Erratic Water Temperature Gauge

Allen_TAllen_T Posts: 1Member
My water temperature gauge is erratic.  When i first launch the boat, it reads normal.  Then after i take off, it drops down to nothing.  When I run it on the trailer with a garden hose, it reads normal.  I checked the gauge by removing the signal wire that runs to the sending unit, and jumping that terminal to ground, and the needle goes all the way to the right.  I am getting around 12V on the wire to the sending unit.  I replaced the sending unit yesterday, thinking it was the culprit - no such luck.  When i am on the water, and have the engine off - i can turn the key to the run (not start) position and the gauge will read normally.  I checked the main ground from the battery to the motor, and it's tight.  Any other ideas?

My Boat:  2000 Rinker Captiva 192, 5.0L Mercruiser, Alpha Drive.
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