A construction-materials distribution yard reported that a Linde E25 2.5-ton electric counterbalance forklift had developed a progressive travel-performance fault over a two-month period. When new, the truck accelerated briskly from a standstill and maintained 18 km/h top speed. Over the past two months, acceleration had become progressively sluggish, and top speed had dropped to roughly 12 km/h. When carrying a full 2.5-ton load up the yard's slight gradient, the truck would slow noticeably and the operator reported a faint "electrical" smell from under the seat panel. The truck's battery discharge indicator showed a faster-than-normal charge depletion during shifts - indicating the truck was consuming more energy than expected for the same work.
The technician first connected the Linde diagnostic tool and retrieved live drive-controller data. During a standard acceleration test, the drive controller was requesting maximum motor current - the controller's output current read 220 amps - far above the normal 130-amp range for this truck under similar conditions. This pattern indicated the traction motor was consuming excessive current to deliver reduced torque output, consistent with either motor-internal resistance losses or mechanical drag on the drive axle.
He inspected the "Traction Controller" first. The controller - the power electronics unit mounted under the seat panel that converts battery DC power into variable-voltage output for the traction motor - showed a cooling fan that was functional but whose intake grill was heavily clogged with fine dust and paper debris. More significantly, the controller's heatsink showed a layer of dried, cracked thermal paste between the heatsink and the controller's power semiconductor modules - a sign that the controller had been operating at elevated junction temperatures for an extended period. He then inspected the traction motor. The E25 uses a DC series motor with carbon brushes; he removed the brush caps and found all four carbon brushes below their minimum-length specification, with two brushes showing weakened spring pressure and one brush showing a chipped edge where the brush's carbon had cracked under a worn spring's uneven pressure. The commutator surface showed visible carbon buildup and light pitting on several segments.
The root cause was traced to a combination of controller cooling restriction and motor brush wear, both accelerated by the yard's dusty environment. The clogged controller heatsink had elevated the controller's operating temperature, triggering progressive thermal derating that reduced available motor torque; the worn brushes had increased motor-to-commutator resistance, drawing additional current to deliver the same torque - compounding the controller's thermal load and the truck's overall energy consumption.
The repair involved cleaning the controller's cooling fan, intake grill, and heatsink with compressed air, removing the dried thermal paste and applying a fresh layer of high-conductivity thermal compound, and replacing all four traction-motor carbon brushes with new units. The commutator was dressed with a commutator stone to remove carbon buildup and restore a smooth brush-track surface, and the brush holders were cleaned of carbon dust. After reassembly, a full loaded acceleration test showed motor current restored to the 130-amp normal range, top speed recovered to 18 km/h, and the electrical smell did not return during a full-shift test cycle. Controller heatsink cleaning and carbon-brush inspection were added to the truck's 250-hour maintenance checklist.
