A quarry operation reported that a John Deere 844K wheel loader was "hunting" between gears while traveling up a haul road. The transmission would shift from 3rd to 2nd and back again rapidly, causing a jerky ride and a severe loss of momentum. The transmission temperature was also running higher than normal.
The technician connected John Deere's Service Advisor diagnostic software. The fault logs showed intermittent "Loss of Output Speed Signal" faults. The 844K transmission uses the output speed sensor to calculate the appropriate shift points and lock-up engagement for the torque converter. If the signal is lost or erratic, the Transmission Control Unit (TCU) defaults to a "limp home" mode or attempts to re-engage the gear aggressively.
The output speed sensor, located on the transmission housing near the drive shaft, was removed. It was covered in a thick layer of metallic sludge from previous internal transmission wear. The sensor was cleaned, but the resistance reading was still out of specification compared to a new sensor. The sensor had been weakened by heat and vibration.
However, the metallic sludge pointed to a deeper issue. The transmission oil pan was dropped, and the internal filter was found to be clogged with clutch material friction discs. The sludge had restricted the fluid flow to the shift solenoids, causing them to react sluggishly. This slow reaction time meant the clutches were slipping during shifts, generating heat and debris, which further coated the speed sensor.
The repair involved replacing the faulty output speed sensor. The transmission valve body was removed and flushed to clear the debris from the shift solenoid passages. Because the clutch material in the filter indicated advanced wear, the technician recommended a transmission oil change with a flushing agent, but the clutch packs were left in place as the machine was due for a rebuild cycle. After replacing the sensor and flushing the system, the TCU was recalibrated. The shifting stabilized immediately, though the machine was flagged for a future transmission overhaul due to the clutch debris.
