Yup.

I bought it 4 weeks ago, used the car 3 times since and all was great. Then yesterday, I changed my tranny, today, I start the car and the gauge shows 210F as soon as I start the car...Any idea why? Could it already be broken?

Its a Water temp autometer gauge.

 

it sounds like you have the sender wire pinched somewhere and shorting to ground
I would look at where the bellhousing bolts to the engine and look for pinched wires

 

Yeah, I'm going to go with a wiring problem.

 

maybe you melted the insulation off the sender wire?

 

Were it dead shorted wouldn't the gauge read full hot?

 

it's probably just a ground issue related specifically to the sender. Check your wiring.

 

If it was a dead short you would be correct but a pinched wire may not cause a perfect connection and have some resistance causing it to read lower than full hot.
bad ground problems with the sender or open circuits with the wiring will cause the guage to read low not high.
The reason I suggested looking for pinched wires where the trans bolts to the engine is because thats the most likely cause after having the trans out.
I have been a mechanic (I still prefer that over tech) for 28 years (yeah I know I am old) but honestly nobody can say for sure what the problem is without looking at it , just give the most likely causes.

 

ground got unplugged, problem solved.