High and Low Range

High and Low Range







High and Low range are selected by the driver from a dashboard mounted switch on manual vehicles and via the selector lever on automatic vehicles. Transfer neutral is selected by inserting a 5 Amp fuse in position 11 of the driver's seat fuse box. Refer to Owner's Handbook.

Drive from the main gearbox is permanently engaged to the sun gear of the epicyclic gear set. When in High range (position B) the sun gear transmits drive directly to the reduction hub. The reduction hub and intermediate shaft rotate at the same speed as the main gearbox output shaft. When in Low range (position A) the reduction hub is driven through the planet carrier. The reduction hub and intermediate shaft rotate at a lower speed than the gearbox output shaft. When in transfer neutral, the reduction hub is positioned between the sun gear and planet carrier.

Changing between ratios is achieved by the reduction hub sliding along a splined section of the intermediate shaft to engage with the required gear. The ratio change mechanism comprises the ratio control motor, reduction hub, interlock spool and selector fork.

The interlock spool consists of a shaft which locates in the front casing and protrudes through the rear casing to engage with the ratio control motor. The shaft carries an aluminium casting with a helical cam track and two springs, which are anchored at one end to the shaft and at the other to the cam track casting.

The selector fork is mounted on a second shaft located within the front and rear casings but is able to slide in the casing mountings. A cam follower on the selector fork engages with the cam track of the interlock spool. When the ratio control motor rotates the interlock spool, the cam follower of the selector fork follows the cam track of the interlock spool. This converts rotational movement of the interlock spool into linear movement of the selector fork.

The selector fork is engaged to the reduction hub. Linear movement of the selector fork is transmitted to the reduction hub, moving it between High, Low and neutral positions. In the event of the reduction hub gear failing to mesh with the epicyclic gear set, wind-up of the interlock is prevented by the interlock spool springs. The springs apply a constant torque to the cam track casting until the reduction hub engages with the epicyclic gear.

Ratio control motor
The ratio control motor drives the selector mechanism and is controlled by the transfer box electronic control unit (ECU). The ECU monitors a number of variables including the position of the ratio control motor, the speed of the vehicle, and the drive ratio selected by the driver. When a ratio change is requested, the transfer box ECU checks that conditions are favorable for the change, for example that the vehicle speed is sufficiently low to permit engagement. The ECU then drives the ratio control motor to the required position. When the conditions for a ratio change are unfavorable, the transfer box ECU communicates instructions to the driver through the Message Centre and will not attempt to change ratios until the correct conditions are met.