Dear friends

Have a 2008 ML 350 with fuel level not reading.
Instrument cluster shows either full or 172 a tank all the time but never got an accurate reading.

Fuel sending unit has been replaced with same problem as well.

Is there any wiring diagram available in this vehicle?
Is there any tip or trick in how to make it work?

We have double check fuel level sending unit with 100% confidence it is working properly.

Please help here whit this electrical gremlin.