A lumberyard reported that a Combilift C4000 4-way electric sideloader ran normally for 10 minutes, then the traction motor shuddered violently and the truck lost all drive. The dashboard displayed a "Traction Controller Thermal Shutdown" fault. After 15 minutes of cooldown, the truck would drive again for 10 minutes before the fault recurred. The hydraulic steering and lift functions were unaffected.
The technician first inspected the "Traction Controller" housing mounted under the battery compartment. The controller's external aluminum heat-sink fins were packed solid with sawdust and wood shavings, completely blocking the airflow. The controller uses a forced-air cooling fan to dissipate the heat generated by the MOSFET switching bridge. With the fins blocked, the controller's internal temperature exceeded 95°C, triggering the thermal shutdown to protect the MOSFETs from thermal runaway.
However, the technician suspected a secondary electrical fault, because a clean controller on this truck typically maintained 60°C. He removed the controller's output phase cables and used an oscilloscope to monitor the three-phase PWM output while driving the motor on a dynamometer. The U-phase waveform was intermittently dropping out for 20 milliseconds every 2 seconds, indicating a failing U-phase MOSFET transistor. The MOSFET's gate was partially shorted, causing it to switch off mid-cycle. When the U-phase dropped out, the V and W phases continued driving, creating a massive torque ripple that physically shuddered the motor and spiked the phase current, generating additional controller heat that accelerated the thermal shutdown.
The root cause of the failing MOSFET was traced to the sawdust blockage. The blocked airflow had allowed the controller to run hot for an extended period, degrading the MOSFET's thermal paste interface and eventually causing the U-phase transistor's internal gate oxide layer to break down under sustained high-temperature switching.
The repair involved replacing the complete traction controller (the internal MOSFET bridge was not field-repairable) and fabricating a steel sawdust shield with a coarse pre-filter screen over the controller's cooling-air intake. The new controller's heat-sink was coated with fresh thermal interface compound, and the cooling fan was tested for full airflow. After reassembly and a phase-current relearn calibration, the sideloader ran continuously for 90 minutes under heavy load with no thermal shutdown or motor shudder, and the controller stabilized at 62°C.
