Voltage guage moving around 2007 W9 Cummins ISX

Study up on how to do voltage drop tests. Search isolate conquer spend some time and little money. Some times some jumper wires can be used to help isolate oddball stuff. Good Luck!

 

Use a multi meter to test voltage at alternator and at batteries to confirm if it is the charging system at fault or something else.
some times the ground wire for the cecu (that controls the gauges) will need to be checked, its on the firewall behind the tach/speedo cluster, make sure its tight inside and outside too.
the pick up sensor for the gauge could also be at issue, its an inductive pick up that is a circular device that the wire pass's through, has 3 wires running into it, might be found in the battery box or on the firewall outside by the back of engine, check for rubbed wires, tight connection and clean.

 

I suggest going around jiggling all the tail lights and marker light connections etc, gently of course, and seeing if there's one that has a bad connection or has a light that's on its way out and is acting as a parasite. One time I had an electrical drain like this and it ending up being a side marker light on my truck. After I replaced the little bulb my voltage leveled back up to where it should.

Good luck