Water temp gauge erratic when running, steady with engine off

[US6x4]

The title describes what I'm seeing which is a new behavior ever since I replaced the voltage regulator. The needle bounces back and forth from far left up to actual engine temp with the engine/alternator running. When I shut off the engine but keep the master switch on the temp gauge is steady on the actual temp. This did not happen before the alternator fix and none of the other electric gauges are doing this.

Has anyone else seen this behavior?

 

[GopherHill]

Just a wild stab at it, could your new alternator be feeding ac through bad diodes?

 

[US6x4]

Anything is possible. It's an old alternator with a new voltage regulator that was burnt up inside...

 

[charlesmann]

It could also be the gauge is deciding to crap out or a combo of both alt and crapping out.

My coolant temp gauge was bouncing and will sometimes show actual temp, but for the most part, it pegs out on the high side. Iv just stopped worrying bout it since the mech gauge i hang tested showed pretty accurate (-/+ a few degrees of the temp gun)

 

[US6x4]

The gauge could be crapping out but it's suspicious that it only bounces when the alternator is spinning.

 

[US6x4]

I briefly ran the truck yesterday just parked in the yard and the gauge was steady as could be. Alternator output was also steady at 28.52 volts. Could be the previous behavior was a fluke or I'm starting to see intermittent erratic gauge behavior which would point to a failing gauge, right?

 

[US6x4]

OK, so I think I found the source of the problem. I wiggled and twisted and made sure the wire connector that plugs into the water manifold sending unit was fully seated and I gave had no more gauge issues. The connector felt fully seated but something in that connection needed some attention.