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Calculating Fuel Sender Voltages

EFI Wiring Fundamentals

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My electrical maths is a bit foggy.

I have a 0-90ohm fuel sender in an 80 litre tank which I tested so I have the actual sender resistances for each 5 litres of fuel from empty to full. e.g 89.8 ohms @ 80 litres, 76.8ohms @ 75 lites, etc.

The sender is wired into a Plex SDM700 dash analog input which requires a 0 to 5v signal so the pullup resistor option selected. The pull resistor, according to the documentation is a 1K resistor.

How do I calculate the voltage output from the sender for each of my resistance points so I can use them to properly calibrate the input table?

The formula for a voltage divider is this:

Voltage = Reference Voltage x (sensor resistance / total resistance).

The total resistance will be the pullup value (1000 ohms) plus your sensor resistance.

So if the pullup is to a 5V source, the formula will be:

Voltage = 5 x (resistance / (1000+resistance)).

Voltage @89 litres = 5 x (89.9/1089.9) = 5 x 0.0825 = 0.412v

Voltage @75 litres = 5 x (76.8/1076.8) = 5 x 0.0713 = 0.356v

This online calculator is nice and easy to use:

Voltage Divider Calculator (ohmslawcalculator.com)


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