A large distribution center reported that a Jungheinrich EFG 320 48V reach truck completely lost power mid-lift. Cycling the key switch resulted in a "Main Contactor Welded" fault on the dashboard, and the truck refused to drive or lift.
The technician first checked the main contactor located under the battery compartment. He attempted to separate the contactor tips with an insulated tool, but they were physically welded together. A contactor welds when it experiences a massive inrush current upon closure, typically caused by a dead short downstream. To find the short, the technician cut the welded contactor tips apart and used a multimeter to test the hydraulic pump motor. The traction motor controller tested fine, but the resistance between the hydraulic pump motor phases was near 0 ohms (a dead short).
Upon removing the hydraulic pump motor's rear end bell, the cause was clear. The "Carbon Brushes" had worn down so severely that the bare, energized brush springs were resting directly on the commutator. The exposed springs had arced across the commutator bars, creating a phase-to-phase dead short. When the Battery Management System (BMS) closed the main contactor to complete the pre-charge circuit, the massive short-circuit current instantly welded the contactor tips together. The electrical arc had also heavily burned the commutator surface.
The repair involved replacing the main contactor assembly and the complete hydraulic pump motor. The commutator was unrecoverable due to arcing damage, so a brand new motor assembly was installed. The contactor housing was cleaned to remove arc soot. After reassembly, the BMS was reset, and the truck returned to normal operation. The root cause was traced to a missed preventive maintenance interval; the 3,000-hour carbon brush inspection had been overlooked, allowing the brushes to wear to the point of catastrophic electrical failure.
