2016 Mack Pinnacle - Allison 4500 RDS Shifting Hard and Hunting

Question

My Mack is having a hard time deciding which gear to be in. When accelerating, it slams into 3rd gear and sometimes the speedometer drops to zero for a split second while I'm moving. It feels like the transmission is confused.

Fluid level is correct and smells fresh, not burnt. I'm getting a code for Output Shaft Speed Sensor circuit no signal. I'm wondering if it's the sensor or the wiring harness going to the Allison TCM.

GearheadGus · Answer

✅ Confirmed fix: This was a failed Output Speed Sensor. The internal magnet had cracked. Replaced the sensor on the tail-shaft of the Allison and the shifting returned to smooth transition. Very common on vocational trucks that see high vibration.

WesternStarWill · Answer

Check the battery grounds. Allison transmissions are extremely sensitive to voltage drops. If the ground to the TCM is loose, it will cause sensor signal dropouts that look like component failures.

2016 Mack Pinnacle - Allison 4500 RDS Shifting Hard and Hunting

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