Workplace training and safety blog
Posted in Defensive Driving

How does centre diff lock work?

Centre diff locks are only applicable to all wheel drive (AWD) or constant 4WD vehicles or vehicles that have the capability of delivering that sort of drive. With a Part Time 4WD like a Patrol or Landcruiser, the front wheels aren't driving until the driver selects 4H or 4L and the transfer case moves into 4H or 4L, the drive is shared via the transfer case on a 50/50 basis front to rear wheels.

Your Toyota 4WD may look like this.

    How does centre diff lock work?

Driving a part time 4WD in 4X4 on a hard surface like a sealed or bitumen road will mean that the different speeds and distances travelled by the front and rear wheels will act just the same way as when a manual cross axle diff lock is locked.  The only way you can turn or go up or down the smallest of inclines is to scrub tyres &/or push massive forces back thru the transfer case, this is known as transfer case "Wind Up" and it can destroy a transfer case very quickly, so don't do it.  On an AWD, the transfer case has a centre differential built into it in order to divert some drive to the front or rear diff all the time and yet still allow them to run at different speeds so you can turn or climb hills etc, usually an open diff but always (of necessity) a somewhat smaller device than your cross axle diffs at each end.  Learn more about 4WD training

When the Centre Diff is unlocked, it is often an open diff, so it really only delivers drive to whichever end of the vehicle has the least traction, although that can change, In effect, an All Wheel Drive Vehicle with the Centre Diff UN-locked is really only ever driving one wheel at any given instant (the one with the least traction).  But it gets to choose from any one of 4 due to the nature of the three differentials fitted, front, rear, & centre.  A part time 4WD with no other diff locks will only be driving one of two wheels when it is in 2WD, or any two of four wheels once 4WD is selected.  So effectively an AWD is not in 4WD until the Centre Diff Lock is locked, just as a Part Time 4WD is NOT in 4WD until you select 4WD with the selector.

James Gorrie has been driving 4WD in the NT for over 20 years See James profile