The most common cause of speed control not working is either a faulty brake pedal switch, or the speed control deactivation switch. Make sure your brake lights work properly. If they do, that switch is fine. If they don't, fix them first. Secondly, if the brake lights are ok, check the other switch - it's mounted above the brake pedal switch on the brake pedal lever. There should be power on both wires to that switch all the time with the pedal released, and with the pedal depressed, power should only be on one wire. If you only have power on one wire with the pedal released, replace the switch.