AUTOMATIC SPEED CHANGE GEAR

PURPOSE:To make it possible to shorten the time of gear shift of an automatic speed change gear having no synchronizing mechanism, by abruptly decreasing the rotational speed of an engine with the use of an exhaust brake during high load operation in accordance with the detected torque value of the output side. CONSTITUTION:In an automatic speed change gear, a shift valve 17 actuates selector valves 19 through 22 through a shift shaft 18 so that sleeves 13a through 16a in drag clutches 13 through 16 are shifted to selectively engage one of gears 3b through 9b, thereby, the rotation of a counter shaft 10 is transmitted to a main gear shaft 11 through a predetermined gear ratio. In this arrangement a load torque is calculated from the phase difference between pick-ups 27, 28 for measuring the rotational speed of the main shaft 11. Further, in order to initiate a gear shift, at first the speed change gear is set at its neutral position, and then those of the select valves 19 through 22 at a desired position are driven to operate an exhaust brake during high load operation so that the rotational speed of the engine is lowered to allow a gear shift.