JLG Articulating Boom Lift 31 Fault Code: Contactor Did Not Close When Energized

What does JLG Articulating Boom Lift error code 31 mean?

Fault code 31 is not a single fault on JLG articulating booms. It is reused across at least three different systems: the traction contactor circuit, the engine coolant temperature warning circuit, and the fuel level/engine shutdown logic. Read the description text on your display or in the fault log alongside the code number, because the corrective steps are completely different for each version.

In the contactor version, the control system checks whether the line contactor or a direction contactor actually closes when it is told to. It does this by watching a point in the traction circuit called traction point A and by watching whether the capacitor bank charge rises to match battery supply. If the expected voltage change does not happen, or happens at the wrong time, the controller logs this fault because it cannot confirm the contactor closed properly.

In the coolant temperature version, the fault simply means engine coolant temperature has climbed past the warning level programmed into the control system. This is a functional warning, meaning the machine keeps running but limits certain functions, and the fault clears itself once coolant temperature drops back below the recovery level. In the fuel version, the engine has already shut down because the fuel tank ran empty, and the control system locks out cranking and throttle until fuel is added.

Common causes of 31

• Line contactor and direction contactor coil wiring swapped at installation or after a repair, so traction point A goes high and capacitor bank charge fails to rise to battery supply when the line contactor closes
• Open circuit in the forward direction contactor or its wiring, or an open circuit in the traction motor itself, so traction point A never goes high when the forward contactor is energized
• Open circuit in the line contactor or a power wiring fault, so capacitor bank charge does not rise to battery supply when the line contactor is energized
• Open circuit in the reverse direction contactor or traction motor, or a wiring error, so traction point A never goes high when the reverse contactor is energized
• Coolant temperature has risen above the programmed warning level, often from low coolant, a cooling system problem, or a faulty coolant temperature sensor or its cable
• Fuel tank has run empty, triggering an automatic engine shutdown and lockout of cranking and throttle functions

How to troubleshoot JLG Articulating Boom Lift 31: first checks

1. 1Check which fault description is showing on the display (contactor, coolant temperature, or fuel level) since the troubleshooting path is different for each
2. 2For the contactor version, visually trace and verify the line contactor and direction contactor coil wiring is not swapped, especially after any recent electrical repair
3. 3For the contactor version, check the traction motor and contactor power wiring for open circuits or loose/corroded connections
4. 4Check engine coolant level and condition, and inspect the coolant temperature sensor and its cable for damage or poor connection
5. 5Check the fuel level in the tank and add fuel if it is low or empty, then attempt to restart once fuel is added

How the code clears

For the coolant temperature warning, the fault message clears on its own once coolant temperature drops back below the recovery level programmed into the control system; there is a delay before fill limitation is reapplied if the machine is used again. For the fuel level shutdown, add fuel to the tank to clear the cranking and throttle lockout. No separate clearing step is listed for the contactor version beyond correcting the wiring or open-circuit fault that caused it.