What would be the complaint or symptom in an Allison AT series transmission if a trimmer valve were stuck open?

What would be the complaint or symptom in an Allison AT series transmission if a trimmer valve were stuck open?

• A. The clutch that the trimmer controls would not engage.
• B. The clutch that the trimmer controls would shift at a lower road speed.
• C. The clutch that the trimmer valve controls would have a harsh engagement.
• D. The clutch that the trimmer controls would slip and burn out.

Answer: (D)

The main idea here is how the trimmer valve regulates the pressure that applies a clutch in the Allison AT series. When the trimmer valve is stuck open, it can’t hold enough pressure to fully engage the clutch, so the clutch stays partially released as torque is applied. That partial engagement means the clutch slips instead of locking up, which generates heat and accelerates wear, potentially leading to burnout of the clutch pack. If the valve were stuck open, you wouldn’t necessarily see the clutch fail to engage entirely (that would point to a different fault), nor would you expect an abrupt, harsh engagement (that would suggest an overpressure situation). The slipping and overheating description best matches the consequence of insufficient, uncontrolled apply pressure from a stuck-open trimmer valve.

The main idea here is how the trimmer valve regulates the pressure that applies a clutch in the Allison AT series. When the trimmer valve is stuck open, it can’t hold enough pressure to fully engage the clutch, so the clutch stays partially released as torque is applied. That partial engagement means the clutch slips instead of locking up, which generates heat and accelerates wear, potentially leading to burnout of the clutch pack.

If the valve were stuck open, you wouldn’t necessarily see the clutch fail to engage entirely (that would point to a different fault), nor would you expect an abrupt, harsh engagement (that would suggest an overpressure situation). The slipping and overheating description best matches the consequence of insufficient, uncontrolled apply pressure from a stuck-open trimmer valve.