first get out the potentiometer from the steering column and move it with your hand. then use a multimeter to take the voltage in the viper pin moving the pot in both sides.
there are two IGBT transistor. you can interchange it in order to know if the problem is the transistor or the chopper. first you must check that the chopper is OK!! i don´t know if you can do it.
if you can send me a photo of the chopper and transistor and i will tell you where you can find the problem
silvio_jorge A hotmail com