Internal ground level

Hello,

Can anyone explain to me what is internal ground level of the ECU?

Any circuit has it's own internal ground. Normally this would be represented as the sensor ground, but it could be the chassis ground, or a completely different value depending on the circuit design. The CAN bus is designed so that the signals are differential (ie, they are supposed to be measured relative to each other), to avoid any dependence on CAN devices needing to share the same ground.

What is the reason for your question? Perhaps if you provided some more background (ie. what problem are you trying to solve, or issue you need to understand), a better answer could be provided.