hmm strange if it worked before tthese earlier dc trucks usuall y had some kind of earth leakage to frame when there was a fault so what you need to determine is that this is not the case so two things disconnect battery and connect multi meter to machine plug to see if there is any grounding of the unit to the frame and also when plugged in connect meter to battery earth and the put probe to frame see if you get areading which i suspect you will see if it is surface or hard voltage if you get a reading try this test with a 12v test light from batt neg to frame with battery plugged in if you follow me last resort remove battery from unit and plug in to see if unit will start up to give a ne example of what can occur some years back a tech i know went to servic a similar yoyota unit the battery pack was leaking to frame when he opened the service door some paper clips fell into the contactor control board area and shorted out the cpu board due to the battery leaking to the frame so its important that there iis no earth leakage to frame or battery leakage to frame and also sometimes one of the motors can also create a short to frame which if the case canbe isolated by diconnecting the cables leading to the control boards which isolates the motor unfortunatley its a process of elimination but i fear that maybe cpu board has been shorted somehow good luck