What is a diff lock? How differential locks work and when to use them

A diff lock (differential lock) is a feature that locks both wheels on an axle together so they turn at the same speed. This helps your 4WD maintain traction when one wheel loses grip, making it much easier to drive through rocky terrain, mud, ruts and other challenging off-road conditions.

In this guide, we’ll explain what a diff lock does, how it works, when you should use it, when you shouldn’t, and whether your 4WD has one.

What is a diff lock?

A diff lock (short for differential lock) disables the differential, forcing both wheels on the same axle to rotate at exactly the same speed. This prevents drive from being sent to the wheel with the least traction, helping your 4WD to keep moving when one wheel is in the air, on mud or on loose rocks.

The trade-off is that the wheels can no longer rotate at different speeds when cornering, which is why diff locks should only be engaged when extra traction is needed.

That’s why diff-locks can be engaged or disengaged. You have it engaged when you need it, for example, ascending a steep, rocky climb, and disengaged when you don’t, for example, normal road driving.

What does a diff lock do?

A diff lock improves traction by ensuring both wheels on an axle continue turning together, even if one wheel loses grip.

This is particularly useful when driving over rocks, climbing rutted tracks, crossing uneven terrain or tackling muddy sections where one wheel may lift off the ground or spin freely.

By locking the axle, drive is maintained to the wheel that still has traction, helping the vehicle keep moving.

When should you use a diff lock?

Don’t fall into the trap of thinking that whenever the going gets tough, you engage your diff-lock and life is always better. Sometimes it’s best to leave it disengaged.

There are so many off-road situations that it’s not possible to give specific advice, and you will need to break each of the rules below at some point, but here are some general tips.

Use lockers when:

• Your progress is likely to be impaired by having a wheel on the axle with lockers spin. For example, a rutted hill climb or some muddy ruts. The Jeep Wrangler Rubicon pictured below has its lockers engaged as it ascends a hill. You can tell because the front right wheel isn’t spinning.
• Descending a rutted hill where one wheel is likely to be in the air or lose traction.
• Generally, anywhere where you’re moving in low range first or second, you’re likely to lose traction on one or more wheels, but there’s good traction on the other wheels, and you don’t need to turn tightly.

Don’t use lockers when:

• You need to turn tightly, for example, on a sharp switchback, because the lockers will increase the turning circle.
• All four wheels are on the ground, with roughly equal weight on each of them, or with equal weight distributed across two wheels on an axle. The wheels are likely to have similar grip, so preventing one wheel spinning isn’t a problem.
• On side slopes. The vehicle will crab sideways due to the differential not allowing wheel slip across an axle.
• You’re going relatively fast and need to manoeuvre, for example, in sand, and in some situations, in mud and snow. The Jeep Grand Cherokee below needs counter-steer to continue heading up the hill, and lockers would make that harder. It’s also going relatively fast.

How does a differential work?

To understand why a diff lock works, it helps to understand what a differential actually does.

Imagine you’ve got two wheels on an axle, as shown in the picture below.

When that axle turns a corner, the inside wheel describes a shorter arc than the outside wheel. Yet, both wheels take the same amount of time to complete the turn. This is fine if the wheels just rotate freely on the end of an axle. It becomes a problem if we want to drive the wheels with the engine.

We could simply put a couple of cogs in and have a driveshaft turn the axle. But that would mean both wheels have to rotate at the same speed. If we then tried to go around a corner, the inside wheel would rotate at the same speed as the outside wheel, and the inside wheel would hop, skid and jump as it is forced to rotate at a much faster speed than it should for the little distance it has to travel.

The problem with this is stress on the axle, tyre wear, poor handling, and a tendency to ‘understeer’, which means the nose of your vehicle will push wide in corners.

So, how do we fix it?

What’s needed is a way to drive both wheels while allowing them to turn at different speeds around a corner. There is a solution, and it’s called a differential, or diff for short. It is a complex series of cogs that allows the inside wheel to slow down relative to the outside wheel. Yet both to be driven.

Exactly how the cogs do this isn’t important from the driver’s perspective; what’s important is the effect.

The diagram below shows that the inside wheel is following a shorter circle than the outside, and rotating slower. The purple dot represents the differential, which allows this to happen, yet drives both wheels. The blue arrows show the relative speed of each part of the driveline.

Why does a diff lock improve traction?

So, differentials solve the cornering problem. But they come with a huge disadvantage for off-road driving, which is shown in this picture:

You see in the picture that the front right wheel is spinning madly, but the front left isn’t moving. That’s because differentials are lazy; they direct the drive to the wheel that’s easiest to turn. To be precise, they equalise torque (turning force) between two wheels on an axle.

So if we have a wheel with little grip compared to the other one on the axle, for example, in the air or on mud, that’s pretty easy to turn. It gets very little turning force, and that’s how much its partner wheel on the other side of the axle gets. That’s often nowhere near enough to move the vehicle, and we call this the “differential problem”.

A diff lock solves this problem by disabling the differential and forcing both wheels on an axle to rotate at exactly the same speed. However, it also eliminates the benefit of a differential, which allows the wheels to rotate at different speeds when cornering.

That’s why diff-locks can be engaged or disengaged. You have it engaged when you need it, for example, ascending a steep, rocky climb, and disengaged when you don’t, for example, normal road driving.

Understanding the differential problem is one of the fundamentals of off-roading. It is the key to understanding not just what a diff-lock is and does, but lots of other traction aids and driving techniques too.

Manual vs automatic diflocks

There are two basic types of diff-lock, manual and automatic. Most are manually engaged and disengaged. But there are some designs that are automatically engaged and disengaged by mechanical means or computer control.

Lockers are for slow-speed off-road work on loose surfaces, so losing the differential effect around corners isn’t too much of a problem. You just notice the turning circle is bigger than before.

Centre diff lock vs cross-axle diff lock

What we’ve talked about so far is not just a diff-lock; it’s a “cross-axle locking differential” to give it the full name. That’s because it’s a differential lock that works across two wheels on an axle.

Exactly the same concept applies to the relative rotation of the front axle to the rear. That’s called a “centre differential”. Just be careful to know that a cross-axle locker is different to a centre diff-lock, even though the concept is the same.

We also call a differential that has no locker an open differential, or open diff.

What does the diff lock light mean?

The photo below shows a Toyota Prado. It has its centre diff-lock engaged, which is shown on the speedo by the orange icon of four wheels with an X in the middle. It also has its rear cross-axle locking differential engaged, shown by the equivalent red icon but with an X between the rear axles.

Can I fit an aftermarket diff-lock?

Maybe it already has one or two as standard. If not, the answer is perhaps. There are a number of aftermarket companies that supply diff-locks. But not for all makes and models, only those most popularly used for off-roading.

If there is a locker available it can be fitted by a mechanic. Examples of aftermarket locker companies are ARB and Eaton.

Diff-locks are definitely not the first things to buy for an off-roader. Safety equipment such as a fire extinguisher and first aid kits are a higher priority. Then suspension, tyres, snorkels and the like. Once you have everything else sorted, then consider diff-locks.

If your vehicle is a modern one with good electronic traction control, then you have less need for diff-locks compared to an older vehicle with no electronic traction control. That’s because an electronic traction control system mitigates the “differential problem” in a similar way to a diff-lock.

The photo below shows the driver’s controls for front and rear ARB Air Lockers. They are called air lockers because the mechanism to engage and disengage is driven by an air compressor.

Most owners fit aftermarket lockers to the rear of the car as that is typically where the most benefit is when driving off-road, and the rear axle is stronger than the front axle, not least as it doesn’t have CV joints.

However, some owners of vehicles such as the Nissan Patrol fit them to the front axle, on the basis that the rear already has a strong limited-slip differential (LSD). An LSD is partway between a locker and an open differential.

Which 4WDs have diff-locks as standard?

It could well be that your 4WD comes with a diff-lock on the rear axle, or on both axles. Examples of cars that have rear lockers are the Ford Ranger, Pajero Sport and Nissan Patrol Y62.

For front and rear lockers, examples are some Land Cruiser 70 Series, Mercedes-Benz G-Wagen and Jeep Wrangler Rubicon. We call vehicles with front and rear diff-locks twin locked.

Not all versions of all 4WDs have diff-locks.  Look for a button like this on a Nissan Y62 Patrol. On the right hand side of the picture there’s an icon of four wheels with an X between the rear axle. That’s the button to engage and disengage the rear locker.

There are no production 4WDs that have lockers on the front axle and not the rear. It’s always rear axle only, or both axles, and you have to engage the rear before you can engage the front. That’s to minimise the risk of stress and damage to the drivetrain. Lockers can easily produce significant extra stress on transmission components such as axles.

So, to wrap this all up

A diff lock is one of the most effective traction aids you can have in a 4WD. By locking both wheels on an axle together, it prevents drive being lost to a spinning wheel and helps maintain forward progress on difficult terrain.

While many modern 4WDs rely on electronic traction control, factory and aftermarket diff locks still offer significant advantages in challenging off-road conditions. The key is knowing when to use them, and just as importantly, when to switch them off.

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