temperature gauge pegs out on Kodiak/topkick
 

I am helping a friend of mine, who has actually a 1998 GMC Topkick with 366 gas engine.
When he starts it up, within 1 minute or less, the temperature gauge goes from cold, to pegging out to the hot side of the gauge.
Thinking it was a sender on the motor, we checked the sender, and found the yellow wire that comes from the sender, (It's a 2 wire sender, black, and yellow wire) is grounded to the intake manifold.
I haven't seen any wire that has been cut, but from what I've seen of the black wire being crimped to a black wire from the harness, that something is wrong.
Should the sender be a 1 wire sender, or 2 wire. The truck has a gauge, and not idiot light.
Does anyone have a schematic for this, so I may be able to trace, and find the Yellow wire circuit.
Any and all answers will be appreciated.