Erratic Gauges

[N162RV]

I was at just about dark yesterday when I turned on the Nav Lights and my Vans oil temp and fuel pressure gauges would peg out. I have seen some minor fluctuations in the past but this was by far the worst.

At the time I had been flying for about an hour or so and it was pretty hot outside as well. Im thinking it has something to do with the ground for the nav lights. The lights are located in the fiberglass tips and have a grounding wire attached to an end rib. Any thoughts?
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[airguy]

Definitely a voltage issue - either a floating ground or a true low-voltage scenario on the supply side of the gauges. Can you reproduce it on the ground?

[Kevin Horton]

Where are the gauges grounded, and where are the senders for the gauges grounded? This smells like a condition where there is a bit of current flowing between those two locations, which would put the two grounds at slightly different voltages due to the resistance along that current path - i.e. a ground loop.

If various grounds are all in different places, then the return current (i.e. the current that flows from the device through the ground back to the battery) will cause different parts of the airframe to be at slightly different voltages. This is not a problem for most systems, but some systems (some instruments and intercoms, etc) require all parts of their system to have a ground at the same voltage.

If you can reproduce this on the ground, what happens if you use a piece of large wire to tie those two grounds together?