temp gauge pegged in red zone- but it's not hot !

FINALLY replaced temp sending unit and harness connector (to instrument cluster) after 2 1/2 years driving with 'broken' temp gage- wired it in last week-it was working fine but the installation was 'incomplete' until today, when I was able to use my heat gun on the marine heat-shrink tubing with which I'd covered the splice. Later, started engine to warm it up and confirm I'd eliminated an unrelated issue been working on for days-some few minutes later I looked at the dash and the temp gage needle was pegged all the way in the red 'hot' zone. Shut it off (half-panic-stricken)-let cool completely, needle back down below 'c'-turned key on, needle instantly jumped back into red 'H'. Other than the temp gage there were no other indications of overheat/running hot.
Did I trigger this by using the heat gun on the wires? Now what do I do?
Thanks in advance for your input.

 

disconnect the wire from the temp sensor and see if the gage still pegs "H", if so, the wire for the gage is shorted to ground somewhere.

 

I'll try that, thanks for taking the time to answer.

 

so I did as suggested: unplugged, plugged back in. Took a moment or so but gage pegged 'H" again. That sensor only has one wire, AFAIK the sensor itself provides the 'ground' connection where it's threaded into the block. D'ya think I somehow fried the sensor or .....?

 

So just to make sure we understand correctly, you originally did a temporary install of the new sending unit and the temp gauge marked where it should around the halfway mark. Couple weeks later you came back to clean up some wiring of the sensor, adding heat shrink wrap to wiring. Reassembled and jumps to H quickly again without engine overheating.
If so, I think the heat of the torch must've affected the temp send unit.
Well yeah now that I think about it. That sensor only sees like up around 200-230 degrees huh? I guess heat from a torch could easily affect it.
Do you have a digital multi-meter?

 

I am sorry if I didnt explain fully what I wanted you to do.

The temp sesnor is nothing more variable resistor whos resistance changes with temperature. If you take the wire off the sensor and ground it the coolant gage should go to H. If you leave it disconnected is should go to C. If the wire is disconnected and the gage still shows H then there is a problem with the gage or the wiring for the gage.

 

thanks so much for the replies guys! Will follow instructions exactly and return here...…..

 

yup, evidently I fried the sensor.........(cursing, tryin' to kick my own behind).................and suddenly the truck won't start.

edit:
d'oh!
Rookie move: Loose ground wire(s)=engine no start. Took me a few minutes of backtracking to figure out that although I'd put the engine ground connections back in place (after fixing the vacuum leak at the fuel injector) I had failed to properly tighten them down.