Haldex: Just hot air or world class hydraulic systems for heavy duty vehicles

Haldex: Just hot air or world class hydraulic systems for heavy duty vehicles

How does Haldex AWD work and what are the benefits and drawbacks of it?

From their very first iterations in the early 20th Century, cars have evolved in a big way. From changes to the way the engine works to better brake systems, everything has developed around making the driving experience better for drivers. One of the biggest changes we have seen in recent years has been the development of vehicles’ drivetrains and how power is distributed to the wheels. While those first cars were mostly rear wheel drive, many modern drive trains are either four-wheel drive (4WD) or all-wheel drive (AWD). Thanks to developments in clutches and couplings, some vehicles come equipped with systems which allow power to be shifted between the front and rear wheels, or engaging all wheels when necessary, to maximize a vehicle’s potential and make driving easy.

As a result of these systems that allow users to choose (or situations to dictate) if power should be transferred to four wheels or just two, car owners get better safety and better fuel economy. They can, for example, benefit from the economy of a front-wheel drive vehicle during everyday driving conditions and the performance and handling benefits that come from a 4WD car when the going gets tough. One of the most famous coupling systems is Haldex. Though there are many AWD (all-wheel drive) systems, for example Audi’s quattro or Subaru’s Symmetrical All-Wheel Drive, the name Haldex pops up more than any individual system.

You may have heard of Haldex AWD before and you will have definitely heard of some of the cars equipped with it, for example the Audi S3, Volkswagen Golf R, or Mercedes-Benz A45 AMG. Though each of these companies have their own names for their drive train systems, they are all linked to Haldex. So, what is this AWD system, how does it work, and what are the positives and negatives of it?

What is Haldex and how does this system work?

What is Haldex and how does this system work

Haldex Traction is a product of Swedish company HaldexAB, which has specialised in converting front-wheel drive systems to all-wheel drive systems for many years now. Haldex 4WD typically takes a front-wheel drive set-up, such as one found in the VW Golf or Audi A3, and converts it into an all-wheel drive, normally by using electronic sensors and differentials to redirect power to the rear wheels when necessary.

A ‘Haldex’ system is present on the majority of small- to mid-sized 4WD vehicles. Basically, it utilises an oil pressure-activated clutch coupling system on the rear differential to allow the car to instantly switch from FWD to 4WD as and when the ECU decides that it is necessary, without input from the driver.

Haldex AWD is not actually ‘proper’ all-wheel drive as in AWD vehicles the standard setup of the drivetrain drives all wheels of the vehicle. Haldex is technically a four-wheel drive system as the default setup for Haldex AWD drives the front wheels only, so it should really be called Haldex four wheel drive. It is only when wheel-slippage is detected that the Haldex computer systems redirect torque to the rear wheels to compensate for the loss of traction at the front, driving all four of the wheels. Under regular driving circumstances, Haldex equipped vehicles will operate as front-wheel drive only. Because this shift happens without the driver doing anything, the system detects it itself, the system is often described as an automatic all-wheel drive system. Essentially, a car equipped with a Haldex system is a front-wheel drive car which has the option to use all four wheels if the situation dictates it.

The Haldex 4WD coupling is mounted on the rear axle differential and is driven by the drive shaft. Engine torque is transmitted to the drive shaft through the gearbox, the front axle differential, and the front axle drive. The drive shaft is connected to the input shaft of the Haldex coupling. Torque can only be transmitted to the rear axle differential when the Haldex coupling clutch plates ae engaged.

Different generations of Haldex

Haldex itself has evolved a lot since its initial inception and there have been 5 different generations to date.

Haldex gen 1 is the blueprint for all other Haldex generations. The main components of the system are a mechanical-hydraulic pump driven activated when the wheels slips, a wet multi-plate clutch, working pistons, and a controllable throttle valve.

The unit is essentially a hydraulic pump with two shafts connected to it, one leading to the front axle and one to the rear (input and output shaft). These two shafts are connected via the wet multi-plate clutch pack and this clutch is normally ‘unloaded’, meaning no torque is transferred between the two. When both shafts are rotating at the same speed, the pump is inactive.

When a difference in speed between the wheels on each axle is detected, between the input and output shafts, the input shaft pumps a piston and build up oil pressure in the pump. As oil pressure is diverted via an oil duct to the working piston, this forces the working piston to compress the clutch plate set. The input shaft and the output shaft of the clutch are now interconnected, and thus the both the front and rear axles are connected. We now have all-wheel drive!

Under normal driving conditions (driving at the constant speed with no slippage), the torque distribution is 90/10 front to rear. When the clutch is locked, torque is evenly distributed between the axles (50/50).

Haldex gen 2 still uses the mechanical hydraulic pump and works in precisely the same way as gen 1, with some minor improvements. This system is still reactive.

The biggest change for Haldex gen 3 and Haldex gen 4 is that they are proactive rather than reactive systems. This means that the electronics in the ECU can ‘pre-tense’ the clutch and activate all-wheel drive before a wheel slip occurs, making it far more effective and increasing the safety for the driver as well. Gen 4 does not use the mechanical pump, but rather uses a feeder pump instead.

In Haldex 5th generation couplings, a new electro-hydraulic clutch actuator uses a centrifugal overflow valve to help accurately distribute power between the front and rear axles. Makes the system run more smoothly and eliminates the need for some components, streamlining the process. To help with this there is also an integrated electronic control unit.

What are the benefits of Haldex?

Haldex does provide several benefits, especially for manufacturers without unlimited budgets. Rather than researching and developing their own all-wheel drive systems, they can simply use Haldex systems. They can be equipped to almost any front-wheel drive vehicle and as such are popular with many manufacturers as FWD is the standard drive system. This makes it easy for companies to produce a sportier, AWD model of a car which previously was only front-wheel drive.

One of the main benefits for drivers is that Haldex provides greater fuel efficiency as it drives only 2 wheels for a large percentage of the vehicle’s use, only resorting to 4WD when circumstances require additional traction.

The system of course makes driving a lot safer. With the same sort of working principle as ABS systems, recognising when a wheel is slipping can mean that a car has better traction on slippery surfaces and eliminates some of the risks when driving on difficult terrain. This is true of all 4WD or AWD systems, but many of these must be activated by the driver. Given Haldex is automatic, as the Haldex controller decides when the system should be activated, it makes it stress-free and better at dealing with unexpected changes in the road surface.

What are the negatives of Haldex?

Because the Haldex system is rather complicated in its nature – using specific oil, relying on high tolerances, and generating a build-up of friction particles over time – it must be kept in the best condition and requires special oil and maintenance. Haldex repairs are hard to do yourself (and not recommended!) so regular upkeep can be expensive.

Not just any old oil can be used in the system; you should ideally only ever use the Genuine VAG Haldex oil designed for your specific vehicle and this can be expensive. The Haldex oil is a non-lubricative oil to promote friction of the clutch plates and using any other oil can result in a total loss of drive from the rear effectively converting your car into a front wheel drive. This means that Haldex oil changes are extremely important in the effective operation of the system.

There are also some performance issues for cars equipped with Haldex systems. As Haldex is primarily front-wheel drive, under heavy loads, Haldex equipped vehicles have a tendency to understeer massively, as the computers are often too slow to apportion torque to the rear axle, and as the default torque distribution remains front-wheel orientated. This was a common problem with vehicles such as the Audi RS3, known for its chronic understeer.

Because the oil is designed specifically to promote friction between the clutch plates, this friction causes more particles of material to slowly build up within the oil and means it can become ineffective fast, possibly requiring more frequent expensive changes.

When should I have my Haldex serviced?

When should I have my Haldex serviced

On vehicles with a Haldex filter, meaning the first and second generations, it is a good idea to change both the oil and the filter at 20,000 mile intervals. For vehicles which don’t have a filter, changing oil after every 10,000 miles is a good idea, as the strainer on the pump can become blocked, eventually leading to pump failure. A garage will be able to run a more comprehensive and accurate Haldex diagnostic test for you.


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