Fuel guage sending unit voltage range?

[Stu]

I'm thinking of creating my own tripminder. Does anybody know what the voltage range is of the fuel guage sending unit? Since this is a carbureted car it won't be exact, but it should be an interesting experiment. Right now the plan is to measure fuel levels over a measured distance from the speed sensor and continously update it, thereby creating an average.

[Stu]

Nevermind... 14-18 ohms is empty, 155-165 ohms is full. That will work...

[Grand Marquis GT]

Stu,

You can actually pull the sending unit out pretty easy, just make sure you have 1/4 or less of a tank!

I learned the hard way!

But it is located on the front of the tank.

That way, you can pull out a multimeter, and test the resistance.

[Stu]

Thanks for the location, haven't looked to be honest. Now I'll know where to tap into. Do you know if it has 12vdc going to it or does it use a difference voltage?

[Grand Marquis GT]

I aint really sure if it even uses voltage.

I just know it is a variable resistor.

[85crownHPP]

sounds interesting, and more do-able than the MCU OBD idea.

To me the only hard part would be to correlate readings from the sender to a numeric volume.
What I'd do is just take readings with a VOM at full and empty, or (if you wanted to get this involved) would be to semi-permanantly install a VOM to the sender and fill her up at the gas pump stopping in gallon increments and recording the data readings.
Id assume the response is linear, so you could plot a line to your data and use that function for your calculation.
You'd have to adjust your 'max' to account for the portion of gas you can squeze in after the sender maxes out (if its even neglegable).
I guess you could always note where the reading stops, and use that as your 'max'.

I dont know what volatge range the sender works on myself, or even if it used the voltage reading. Regardless I would do the above for this sort of project anyway, and Id guess that resistance would be a better reading to take for this, in case it only works over a small range of volts.

This would have been a cool project for the Instrumentation and Measurement course I took in college.

[Stu]

Thanks for the input.

[85crownHPP]

a varying resistance will also 'show' a varying voltage on a meter, but it probably uses the resistance for the reading, most all sensors do.

[gadget73]

I think there is a voltage regulator system under the dash for the fuel guage. also, the response isn't linear on these things at all. most cars are calibrated to have a larger amount of fuel from 1/1 to 1/2 on the guage than from 1/2 to 0/0. Mine for example is exactly 10 gallons down at 1/2 tank, and its a total 18 gallon capacity. From the 1986 shop manual
"The instrument voltage regulator (IVR) used with the fuel guage controls and and maintains and average pulsating value of 5.0V between input to guage system and system ground (sheet metal)"

full 8-12 ohms
empty 60-86 ohms.

[Chachi]

the sending unit does not use dc+ .. the resistance is mesured id dc-.. Or in other words Ground potential..

[gadget73]

right, it varies to ground like an oil pressure or temp sensor does. but the + side in this case is +5v instead of +12v.