I've a numeric dashboard project for my old truck that has a faulty dashboard at the moment (fuel level and water temp has wrong value, the sensors are good). In this project, I've 4 sensors that are basically a resistor that changes with the value of the content (NTC400 water temp sensor, fuel capacitive sensor and 2 pressure sensors for air tank). But today I've a problem with some reverse voltage in measuring sensor ohms value. This project is based on Arduino, It use a voltage divider to get the sensor resistance value.
For more context, I've tested air tank pressure sensor to get the value in ohms @ X bars. The sensor is orginally connected like that on the old dashboard: dashboard sensor output => sensor => ground (I assume, it's an ohms mesurement with a voltage divider as the old dashboard gauge is only made from resistors and other small stuf, not ADC or anything else from what I can see inside). All tests was done with a calibrated multimeter and not with the Arduino (the value in Arduino was not good until I set an offset, but like explained below, the reverse voltage value is not always the same).
With the master relay OFF I've correct values from 10 to 140 ohms (0 to 7.5 bars) and when I engage the master relay, the value became totally broken (50 ohms to 80 ohms @ 7.5bar)... When I check the voltage between ground and the dashboard input of the sensor I get 65 mV (Ground => Voltmeter => sensor input => ground).
This value changes as I engage some equipments (not all of them) from 50 to 95 mV on this input (with NTC400 water temp sensor the value isn't the same, about 20 mV when he other one is at 65 mV).
When I check between truck ground and earth (my field) I've 330/350mV. This value only changes with the wind and not with equipments activation (electrostatic charge I think).
So, how can I "suppress" this reverse voltage (I don't know if it's the good name for this) to get the real good sensor value (ohms) like when master relay is not energized? Or another way to get my sensor value maybe ? I can set an offset in the arduino (it work as the equipement activated not change), but I think it will change with humidity or other parameters (Equipements activated). However, the old Analogic dashboard work perfectly for the air pressure values (the water and fuel gauges are broken and gave wrong value..., that's why I've this numeric dashboard project) but I can't use it anymore and a new one is few thousand euros...
EDIT : This values doesn't change when engine is running ! Here is a schematic of how I connect the multimeter.The ground I used is the ground on the battery => 702A is the dashboard plug (disconnected from it)
