Why does the cruise control light show its on but it wont set

Cruise control can be controlled a few different ways on vehicles. It's possible that it's vacuum controlled and if so, there may be a vacuum leak which would allow it to show as set but it not hold enough vacuum to actually maintain speed. Another scenario could be that it is cable driven and the cable is set, but perhaps not connected correctly. These are just a few examples, we would love to check it out for you to give you a more thorough diagnosis. Give us a call at Brotherhood Garage.