I was changing out an old mercury switch dial thermometer for my AC to a chromotherm IV T8600D2028. I wired it as instructed with Y to Y, G to G, and R to Rc with a jumper to R. They also give a resistor to wire between Y and W. With this configuration the thermostat activates the AC but then goes blank. If I wire it without the resistor the same thing happens. Is there a common I should have connected to the thermostat? And if so how do I find it? Any thoughts?

You may need a common wire to the thermostat.