Wrong gas gauge resistor

[Angie]

way back when i did the tank on my truck, i placed in the wrong resistor for the needle's full e marks. when tank is full it reads full. but when tank drops about an 1/4 it goes to empty. is there a way to place an in line resistor to correct the mix up? I can't take the box off to get at the sender unit.

if not i will continue with the old pen and paper / mileage writing down way of general guessing. thankful for spare rear tank for emergencies.

cheers

 

[franklin2]

You must not have a trip odometer in your speedometer? If you did, it's very easy to use that and its much more accurate than a fuel gauge if you use the vehicle all the time on the same round trips.

So what happened exactly? You used the sending unit for the rear tank in the front? Or the wrong year sending unit in the tank?

 

[RonD]

No probably not, as far as adding a resistor

Which year Ranger, the resistance changed in 1989?

1989 and up, is 16 ohms Empty, 160 ohms Full

1988 and earlier, was opposite and then some, 80 ohms Empty, 10 ohms Full

And it could be the angle of the float arm, long shot, does it just show Full, or OVER Full, when you fill it up

 

[Angie]

hi frank, 1983, so no trip meter. the front tank was new on this build and the float section was new and i didn't think the voltage/ohm setting would be that big a deal when i stuck it in the tank. they are/were hard to find when i built this truck.

Hi Ron, yeah i think the 80 ohms sounds familiar or was it 70, i need to go back and see if i have the diagrams for it still... i didn't think it would matter much back then. full is full. not over the lines. but when it gets to about 3/4 full it drops to empty or bounces around the bottom 1/4. the float was all good so it should be fine.

had to get the tank and float through LMC trucks way back. being 83 they changed everything for these years if i remember correctly. as to why i couldn't get something that was 100% for it all.

that was why i was wondering about a resistor to see i that would drop the ohms or something.

not a game changer or killer for driving.

cheers thanks

 

[RonD]

Resistor increases Ohms

0 ohms is no resistance, a direct connection
Infinite ohms is no connection

Ford fuel gauges use the Ground side as the control
The gauge is powered with 5volts, pulsed usually, and its needle shows the resistance to ground thru the sender in the tank

So in the case of a pre-1989 fuel level system a short to ground would be 0 ohms(under 10 ohms) so shows Full on gauge
A disconnected wire is infinite ohms(higher than 80 ohms) so would show Empty

Yes, the specific Empty ohms is 78, I just use 80 because its easier to remember, lol, and the difference is not a big deal in this type of system

The sender in the tank is just a coil of wire that is grounded at one end, the coil has a resistance of 10 ohms at grounded end and 80 ohms at the far end
The wire that goes to the gas gauge is attached to a metal arm inside the sender and to the float
As you fill the tank the metal arm is moved closer to the end of the coil of wire that has the ground, until gauge sees 10 ohms to ground
As the fuel level drops the metal arm moves farther away from grounded end so resistance thru the coil of wire gets higher

Image here: http://www.fordification.com/tech/wiring/fsu08.jpg

IF...........there is any corrosion inside the sender, on the coil, that will increase the resistance to ground when the arm moves to that area of the coil, so gauge drops lower than it should
These can be opened and cleaned, and tested with ohm meter, but would involve pulling the pump assembly

And it could be the gauge itself
You could get 80, 60, 40 ohm(approx.) resistors to test if gauge is showing correct needle movement at the lower end, and then stability with fixed resistance

In 1983 the fuel gauge to sender wire, should be an Orange wire

 

[franklin2]

I would run the truck and carry a 5 gallon can of fuel in it, and run it down very low or out. Then put a little fuel in it to get home and then pull the tank down. I don't see any way of diagnosing it any further without pulling the sending unit out of the tank.