On Allison transmissions, which valves direct clutch apply pressure to the clutch packs?

On Allison transmissions, which valves direct clutch apply pressure to the clutch packs?

• A. shift relay
• B. modulator
• C. shift signal
• D. trimmer

Answer: (A)

In Allison transmissions, clutch packs are energized by hydraulic pressure that is routed through the valve body to the appropriate clutches. The valve that directly directs this apply pressure to the clutch packs is the shift relay valve. It receives the shift command and opens passages that send line pressure to the clutches needed for the selected gear, while other clutches are relieved or held off. The modulator valve mainly adjusts overall line pressure in response to engine load, not which specific clutch gets pressed. The term shift signal refers to the gear-change input, not the hydraulic path that actually directs clutch pressure. The trimmer valve is for fine-tuning pressure in certain circuits, not for directing clutch apply. So, the shift relay valve is the correct one for directing clutch apply pressure.

In Allison transmissions, clutch packs are energized by hydraulic pressure that is routed through the valve body to the appropriate clutches. The valve that directly directs this apply pressure to the clutch packs is the shift relay valve. It receives the shift command and opens passages that send line pressure to the clutches needed for the selected gear, while other clutches are relieved or held off.

The modulator valve mainly adjusts overall line pressure in response to engine load, not which specific clutch gets pressed. The term shift signal refers to the gear-change input, not the hydraulic path that actually directs clutch pressure. The trimmer valve is for fine-tuning pressure in certain circuits, not for directing clutch apply. So, the shift relay valve is the correct one for directing clutch apply pressure.