Fuel level sensor calibration (uc10/nsp)

Hi, Im having an issue with the calibration of the fuel level sensor. Brand new PHR sensor for supra mk4.

I have an haltech r3 + UC10, sender is wired directly to the dash (sensor ground and avi 1, internal pullup 240 ohm enabled). I watched the haltech ressources and tutorials.

My issue is that the voltage when I have the key ON and when the car is running is totally different therefore my calibration is false.

For example, 30 liters in the fuel tank ,key ON position= 1,46v , when the engine is running I have 0,86v.

Any advice ?

Thanks !

So most likely cause is difference in the ground source for the dash and the tank. Remember voltage is a difference between two things.

The dash for fuel level is basically refencing ground via the tank on that input. The pull up resistor is there to give it something to measure against by pulling the ground up to say 5V or whatever (doesn't matter), then measuring the difference between that 5V and the ground the dash's own ground. It then measures the difference between the dash ground and the pulled up fuel level input. That difference is what you are reading, 1.46v or 0.86v.

If there is a difference in the grounds (which are the reference its comparing) then it would change the reading.

I had a similar issue a while back on a WRX.

Solution was to run a second wire from the dash, specifically from its sensor ground output, run that to the fuel level sender, then regardless of car being on or off, the dash will always have the same ground reference. And the pulled up voltage you are measuring and thus fuel level should be consistent even when engine is off or on.

Short version, connect dash sensor ground to the other pin on the fuel level sender.