A reach truck equipped with a hydraulic fork positioner (which allows the operator to adjust the width of the forks from the seat) was crushing expensive product. The operator would set the forks to the correct pallet width, drive into the rack, and as they lifted the load, the forks would slowly and automatically close inward, crushing the product.
The shop replaced the hydraulic valve block and the cylinder seals, assuming internal bypass was causing the drift. It didn't fix the issue. We looked at the electronic control system. Modern positioners use a Hall-effect sensor mounted on the cylinder to tell the controller the exact width of the forks. A magnet on the piston rod moves past the sensor, generating a voltage signal proportional to the position.
We removed the sensor from the side of the cylinder. It was covered in a thick, greasy, black sludge. The cylinder's rod seal had been weeping a microscopic amount of oil, which mixed with the metal dust created by the positioner sliding on the mast rails. This magnetic metal dust had migrated into the sensor recess and coated the Hall-effect chip.
The metal dust was distorting the magnetic field. When the operator set the forks wide, the controller read the correct voltage. But when the load was lifted and the truck vibrated, the metal dust shifted on the sensor. The controller suddenly received a false voltage signal indicating the forks were wider than commanded, so it automatically pulsed the "close" solenoid to correct the position, crushing the load. We cleaned the sensor, fixed the weeping rod seal, and the positioner held its width perfectly. When a position sensor acts erratically, always look for magnetic contamination before replacing the ECU.
