If you have a fully charged battery and good connections, I would guess you are witnessing the death of a sonar unit.
Just in chase:
Remove the display from the mount, very gently and very little, adjust the two power cord pins in any direction.
Do it so little you cannot tell you did. Those pins still have to line up with the tiny holes in the display receptacle--LOOK at them.
Use a pencil point.
Replace the display in the mount and check the operation.