The governor spring is weak or missing.

Replace the governor. Refer to Allison service manuals.

The mechanical actuator is kinked, broken, or not properly adjusted.

Replace or adjust the cable. Refer to Allison service manuals.

The adjustment of the shift signal valve spring is too loose.

Tighten the spring adjusting ring to the specification. Refer to Allison service manuals.

The modulator valve is stuck open.

Check the spring adjustment. Refer to Allison service manuals.

The modulator valve is stuck open.

Replace the modulator. Refer to Transmission Modulator Replacement .