The Haldex AWD System

Thursday, 18 April 2019  |  Ravenol Marketing

If you own a modern all-wheel drive (AWD) car, chances are it will actually be a front-wheel drive, transverse engine system connected to a Haldex (or similar) coupling unit which uses electronic sensors and differentials redirects power to the rear wheels when it’s needed.

A simple Haldex system consists of a hydraulic pump, a clutch and an electronic valve. This diagram from Haldex Repairs shows it pretty simply:

A simple Haldex unit

“Haldex” is actually a brand name of the Swedish company HaldexAB (now owned by BorgWarner) which has specialised in making these electronic front-drive couplings for years. Whilst units from other manufacturers, such as GKN and JTEKT, are now widely used, Haldex is still the best known.

The early Haldex units purely reacted to the front wheels slipping – the difference in speeds between front and rear activated a pump to engage the rear drive. Later generations were able to anticipate when more power needed redirecting to the rears, whilst the latest 5th generation of Haldex unit continually sends 10-15% of power to the rear wheels, plus more when is needed.

Haldex all-wheel, quattro and 4motion drives are commonly found in AWD vehicles from Ford, Volkswagen, Audi, Seat, Skoda, Vauxhall, Land Rover, Volvo and many more. The units were widely adopted by manufacturers because they are less bulky than Transfer Boxes and consume far less fuel than traditional four-wheel drive systems when driving normally along roads and motorways because the rear shafts aren’t spinning.

No matter which type of Haldex unit your vehicle has or where it is located, it will still need regular oil and filter changes, just like your engine does. If you are using RAVENOL AWD-H Haldex fluid, we recommend a service every 37,500 miles, though with other brands you may need to change it more frequently, from every 20,000 miles. In addition, the pump is prone to failure on high mileage vehicles so it’s reassuring to get it looked at by an expert mechanic during a service.

A Haldex unit is a complicated piece of engineering with very fine tolerances so regular maintenance is vital to keep it in working order. If the Haldex fluid is dirty it can damage components and affect performance. Dirty fluid can also cause the coupling to overheat and the clutches to burn out. In severe cases, it can also burn out the Haldex Pump leading to loss of rear drive and failure of the whole unit.

Another point to note about Haldex systems is that the fluid inside them is not a lubricating oil, like you’d find in your engine or transmission. The fluid is a very special product which actually promotes friction between the clutch plates. Because of this, over time particles will start to build up in the oil, which is why a regular service of both the oil and filter is vital. In modern units with an oil strainer instead of a filter, this will still need cleaning during the service.

RAVENOL’s AWD-H Haldex fluid was specially engineered for Haldex's Allrad, Quattro and 4motion powertrains and is very popular with our Land Rover customers, although it’s suitable for many other systems as well. With our high quality fluid at very affordable prices and extended fluid change intervals compared to other brands, you can save money as well as know your car’s in good hands.