Strange temp gauge

[K10bolts]

I have a strange problem with my temp gauge in my 83 K10. The gauge only registers when the key is in the off, or accessory position. If I turn it to run, the gauge zeros out. I'm really stumped.

Anyone have any suggestions on where I should start my troubleshooting?
Thanks, Matt

 

[MarcS]

Sending units bad.

 

[dyeager535]

You can unplug the sending unit wire, with the key in "run", ground the wire. Gauge should peg out if that is all ok.

 

[K10bolts]

I have tested the sender and it is good. The problem is somewhere else in the circuit. I will have to check the wires and the circuit board under the dash I guess.