I f the stereo in the boat causes the temp gauge to beat to the music, louder you turn it up the more pronounced it is. Also if you turn the stereo off the gauge drops 15 degrees, would bad grounds cause this?
Yes, a bad negative (ground) circut could cause that. Radio tries pulling ground through all other equipment if it draws more amps than those other items.
Thats actually a fairly common issue. Check all your connections first for corrosion & tightness / then look for corroded or damaged wires.
If needed;
You can run a spare ground wire from the engine to the dash area to verify it or resolve a hard to find connection problem. Just be sure the gauge of the wire is as large or larger than the original (for fire safety reasons).
