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What’s up guys, I have an ic7 dash in my fc rx7 and I’m having a little bit of a hard time calibrating the fuel level to get it to display correctly. So it seemed pretty straight forward. The pull-up is on. I set up the avi raw voltage reading on the avi channel that my fuel level is on. I took out the fuel pump assembly and moved the float to the full position and then to the empty. At empty I’m getting around 0- .5v ,at full I’m getting .37v with fluctuations. So input those numbers and the reading is way off I’m seeing about 90% full when my tank is about 50% filled. I emailed haltech and they recommended I separate the wire from the fuel level sender due to the possibility interference. So I did that and still nothing changed. Someone recommended I get haltech’s signal conditioner. Would this help?
Do you know how the resistance varies between full and empty? If you measure the resistance with an ohm meter, what do you see at those full and empty? You may need to use a different value pull-up resistor than what is built-in to get enough voltage swing to measure accurately.
The pull-up resistor makes a voltage divider with your sensor being a variable resistor. You can you can calculate the resulting voltage with this:
Volts = ReferenceVolts * R1 / (R1 + R2). ReferenceVolts is what you've connected the pull-up resistor to (typically 5V), R1 is the resistor between the signal and Ground (so your sensor), and R2 is the resistor between the signal the the Reference Voltage (the pull-up resistor).
Now, if your sensor measures close to 0 ohms, you have to be careful not to use too small of a pull-up resistor, as the overall current will be too high. A good minimum overall resistance is about probably 200 ohms (V=IR, so I=V/R or a max current of 25mA).
Thank you for the response David. I have checked the resistance for the sending unit and the numbers I have are as follows. Empty=110omhs Full=.9 ohms. I’m not sure If I’m doing your calculation correctly to find voltage. So This is what I came up with.
5(reference volts) * 110(r1) / (110+ 1000) = .495v
5 * .9 /( .9+1000)= .005v
Your math looks perfect. So is the pull-up resistor in the IC7 dash a 1000 ohm (1k?). Is the Reference Voltage 5V (or maybe even 3.3v if it's internal)?
So you could just enter the values of .495 full, and 0.01 empty (you may want to go a bit above that, 0.025 to have a " 5% reserve").
If you provided your own external pull-up resistor of say 402 ohms (that is a standard resistor available in 1%) to a 5V supply, then you would have a maximum voltage of 1.07 volts, and a minimum voltage of about 0.01v.
A resistor like that would cost 11 cents -- of course you'll pay more for shipping. Get a few for your friends!
Wow thank you so much! Yep the pull-up on the ic7 is 1k and the reference voltage is 5v. So if I wire in my own resistor I would then disable the pull-up in the ic7 software correct ? And thank you so much for that link I’ll buy a few of those and do some testing between the 1k and 402. Thank you so much
Yes, disable the internal 1K. If you leave it in, then you would have two resistors in parallel, and their resistance would be 1 / (1/1000) + (1/402)) or 1 / (0.001+.00248) or 287.35 ohms. You know how to figure out what voltage range you would get...
Once again thank you so much. I’m gonna try that and see what happens
You are welcome. Happy to help.
so here’s a bit of an update. I calibrated it using the numbers I found with the equation. So here’s the thing, key on engine off I’m getting a reading that is close to the amount of fuel I have in the tank(half tank), With engine on the reading then drops close to 0. So then I doubled the numbers in the calibration because the voltage reading doubled when the engine was running. and it gave me the proper reading. Would this be acceptable? Or did I scale it out wrong ?
I think you'll just have to study it to see what happens.
I’ll definitely keep an eye on it thanks for all your help David I really appreciate it