The thumb warmer usually wears out long before the grip warmers do. Then all of the electricity is going to the grips and makes them feel warmer. It's a common problem. I've replaced 4 or 5 thumb warmers on my Vmax-4, still using the original grip warmers. I've also replaced 3 or 4 thumb warmers on my Exciter, still using the original grips.
Shark