Diff Locks and electronic traction control explained

Many people believe that a permanent four-wheel drive system does just that – all four wheels are permanently turning to get you out of trouble.  In actual fact this is only true in areas of good traction – a dry road for example, but in a muddy field you could flounder with only one wheel spinning.  Why?

The wheel offering the least resistance to grip will start to spin, that automatically negates the drive to the wheel on the opposite side of that axle, and power will always go to the wheel offering the least resistance.  You now only have two wheel drive and then the second axle does the same as the first and traction is lost, due to the power being applied to the two spinning wheels that are offering the least resistance.

In order to overcome these problems Land Rover fitted a centre Locking Differential in the transfer gearbox to allow the front axle to be locked to the rear one.  This means that now there is less chance of the power being distributed unevenly and therefore more chance of maintaining forward momentum as if one axle has grip, the other will keep turning at the same speed and keep you moving.  Below is an explanation of the different systems fitted to Land Rover vehicles.



Part time four wheel drive
This was fitted to all series Land Rover products with the exception of the early series one 80 inch models and the stage one V8.  It was designed and fitted because at that time it was the easiest and cheapest way to turn a two-wheel drive into a four-wheel drive system.

What does it do and why have I got it?
It works by taking power from the gearbox to both front and rear prop shafts via a two-speed transfer box bolted to the rear of the main gearbox.  Its default setting is to send drive to the two rear wheels only.  Using the two extra gear levers located in the driving cabin you can enable four-wheel drive to be engaged in either high or low ratio.  High ratio gives the same ratio as normal two-wheel drive and low ratio gives a reduced final drive allowing the engine’s pulling power to be used more efficiently and impose less stress on the both the vehicles engine and transmission.

There are three gear levers in the cab.  Firstly there is the black one, which is used to change from first gear up to fourth and back down again just as you would in any other four-speed car.  There are also two extra levers to operate your four-wheel drive system, these levers work as follows.

The yellow lever which when depressed gives four wheel drive by engaging the front prop shaft, but at the same ratio and road speed as in two-wheel drive.

The red lever has three positions.  In its forward position it will give rear wheel drive only at normal road speeds, (This is known as the default setting). In the centre position it will disengage all drive to either prop shaft or the vehicle will remain motionless.  In the rearward position this engages both front and rear prop shafts and also reduces the final output ratio to give four-wheel drive, but at a lower range, meaning lower on/off road speeds and increased pulling power.

How do I use this system?
Under normal on road use the red lever should be in the forward position and the yellow lever should be in its up position.  (The default setting)  If the yellow lever is depressed you will have all four wheels driving, but in high ratio giving normal road speeds.  THIS CAN BE ENGAGED ON THE MOVE, BUT SLOW RIGHT DOWN BEFORE DOING SO.  In order to remove four-wheel drive when in high range you must pull the red lever backwards which will cause the yellow lever to pop up from the transmission tunnel by about two inches and then return the red lever to its forward position and drive away.  THIS SHOULD ONLY BE DONE WHEN THE VEHICLE IS STATIONARY.
To engage low ratio four-wheel drive firstly you must stop the vehicle, and then simply pull the red lever to its rearmost position.  You should immediately notice the difference in the vehicles performance, as it will have more power and less speed.  To return to two-wheel drive simply push the red lever to its forward position and drive away.  DO NOT CHANGE FROM HIGH TO LOW RANGE WHILST THE VEHICLE IS IN MOTION AS THIS WILL RESULT IN CATASTROPHIC AND EXPENSIVE TRANSMISSION DAMAGE.

When do I use this system?
If you use four-wheel drive for long periods on normal hard, surfaces you will find that the differences in speeds between the front and rear axles will cause transmission wind–up and excessive tyre wear.  This is not conducive to longevity in either your vehicles transmission or tyres.  So in a nutshell only use either mode of four-wheel drive on surfaces that are likely to offer less friction than on normal roads.

Examples of this are off road when the ground is slippery, soft, sandy or there are steep inclines etc.  On the road when there is ice or snow present, but remember that four wheel drive will not prevent a skid if you do not drive according to the road conditions that prevail at the time.  Nor will it guarantee traction and grip on ice.

The main disadvantage of this system is that there is no method of locking the two axles together, which enables the wheels on either axle to turn at different speeds on loose or slippery surfaces.

These were fitted as optional aftermarket applications on series Land Rovers.  If your vehicle has these fitted, four- wheel drive will be ineffective unless the hubs are engaged beforehand.  They work by disengaging the front prop shafts from the fee wheeling hub mechanism, thus allowing the front axle to free wheel when in two wheel drive.  There is a theory that using free wheeling hubs will save both on fuel and component wear and tear in the front axle.  You should engage them at least once a month to lubricate the otherwise stationary parts, as they will not be splashed with oil whilst stationary.

How do they work?
The whole front axle drive flange is replaced by the free wheeling hub mechanism.  A dog clutch system disengages the wheels hubs from the ends of both front half shafts when activated manually by the operator. This enables the front differential and half shafts to remain motionless during two-wheel drive operation and reduce drag, friction and wear in those components.

How do I operate them?
Simply get down on your knees and physically turn the hub in the direction indicated by the arrows on the hubs so they engage.  It will then click into place and engage drive to the front axle.  To disengage them reverse the above procedure by turning them back to the disengaged position.

Any Disadvantages?
Only if you forget to engage them and wonder why you only have two wheels driving.  Some people put a label on the dashboard or on the red lever to remind them to do it.  They need operating regularly to avoid seizure and to lubricate the otherwise stationary parts.  Also they are the weakest point in the front axle drive train system and are likely to fail before any other axle component.  But they are easy to get to so maybe that is a bonus?



This system was fitted to the Classic Range Rover, the Range Rover P38A and all 90’s, 101’s, 110’s and Defender range of vehicles as well as the stage one V8 and the Land Rover Discovery range of vehicles.

Why have I got it?
This system was designed to give better all round traction and grip (tyres notwithstanding) than the previous part time system on earlier Land Rover vehicles.  There is also much less room for driver error as you do not have to make a conscious decision to engage four wheel drive before it is required.  It is also more user friendly and easier to use.

How does it work?
Both the vehicles axles are permanently driven from the vehicles transfer gearbox, which is bolted on to the rear of the main gearbox.  With this system you only have two levers contained within the drivers’ cabin.  The normal gear lever is used for changing gear as you would in any other car and a smaller lever that controls the locking differential (in some cases) and the high and low range features.  When you push the smaller lever forward it engages the low range gears in the transfer box in the same way the red lever would do in a series Land Rover.  By pulling it back to the centre position you will engage neutral drive and when it is rearmost high ratio is then reselected for normal driving.  Also using this lever (on some models) you can push it sideways to engage the locking centre differential.  This device is used to keep both axles turning at the same speed and should only be used in slippery conditions.

How do I operate it?
Four-wheel drive is permanently engaged on these types of vehicle and therefore requires no operation at all.  The only choice you need to make is between high or low range gearing and whether or not you require the use of the locking differential (unless it’s automatic, see below)

Any disadvantages?
Generally the only disadvantage is greater fuel consumption as you are permanently driving all four wheels.


Why have I got it?
Two axles that are permanently driven will revolve at different speeds under slippery conditions just as two wheels would on a single axle.  The centre differential counteracts this by locking the front and rear axles drive speeds together, so if one axle loses grip the other won’t necessarily do the same.  All Land Rovers fitted with permanent four-wheel drive are fitted with locking centre differentials.  These should not be confused with axle locking differentials which are a totally different thing altogether and unless you are really seriously into off road events, personally I do not think you need them anyway.

How does it work?
The system operates in much the same way as an axle differential does in any other axle.  However inside the transfer gearbox there is a mechanical differential which when activated brings the ends of the two drive shafts together connecting them via this differential.  This means equal drive; power and torque will be distributed between each axle whilst they are locked together. 

How do I operate it?
On earlier models 1970 to 1983 this is operated using the pull push switch located to the left of the driver.  To engage the locking diff pull the lever up and to disengage push it down.  On Range Rover models from 1983 to 1989 and all 90 and 110 and Discovery models the locking diff control is located on the smaller of the two gear levers located in the drivers’ cabin.  To operate the centre locking diff merely push the small lever to the side and the dash warning lamp (if operational) should indicate that it is engaged.  You do not have to stop the vehicle to engage the centre locking differential, but be sensible don’t try it at max rpm with everything spinning.

Any disadvantages?
It will only control each axle but not each individual wheel.

All Range Rover models (1989 onwards) do not have the option to lock the centre differential.  The reason is that Land Rover fitted a device known as a viscous coupling to these models and it works automatically when the vehicle sensors detect loss of grip on one axle.  This device is fully automatic and needs no input from the driver whatsoever.  It works by joining discs from one end of each prop shaft.  The space in between is then filled with a viscous fluid, which then thickens up and locks the two axles together. Therefore you will only have high and low range options on your small gear stick when this device is fitted.


Why have I got it?
Traction control works differently.  Modern Land Rover products (Freelanders, Range Rovers Discovery 3 and the later Defenders) do not have viscous couplings or centre locking differentials.  Instead they rely on computer electronics to sense wheel slip and counteract this by applying the brakes to the wheel that is slipping.

How does it work?
This system uses the anti-lock braking system (where fitted to a vehicle) to slow the spinning wheel to the same speed as the one that is offering some grip.  The computer does all the thinking for you by getting information from the wheel sensors as to which wheel(s) are slipping and automatically counteracts the slip.  Thus ensuring momentum is maintained in most cases.  There are of course, exceptions to every rule and ETC does have its limitations.

How do I operate it?
Different Land Rover models work in different ways.  Generally when you have engaged the off road mode, the computer will automatically engage the ETC although some models do require driver input to do this.  You will also have to use more engine revs than with non electronic systems

Any disadvantages?
Electronics, mud and water don’t generally mix well.  This system relies on input from wheel sensors that can and often do get dirty and fail to send a pulse to the computer whilst wet and dirty.  You also need to drive the car with high engine revs to make this system work effectively as it relies on input from spinning wheels to work properly.