Temperature gauge bad?

[atarijake]

Hello all, I recently bought an 85 GMC Sierra with a few bugs...

I've fixed the oil gauge but now it's time to try and fix the temp gauge, it reads hot all the time. I can take my finger and move the needle down to 0 but it shoots up past hot as soon as I let it go.

I replaced the sending unit and still no change. I connected a test light to the prongs on the dash to verify power, ground and they're both good.

I also tested the sending unit wire by unplugging the wire, connecting that to power and testing to see if the test light would light up on the dash.. which it did. So I thought the wire was good, but the gauge still reads hot. So I thought maybe it was grounded but just barely? Creating resistance?

I'm not a mechanic, so I just took the gauge out of the dash and manually hooked it up to the battery positive and negative posts via jumper leads and manually connected it to the temp sender via a jumper... and it STILL reads all the way hot.

Is it just the gauge that's bad? I can't come up with anything else, but before I spend 50 bucks on it I figured I'd check with the experts.

Thanks!