I put a new temp sending unit on my 73 F-350 and now the gauge always pegs all the way over to full hot. The old sender did not work at all, I did not change anything but the sender. Could the new sender be bad or do I need to look elsewhere? Thanks for any help.

"bowties in the rearview mirror"

 

Could it be an idiot light sensor which is on or off, or is it a gauge sensor which is variable?

 

Could also be you put in a switch-type sender instead of a thermistor-type.

 

From my reading, the test for sending temp to gauge is to take the wire off of sender and short it to ground with a jumper wire. Turn on the key (just half way, not to start), if gauge goes to hot then it's your sending unit that's bad. Sounds like your new part might be shorting out.