Anyone know if Fuel Gauge reading is linear with resistance?

The fuel gauge in my Grabber has always been flaky .... but the last two rides it has really been wacky ... it seems to have stopped at slightly below half. I added 6 gallons and it went slightly above half ... drove 100 miles and now it's back slightly below half.

So I started doing some research and testing as per the shop manual.

The range of resistance is 10 ohm = full .... 78 ohm = empty. My sender is currently reading 24 ohm and I have no idea how much gas is in there.

I bought a cheap rheostat this morning .. when set to 10 ohm the gauge reads full and 78 ohm it does read empty ... I'm using a multimeter on the rheostat to verify.

When I set the rheostat to 24 ohm ... it reads slightly below half ... just like from the sender. Now my math says ... 22 ohm should be 3/4 full, 34 ohm should be half and 56 should be 1/4 full based on the 10 to 78 numbers. Of course this match doesn't work if the fuel gauge isn't linear ...

Anyone have any ideas? Could folks check their fuel gauge and then check their sender ohm readings with a multimeter and post the resistance here?

Thanks ... Dan

 

Actually the points would be 10, 27, 44, 61, and 78 from full to empty. I have to check my cluster bulbs this wekend, so I can take some measurements for comparison.

 

I have 7/8 of a tank and a reading of 13.4 ohms.

 

Thanks ... I just set my rheostat to 13.2 (closest I could get) and it lands the needle exactly on full ... Maybe I should be looking for a new gauge? The IVR test as per the shop manual is good ... fluctuating voltage when tested.

 

I think my gauge is wacked .... it is indicating Empty with only 54 ohm resistance ...

 

sounds stupid but is your ground bad?

 

I have a spare fuel gauge just sitting here if you need one. The orange on the needle is a little faded, but otherwise it looks and works fine.

 

Thanks Jamie ... But Greens has plenty in stock ... as well as the regulators ... I'll probably buy both just to make sure ... The sender was new from AK so I am assuming it's OK since I never tested it before installing.

 

Mine doesn't work at all....I jumped the terminals on the plug and it went to full, so I guess I'll have to get a new sending unit for it.

 

Sounds like the sender ... you can check the resistance on the sender itself but if it isn't moving the needle the ohms will read over 80 ...

 

To answer your question, NO, the gauge does not read linear withthe resistance. The gauge works because of the heating of a bi-metalic strip. so it would read linearly with the amount of power not resistance. (power = volts x amps) amps = volts / resistance.
There are only two places that a working gauge reads correctly; full and empty - and then only if the tank is not over filled or under emptied.

 

I disagree with that statement. The other 2 formulas for power are P=I²XR and P=E²/R. Theoretically it should be linear but it may not be just because its an electro/mechanical devise.

 

In electrical theory power is expressed in watts. Watts = Volts x amps or W=IxR

I have never seen any of the formulas that you have expressed. There is a triangle that expresses the formulas:


W
-------- W/V=I W/I=V VxI=W and
V | I


V
---------- V/R=I V/I=R IxR=V
I | R

I hope that comes out properly when it is posted.
For reference see Ohm's Law

 

I can assure you they are correct. The Navy drummed these formulas into my head in both Basic Electricity and Advanced Electronics Schools. Take a simple DC circuit. Apply 12v to a 10Ω resister. Ohms law says I=E/R. So current is 1.2 amps. The Power formula that is most advertised (as you stated) is P=IE. So power would be 1.2x12=14.4 watts.
Using the formulas I posted:
P=E²/R or 144/10 =14.4
P=I²xR or 1.44X10=14.4
The heating of the bi-metallic strip is controlled by current, which is controlled by resistance. Decrease resistance, current goes up, and the metal heats more, which causes it to bend more. In a perfect world the bending can be calculated and precise. So as I stated, theoretically, it should be linear. But we all know how the real world takes over.

And that boys and girls, concludes todays lesson in basic electricity. Go Navy!!!

 

The heat is generated by the wattage - watts - the resistance is controlling the watts but not in a linear manner.

12/10=1.2 amps and it follows watts = 12 x 1.2= 14.4 watts
12/44= .2727 amps and it follows watts = 12 x .2727= 3.2727 watts (the half way point)
12/78=.1538 amps and it follows watts = 12 x .1538= 1.8456 watts

as you can see the resistance at half way is no where near the half way point in the range of watts.

Now, I reiterate that the function of the fuel gauge is not linear.
OK?