Tempture gauge isn't working

[putnam00]

I'm having a problem. I got rid of the dummy lights on my 71 c-10 and a gauge cluster from a 72 GMC, I rewired the harness for the gauges. I am running an lt1 and had to use a sending unit that has smaller threads for the heads. what I was told is the resistance is the same on the sending units. when I turn on the key on it went strait to Hot. I had to ground the gauge to get it to go to Cold but my engine fan turns on at 210 and with it ungrounded it is always hot and grounded always cold. my other question is what is the resistor on the ground for? Please help