The ready light on my Vanguard comes on when the water temp is within 2 degrees +/- of set temp. Example: set at 98, ready light will be on if the tub is within 96 - 100 degrees. If the water temp is 105, then the ready light won't come on until it drops some heat. Figure it is the same for the Grandee.
Good luck! My ready light hasn't been on all summer. The Oklahoma heat has seen to that!