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Gregte 06-30-2008 10:18 PM

possible fuel gage improvement?
I believe that cars of yore used a wire wound rheostat inside the fuel tank as the variable resistor for supplying a signal to the fuel gage. This would give a rather coarse resolution but certainly work for the on-dash gage.

I am wondering if this is still what is used for the fuel gage 'sending unit' or if something with much better ability to finely resolve the level is used, such as a regular carbon pot.

Just wondering if it would be worth the trouble to tap into the fuel gage's signal wire and feed it to a voltmeter, which can resolve to hundredths of a volt, for the purpose of being able to know very accurately your fuel level, such as when parked on your level garage floor.

wagonman76 06-30-2008 11:59 PM

I dont think it would be worth it. There are just too many variables that give a less than perfect reading.

For starters, the shape of most gas tanks is not a rectangular box, but an odd shape designed to make the best use of space in the floorpan. That right there would make the reading be inaccurate at different points on the sender, even if the sender was perfect.

Then there is the level of vehicle, and whether you are accelerating or decelerating usually makes a difference in the reading even with the same amount of fuel in the tank. Also there is the internal baffle around the sender, and Ive noticed that the fuel will read higher if Ive stopped and restarted during a long drive, than if I just drove it straight. Stops and restarts will refill the baffle and give higher readings for awhile, then when the fuel really runs low the gauge drops much faster.

Then there is the geometry of the float arm itself. Assuming all else was perfect, the only way to get a true reading would be at the point when the float arm is horizontal. As it goes higher or lower, some of the float swing is horizontal motion, so the float (and gauge) moves more for the same drop in fuel level. The point where the gauge moves the slowest would be the point that the float arm is attached to the sender. It might be different for all cars.

Some newer cars might have better float systems anyway. I am mainly familiar with my old GMs.

dcb 07-01-2008 12:24 AM

I hope to add "fuel remaining" to the mpguino at some point (mpg gauge), that is why tank size is one of the parameters.

Gregte 07-01-2008 11:37 AM

I was not expecting for it to read a linear value, i.e. n Volts per Gallon. I figured you could start with an empty tank, take your reading in volts then add a half gallon, let it settle, take a reading, and so on. This would give you a table of volts vs. gallons. It would not make any difference what the shape of the tank is or the geometry of the float arm.

I am just trying to determine, accurately, gallons used over short distances rather than waiting two weeks till I fill the tank.

My anticipated use of this would not be for while driving, but rather for when the vehicle is sitting in the garage, still and level.

My only question is whether or not the sending unit of current day cars has the infinite resolution required as opposed to the old days where I believe they used a wire wound rheostat which obviously has very rough and poor resolution.

dcb 07-01-2008 11:53 AM

Can you describe the intended use a bit more? Is it running in the garage or sitting still, yet somehow needing a refill every two weeks?

If you get the mpguino dialed in for your vehicle it will give you instant, as well as how much it thinks you used for an arbitrary amount of time. I don't know where the other fuel is going in your situation.

You could also weigh the vehicle I suppose.

Also maybe you can tee into the fuel line and add a sight gauge:

Gregte 07-01-2008 01:33 PM


Originally Posted by dcb (Post 39984)
Can you describe the intended use a bit more? Is it running in the garage or sitting still, yet somehow needing a refill every two weeks?

I guess I have not made myself very clear. I am only trying to figure out how much fuel is in my tank, say every morning before I take off driving, or every night when I get home. The dash board gas gage already does this except it is a very crude method of telling 'how much' fuel is in the tank. I want to measure my remaining fuel very accurately.

According to the electrical diagram of my vehicle (Chilton's manual) The fuel gage sending unit in the tank is nothing more than a variable resistor. However, I am trying to learn if that variable resistor, in newer cars, is a wire wound pot (poor resolution) or if it uses some other type of variable resistor which might have very high resolution.

The sight gauge should work except I don't want to go to that much effort. It would not be very easily doable on my Sonoma pickup.

I want to measure my gas tank within a fraction of a gallon.

I could just find the wire coming from my sending unit and discover for myself if the variable resistor has the resolution I need but I thought I would ask on this forum first. Maybe someone already knows and can tell me, since it would be a bit of work finding the wire, tapping into it, and then doing the required procedure to determine the info I am seeking.

Red 07-01-2008 02:42 PM

Kinda crazy but why not use some kind of pressure sensor to measure the weight of the tank? As said earlier most tanks are odd ball in shape or internally baffled around the pump.

But in answer to your question, if your sending unit is a Bosch, its a coarse pot, 5v reference signal. Resistance dependent on fuel level. Resistance is inversely proportional to fuel level. I don't believe most cars use a fine resolution sensor, just something that would give them +/- 0.5 gallon resolution.

Gregte 07-01-2008 05:01 PM


Originally Posted by Red (Post 40061)
...I don't believe most cars use a fine resolution sensor, just something that would give them +/- 0.5 gallon resolution.

Thanks! That is just what I wanted to know but unfortunately not what I hoped for the answer to be ;-)

ttoyoda 07-01-2008 07:05 PM


Kinda crazy but why not use some kind of pressure sensor to measure the weight of the tank?

Strain guages mounted on the straps, or maybe just on the bottom of the tank. The tank bottom or support straps will deflect more with added gas. You will only measure weight, not volume. You will have to remove the gas cap to de-pressureize the tank before you measure. Naturally using gauges in all 4 legs of the bridge will help negate temperature effects.
Strain Gages, Accessories and Instrumentation

pcp 07-01-2008 07:50 PM

Hello Gregte, Yes you can read how much fuel is in the tank with the a multimeter. I used to use one in my old Sunbird when my fuel gauge stopped working, I would measure the resistance in ohms, I'm not sure now what the readings were now but if you tap into the wires you will soon find out your full mark and when its empty. Mine was easy to get at I just kept a multimeter in the trunk, cause the sending unit was accessible. hope I helped.:thumbup:

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