Another fuel gauge/sender mystery

[mouserider]

Fuel gauge worked, I replaced sending unit with new, fuel gauge stopped working (pegged out). Checked lvl wire by grounding it - gauge go full, meaning all good with gauge and lvl wire. At the same time I had some fuel delivery problem and decided to simply fix all with new aeromotive fuel tank that comes assembled with pump and sending unit.

So now with 9 gallons fuel gauge reads between E and first line( bellow empty, but a bit above lowest point ) . Resistance at sending unit is 13.5. Basically nothing is right , 9 gallons is bellow mid, but resistance is 13.5, with that resistance fuel gauge should be almost at full instead it’s empty

 

[pnora]

Get a meter match and be done with it.

 

[Canadian1968]

Get a meter match and be done with it.

This is what I did. As long as you know your gague is Calibrated somewhat correctly, and your sending unit is supplying consistent resistances. Use the meter match to make it read what it should .

After market sending unit will tell you when it's full and empty.... everything in between is a guess

 

[64fury74charger]

Meter match is the easiest thing to set up and it works with just about any combination of gauge or sender resistance difference. Worth every penny in my opinion.

 

[mouserider]

oh guys, please help me out, as im completely lost, probably due to the fact that i dont udnerstand the working logic of the fuel gauge

So, what i did today.
1. disconnected sending unit completely and measured the resistant - 75
2. connected sending unit to the lvl wire and checking the resistance - 12
3. ignition on - resistance is chaotic 0,25,40,0,60

So apparently gauge works correctly (almost) and shows bellow empty with 75 resistant.

is this how this circuit works?

 

[LowBikeMike]

No. Resistance is measured with both ends of the wire or component disconnected, isolated from the circuit.

 

[moparedtn]

I've gotten ahold of a used factory 3/8" sending unit finally and will be playing with it on the bench
this weekend. If I recall @dadsbee correctly, I should be looking for 10-70ohm swing in resistance
for the travel of the float, correct?
I'll be splitting open the swivel part of the sender to clean it up a tad and get it smooth-moving...

 

[pnora]

oh guys, please help me out, as im completely lost, probably due to the fact that i dont udnerstand the working logic of the fuel gauge

So, what i did today.
1. disconnected sending unit completely and measured the resistant - 75
2. connected sending unit to the lvl wire and checking the resistance - 12
3. ignition on - resistance is chaotic 0,25,40,0,60

So apparently gauge works correctly (almost) and shows bellow empty with 75 resistant.

is this how this circuit works?

Get the FSM and read.

 

[pnora]

I will try to explain the system. The gauge is fed power through a voltage limiter which lowers the input voltage to the gauge. It will pulsate the voltage to and through the gauge if there is no grounding on the sense wire that goes to the sender. The sender is a variable ground depending on fuel level. So depending on fuel level the ground resistance and value will dictate where the gauge reads. So my simple gauge test is this. Unplug it at the tanks sender. Take a 12 volt test light and make sure it is grounded and put the tester on the wire. Turn the key on and the tester will dimly light and will fluctuate. The filament of the bulb is acting as a resistor so the gauge will climb. If it does light and fluctuate the gauge and wiring are most likely ok. Then I turn the key off and the gauge should return to empty. Then I ground the wire disconnected from the sending unit and turn the key on. The gauge should slowly rise and go past full. If this happens as said the gauge and wiring are most likely ok. There are other tests with a tester or resistors that can show the guage accuracy at 1/4----1/2---3/4-----F. At this time you should not worry about the gauge or wiring along with that last accuracy test. Now to the sender. It should be out of the tank. You hook and OHM meter to it and check the readings and each end of the senders travel. In a perfect world it should be at the specified recommendation for the gauge. This is a approximate 10-70 Ohms. This is where the problem comes in. It most likely will have a range not close hence the inaccurate gauge. That's where the Meter Match comes in. It interprets the senders range and it allows you to calibrate the gauge to the senders value. I have my own way of calibrating the gauge. Empty tank. Add 3 gallons of fuel and calibrate the gauge to be dead on empty. That way I have a 3 gallon reserve. Then I fill the tank and set it to be maybe a needles width past full. That's about it.