length toward the front of the truck and the other half back so that the float could remain in its same plane of travel around the pivot. I also made sure that at the bottom of that travel the float ended up where the worn out float would have ended up. I hope this put a priority on the positional calibration of empty relative to any other reading.

So I declared the conversion to be a success. I now have a very good indication of the lower 3/4th of that actual level in the tank to avoid being stranded at the expense of knowing just how full the tank is.

It turned out the gauge needle swings beyond full. In fact I can put 4.3 more gallons in my 16 gallon tank after the gauge reads full. The gauge needle continues to follow the fuel level past the full mark so far that it looks like I have 5 fourths of a tank (but it is really 3/4ths). The float sits there submerged until I have burned a couple gallons before it begins to float again.

I am going to have to pack a gas can with me and actually run out of gas to know where empty really is. If it is below the E on my gauge, I could add a variable resistor in line with the sender to calibrate the E on the gauge which is my primary concern as I am about to embark on a long trip with my 16 gallon tank. If it is above I could still calibrate it with a series resistor on the battery side of the gauge though it would be difficult to modify the flexible circuit board in the instrument cluster. I will probably just figure out where E really is and live with it.

All this because I was told the senders were no longer being made. I would much rather have continued to search the salvage yard but I am in a bit of a hurry.

 

Who Told you that?

What year is your Truck? EFI or Carb?

The 1980-1984 senders, Front and Rear, are still avaliable new.

 

You should have set it up when you had it out of the truck. The float arm should have a mechanical stop when it drops all the way down. The float should be a inch or two above the bottom of the tank when it hits the stop(You don't want the float banging on the bottom of the tank). In this position, with the wiring plugged in, the key in run, and a wire grounding the unit, you can adjust the assembly till the dash gauge reads on the "E". When it reads on the "E", and the float is a couple of inches off the bottom of the tank(the arm is sitting on the stop), this should give you a reserve of about 3 gallons or so, depending on the size of the tank and how much lower the pickup sock is located.

The gauge going way past full is pretty normal for most cars and trucks I have driven. Some are worse than others though, but all I care about is the "E" area.

 

Thanks Franklin,
I am surprised at myself for not thinking to verify calibration during installation. I did try to duplicate the lowest float height relative to the bottom of the pickup filter but have not yet allowed the tank to get that low.

 

When I swapped EFI on to my 84 F250, I used a fuel tank,sending unit and pump from a 88 truck. Found out the resistance was backwards!! My tank when empty would read full and full when empty. Ole Ford changed it for some reason. I redesigned that 88 tank with my 84 sending unit and that fixed that problem. Word of caution on those brass floats, they will leak and sink! This usuallu happens after working on the sending unit. The brass float is sooooo thin around the mounting arm. I wound up using a foam float I stuffed into that rotted out brass one.