Transfer case

Also called t-case, transfer box, transfer gearbox

A transfer case is the gearbox behind the transmission that sends engine power to the front axle as well as the rear, and usually offers a low range that multiplies torque for slow, difficult ground.

What is a transfer case?

In a two-wheel-drive truck the transmission drives a single propshaft to the rear axle. A four-wheel drive puts a second gearbox behind the transmission that takes that same output and splits it, sending one propshaft back and another forward. That box is the transfer case, and it is what the 4WD selector actually operates.

It does a second job that is arguably more important. Most transfer cases contain a reduction gear set, so you can select a low range that multiplies torque by somewhere between about 2.5 and 4 to 1 and divides road speed by the same factor.

What 2H, 4H and 4L mean

2H is two-wheel drive, high range: rear axle only, normal gearing, what you drive on the road. 4H is four-wheel drive, high range: both axles, normal gearing, for gravel, snow, sand and anything where you want traction at speed. 4L is four-wheel drive, low range: both axles, reduction gear engaged, for crawling, steep descents and pulling.

Low range is the tool most new owners under-use. It gives finer throttle control, more torque at the wheels, and much stronger engine braking downhill, and it means you can idle over an obstacle rather than trying to modulate a clutch or a throttle that is far too sensitive for the job.

Why not to use 4H on dry pavement

A part-time transfer case locks the front and rear propshafts together. Through a turn the front wheels travel further than the rear ones, and on loose ground the difference is absorbed by a tire slipping slightly. On dry tarmac nothing can slip, so the driveline winds up: the steering goes heavy, the vehicle judders, and something eventually gives, usually the transfer case.

This is called driveline binding, and it is the single most common way people damage a four-wheel-drive system. Full-time systems have a centre differential specifically so this cannot happen, and they can be left engaged.