Two technicians are altering shift points in an Allison hydraulically controlled transmission. Technician A says this can be done by changing the spring tension on the modulator valve. Technician B says it can be accomplished by adjusting the spring tension on the individual shift signal valves. Who is correct?

Two technicians are altering shift points in an Allison hydraulically controlled transmission. Technician A says this can be done by changing the spring tension on the modulator valve. Technician B says it can be accomplished by adjusting the spring tension on the individual shift signal valves. Who is correct?

• A. Technician A
• B. Technician B
• C. Both Technician A and Technician B
• D. Neither Technician A nor Technician B

Answer: (C)

Shift points in an Allison hydraulically controlled transmission are set by hydraulic pressure that triggers the shift valves at the right engine and road conditions. The modulator valve uses spring tension to establish the baseline throttle-pressure relationship that governs when shifts begin. Changing that spring tension shifts the point at which the system considers enough load to upshift, so altering the modulator spring changes overall shift timing. Each gear change is also controlled by its own shift signal valve, which has a spring that sets the threshold for that particular shift. Adjusting the spring tension on these shift signal valves changes the exact conditions (speed/load) that trigger that gear change, allowing tuning of individual shift points. Because both the modulator and the individual shift signal valves influence when shifts occur, both technicians are correct. Downgrading or upping those spring tensions must be done within the manufacturer’s specs to avoid harsh shifts, slip, or unintended gear selection.

Shift points in an Allison hydraulically controlled transmission are set by hydraulic pressure that triggers the shift valves at the right engine and road conditions. The modulator valve uses spring tension to establish the baseline throttle-pressure relationship that governs when shifts begin. Changing that spring tension shifts the point at which the system considers enough load to upshift, so altering the modulator spring changes overall shift timing.

Each gear change is also controlled by its own shift signal valve, which has a spring that sets the threshold for that particular shift. Adjusting the spring tension on these shift signal valves changes the exact conditions (speed/load) that trigger that gear change, allowing tuning of individual shift points.

Because both the modulator and the individual shift signal valves influence when shifts occur, both technicians are correct. Downgrading or upping those spring tensions must be done within the manufacturer’s specs to avoid harsh shifts, slip, or unintended gear selection.