hi, have the above truck, actually 2. having some problems with one truck. the motor spins and the wheels will spin without resistance. holding each wheel independently the other will spin. holding both at the same time the motor will continue to spin. doing this on the other truck and it will just out of my hands. i replaced a worn hex on one of the tires. people mention the slipper clutch. how do i know if the pads are bad? took both trucks apart and they look similar. is there another component past the slipper clutch, like in the transmission? stumped.