It depends if it is a control fault or power circuit fault. Start with raising drive tyre off of the ground or removing one of the drive motor cables from the terminal block on the transmission casing. Swap door sw wire to the N/C terminal and then pull down tiller and test either Fwd or Rev, then 2nd speed. If middle contactor doesn't close, then check input voltage at 2nd speed sw, then output voltage from sw. If OK, then check voltage at the 2nd speed contactor coil (middle contactor). If contactor is closing, check power circuit voltage in and out of contactor and then in and out of power resistor that is hidden inside the door behind the steer post. I'll take a stab in the dark and say the 2nd speed micro sw in the handle isn't closing or o/c