A beverage distribution warehouse reported that a Linde E20 386 Series 2-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 its 20 km/h top speed across a full shift. Over the past two months, acceleration had become progressively sluggish, and top speed had dropped to roughly 14 km/h. When carrying a full 2-ton pallet load across the warehouse's polished-concrete floor, the truck would slow noticeably, and the operator reported a faint "electrical" odor from under the truck's hood panel. The truck's battery discharge indicator showed faster-than-normal charge depletion during shifts - indicating the truck was consuming more energy than expected for the same work.
The technician first verified the symptom by driving the truck with a loaded test pallet across a measured course and monitoring travel performance. The truck's top speed was confirmed at roughly 14 km/h, and during acceleration under load the drive motor's case was noticeably warmer than normal after a few minutes of operation.
He connected the Linde diagnostic tool and retrieved live drive-controller data. During a standard loaded acceleration test, the drive controller was requesting maximum motor current - the controller's output current read 175 amps, far above the normal 105-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 motor's cooling system first. The Linde E20's drive motor uses a forced-air cooling design - a small fan mounted on the motor's shaft end that draws ambient air across the motor's case to dissipate heat during operation. The cooling fan's intake grill was heavily clogged with fine paper dust, plastic film fragments, and warehouse-floor debris - restricting airflow across the motor case and elevating the motor's operating temperature during extended operation. The elevated temperature had accelerated the wear of the motor's internal components.
He then inspected the drive motor's "Carbon Brushes" and "Commutator." The Linde E20 386 Series uses a brush-type DC drive motor, with carbon brushes held against the rotating commutator by calibrated springs to deliver electrical current to the motor's armature windings. He removed the brush caps and found all four carbon brushes significantly 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's surface showed visible carbon buildup and light pitting on several segments - indicating the worn brushes had allowed excessive arcing at the brush-commutator interface, which had eroded the commutator's surface and further increased electrical resistance.
The root cause was traced to carbon brush wear compounded by motor cooling restriction. The clogged cooling fan intake had elevated the motor's operating temperature, which had accelerated the wear of the carbon brushes and weakened their spring pressure. The worn brushes had increased electrical resistance at the brush-commutator interface, causing the motor to draw excessive current to deliver the same torque - compounding the motor's thermal load, the truck's overall energy consumption, and the progressive power loss the operator described.
The repair involved cleaning the motor cooling fan's intake grill and fan blades with compressed air, removing all accumulated dust and debris, and replacing all four 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. The brush springs were replaced with new units to restore proper brush pressure. The drive axle was inspected for mechanical drag (none found), and the truck's drive controller calibration was verified with the diagnostic tool. After reassembly, a full loaded acceleration test showed motor current restored to the 105-amp normal range, top speed recovered to 20 km/h, and the electrical odor did not return during a full-shift test cycle. Motor cooling fan cleaning and carbon brush inspection were added to the truck's 250-hour preventive maintenance checklist.
