Sensor Arexx Pro-88msn

I use the Pro-88msn sensor to measure my battery voltage. I measure 1.2 volts with a voltage divider at the connection box JB-T1. The measured value jumps extremely up and down. I get within a few seconds readings between 1.188 to 1.202 volts. The graphic curve moves zigzag. The normal multi meter measure exact 1.200 volts.

Is there an error in the connection box?

Can anybody help?

Thanks

Best regards

T. Bucher