A building-materials distribution center reported that a Jungheinrich EFG DF18 1.8-ton electric counterbalance forklift had developed progressively reduced travel performance. The truck still reached its normal top speed on flat ground, but on slight grades - the warehouse's ramp entrances and dock ramps - travel speed dropped to roughly 60 percent of normal unless the operator commanded full travel. Acceleration response was also noticeably sluggish, and the operator described a distinct "hesitation" when starting from a standstill. Battery voltage and state of charge were normal, verified with the diagnostic tool.
The technician first connected the forklift to the Jungheinrich diagnostic tool and pulled the drive motor's live data. On flat ground, the drive controller limited motor current to 30 amps (within normal range) and delivered normal torque output. However, on a simulated grade test cycle, motor current climbed sharply to 75 amps - the controller was requesting maximum torque, but the motor's effective torque output was extremely low, resulting in excessive current draw from the battery to achieve the torque demand. This pattern - high current draw with low torque output - pointed to internal resistance losses within the drive motor itself.
He inspected the drive motor. The EFG DF18 uses a DC motor with carbon brushes, whose output passes through a single-stage reduction gearbox to the drive axle. He removed the motor's brush caps to inspect the "Carbon Brushes" and "Commutator." Three of the four carbon brushes were below the 4-millimeter minimum length specification (new brushes are roughly 15 millimeters long). One brush's spring pressure was noticeably weak - the spring had taken a permanent set and was no longer applying the required force to hold the brush against the commutator surface. The commutator surface showed pronounced pitting, with deep craters on several bar segments and a dark carbonized deposit layer - consistent with prolonged high-current arcing.
The root cause was traced to brush wear without replacement. Carbon brushes are consumable items in a DC motor that wear with normal use; they should be replaced by a preventive maintenance schedule before reaching the minimum length specification. The forklift's service history showed no brush inspections for years - and as the brushes wore to low length, the reduced spring pressure and reduced brush-to-commutator contact area increased contact resistance at the brush-commutator interface, which increased arcing, pitted the commutator, and progressively weakened the motor.
The repair involved replacing all four carbon brushes with new units, machining the commutator surface (turning) to remove the pitting and carbon deposit and restore a smooth cylindrical brush track. After the repair, the commutator's brush tracks were hand-polished with fine abrasive paper, the brush-undercut gaps were cleaned, and the new carbon brushes were installed at the correct spring pressure. The drive controller's software calibration was verified correct with the diagnostic tool. After reassembly, the forklift recovered full travel speed on flat ground, climbed the dock ramps without noticeable speed reduction, and the acceleration response was restored to immediate. Motor current remained within specification throughout a loaded test cycle.
