just went through this w/ same unit, replaced line contactor & controller, turned out to be internally shorted display/BDI, if this unit has zapi controller, unplug BDI @ controller, lower front left plug on controller, (where you would hook up handset), truck will run w/ this unplugged o.k. untill you can get replacement part, hope this helps.

there is a switch that activates when you pull the handle down to operate the truck, it sounds like this switch is bad?
if this switch test ok then do what brewski says, check wiring and other components connected to controller for faults.

This is a short circuit in a device connected to main controller. Check the wire harness for shorts in main contactor wiring, solenoid valve wiring and brake coil wiring.