Common earth.
All sensors and the device to a common earthing point which ideally should be on the engine - cos it's what you take readings from.
Plus all sensors etc should take power from a common point.
Be careful that anything you are connecting doesn't have it's casing (if metal duh) earthed to the GND line and chassis at same time.
It dosn't matter if that is a GND wire from the engine, point is that it's common.
If it dont work like that, then 9/10 you aint doing it on one common point.
Ditch the extra battery of course.
All sensors and the device to a common earthing point which ideally should be on the engine - cos it's what you take readings from.
Plus all sensors etc should take power from a common point.
Be careful that anything you are connecting doesn't have it's casing (if metal duh) earthed to the GND line and chassis at same time.
It dosn't matter if that is a GND wire from the engine, point is that it's common.
If it dont work like that, then 9/10 you aint doing it on one common point.
Ditch the extra battery of course.