We purchased a JLG scan tool. Ran a system test. The truck failed Steer test. Also checked the Code log under "Help" it showed a Code 7740. Traction Current @ Zero. "Check shunt wiring". P2 on the power module checked fine. I could duplicate Code 77 by disconnecting P2 and powering up the truck. No Motor or wiring problems where found. I swapped Power controllers with another E600J and the code 77 cleared, so I installed the bad power module on the other lift and the code moved to that lift. Ordered new power module. Thank you for your direction.
Hope this helps others who may run into the same problem.

You can download a manual from jlg for this truck....go to jlg dot com and you can dowload the manual...

Hey Kevin,
Thank you for the reply. Very helpful info.
It will help me understand this part of the truck operation once I understand what it is actually saying. I thought it could have been caused by low battery voltage, but the hydraulic boom functions work normally. Or that the hydraulic brake on the hub side of the motors may not be releasing.
Can I check the if the MOSFETs are on or off without a scan tool?

Code 7-7:

CAPACITOR BANK FAULT - CHECK POWER/MOTOR WIRING The capacitor bank is not charging. This is probably due to a motor or power wiring error causing illegal current drain; it could also be due to a very low battery supply. OPEN-CIRCUIT PUMP MOTOR Pump point A is collapsing when the pump MOSFETs are pulsed. This is probably due to an open circuit pump motor or a power wiring error. WARNING: If the power wiring between the power module & contactor panel is wrong, this fault can occur - any attempt to operate the vehicle could damage the power module. OPEN-CIRCUIT DIRECTION CONTACTOR OR TRACTION MOTOR Traction point A is collapsing when the traction MOSFETs are pulsed. This is probably due to an open circuit traction motor or direc- tional contactor or wiring error. NOTE: This fault is unlikely to be seen due to interaction with speed control... PUMP POINT A LOW - CHECK POWER / MOTOR WIRING Pump point A is near 0V when the pump MOSFETs are off. This is probably due to a motor or power wiring error. STALLED TRACTION MOTOR The power module traction MOSFET protection circuit is active. This is due to massive current drain and could be a stalled traction motor or a power wiring error. Flash code 7/7 indicates problems with a motor (continued). STALLED PUMP MOTOR The power module pump MOSFET protection circuit is active. This is due to massive current drain and could be a stalled pump motor or a power wiring error. TRACTION MOTOR OVERLOADED The traction motor is drawing Excessive current for an extended period of time; the system shuts down to prevent destruction of the motor or power module. PUMP MOTOR OVERLOADED The pump motor has been operating in current limit at a low percentage on for a period of time greater than 10 seconds TRACTION CURRENT AT ZERO - CHECK SHUNT WIRING Traction current measurement is at zero. This is probably due to an open-circuit between the current measurement shunt and the power module. TRACTION POINT A HIGH - CHECK POWER / MOTOR WIRING Traction point A is near battery supply when neither direction contactor is energized and the traction MOSFETs are off. This could be due to a welded direction contactor or a power wiring error. TRACTION POINT A LOW - CHECK POWER / MOTOR WIRING Traction point A is near 0V when neither direction contactor is energized and the traction MOSFETs are off. This could be due to a power wiring error.