Arrangement for preventing incorrect shifting in a vehicle gearbox

Arrangement for preventing incorrect shifting in a gear-box preferably intended for heavier motor vehicles, said gearbox having a base portion with a number of gear ratios and a two-speed range portion, coupled to the output shaft of the base portion, and having such gear ratios that the total gear ratio of the gearbox, when the higher gear ratio of the range portion is used together with the lowest gear ratio of the base portion, is higher than that when the lower gear ratio of the range portion is used together with the highest gear ratio of the base portion, the range portion having a gear selector mechanism coupled to a shift unit, said device comprising a first blocking system which is coordinated with the shift unit of the range portion and a gear selector mechanism of the base portion to oppose shifting in the base portion during the shifting movement of the shift unit in the range portion, and a second blocking system which is coordinated with means for sensing the rotational speed of the output shaft and with the gear selector mechanism of the base portion to prevent shifting to a certain gear or certain gears in the base portion when the rotational speed of the output shaft exceeds a certain value or certain values, characterized in that the first and the second blocking systems have a common blocking device, that the first system has means which sense the shifting movement of the shift unit, said means being disposed to activate the blocking device to block the gear selector of the base portion in the neutral position during the shifting movement, and that the second system has means for sensing the lateral position of the gear lever, said means being disposed, depending on the rotational speed of the output shaft, in the lateral position of first and second gear to actuate the blocking device to block the gear selector of the base portion in the neutral position.