A distribution center reported a bizarre issue on a Hyster S50FT LPG forklift. The engine would sometimes start and run perfectly, but other times it would crank without starting, or stall while driving. The fault was completely intermittent and seemed to happen more often when the forklift was traveling over bumps.
The technician connected the diagnostic laptop and retrieved a "Crankshaft Position (CKP) Sensor Signal Intermittent" fault. The CKP sensor tells the engine computer (ECU) the position of the crankshaft so it can fire the spark plugs and injectors. If the signal is lost, the engine dies instantly.
The sensor was replaced, but the problem returned the next day. The technician removed the starter motor to inspect the flywheel and the sensor tone ring (the toothed wheel that the sensor reads). The tone ring is pressed onto the crankshaft. The technician found that the tone ring had cracked and was wobbling on the crankshaft snout.
When the forklift hit a bump, the wobbling tone ring would move away from the CKP sensor, creating a gap too large for the sensor to read. This caused the signal loss and the stall. Worse, the wobbling ring had been striking the CKP sensor tip, damaging the new sensor.
The root cause was traced to a previous starter failure where the starter pinion gear had extended too far and engaged the flywheel aggressively, cracking the interference fit of the tone ring on the crankshaft.
The repair involved pressing a new tone ring onto the crankshaft and installing a second CKP sensor. The starter motor was also inspected for proper pinion engagement depth to prevent the ring gear from being damaged again. After the repair, the forklift started instantly and ran smoothly over the roughest yard surfaces without a single stall.
