Abstract: A compound transmission for a transport vehicle which has a manually operated main gearbox and an auxiliary two-range gearbox tandemly connected therewith and having a high-gear range and low-gear range shifting by a servo actuator. A control mechanism of this compound transmission comprises a device applicable for use as a fail safe device in manual shifting the auxiliary gearbox to low-gear range in case of failure of the servo actuator of the auxiliary gearbox while it is in the high-gear range. The control mechanism is provided with four sliders. The first second and third sliders are shifted by the drive element connected with the manually operated hand lever. The fourth slider is shifted by the servo actuator. The first and the second sliders engage the forward speeds in the main gearbox. The third slider engages the reverse speed in the main gearbox. The fourth slider engages the low-gear range and the high-gear range in the auxiliary two-range gearbox.