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The advent of all-wheel-drive in passenger cars reshaped how automakers designed every vehicle they made, whether pedestrian or performance. Audi paved the way with their quattro and rewrote the rally rulebook simultaneously, leading every manufacturer to develop their own system or seek one out. Haldex Traction, a subsidiary of Haldex, was the owner of an electronically-controlled limited-slip differential patent that, when placed in line with the driveline, could offer all-wheel-drive on demand. By the mid-nineties, Volvo had instituted the Haldex design, with many more manufacturers to follow in the coming years. 

The automotive landscape has changed drastically since the mid-nineties, but the compact nature and continually simplified Haldex design has allowed the system to remain relevant in the newest models of today. Its performance rivals the best systems, and they come in all kinds of packages, from the ultimate hot-hatch Golf R to the safest wagon around, the XC70. If you’re looking into a Haldex-equipped vehicle, here’s everything you’ll need to know. 

Haldex AWD Explained Table of Contents

What is Haldex AWD?

There are more than a few ways of instituting an AWD system into a vehicle, but they’re generally dictated by the engine’s orientation. Longitudinal layouts, like in an M3 or E63 AMG, are traditional and have the transmission behind the engine. In contrast, the transverse layout found in a GTI or Volvo has the engine and transmission next to each other in the engine bay. Traditional layouts generally feature some form of a torque-splitting device attached to the backside of the transmission. BMWs and others use a transfer case and a small drive shaft to send power forward from the gearbox to the front wheels. On the other hand, Porsche, Audi, and Subaru use a center differential on the transmission’s nose that sends power back through the gearbox to the differential inside the trans case. 

FCP_Euro_Haldex_Explained_Audi_TT_Mk1_Diagram

The Haldex design doesn’t use anything like that and instead features its two main components separately. At the rear of the vehicle is the AOC, or Active On-demand Coupling, positioned on the nose of the rear differential. The AOC engages when the front wheels lose traction through direction from sensors on the driveline or through a mechanical drive. As slip occurs, the AOC activates its internal hydraulic clutch system and engages it, transferring torque to the rear wheels. Under normal conditions, the system essentially acts as a front-wheel drive. 

Within the AOC are two sets of wet friction plates or clutches. With one group connected to the drive shaft and the other to the rear differential, engaging the plates together activates the torque transfer sending up to 50% of engine torque rearward. In all Haldex variants, hydraulic pressure locks the two plate sets together, regardless of the activation method.

Later revisions, with their far more advanced sensors and tech, have far more control of lock-up forces and much quicker reaction times. 

Accompanying the AOC is a bevel box. Also referred to as the bevel gear or angle gear, the unit is mounted to the back of the transmission and rotates the transaxle’s force 90° degrees to be sent rearward through a drive shaft. The simple mechanical nature allows manufacturers to reap the safety and packaging benefits of the transverse engine layout while retaining the distinct advantages of all-wheel-drive. 

Volkswagen Mk7 Golf R Haldex

In a few ways, the Haldex design is closer to a Four-wheel-drive system than an all-wheel one; 4WD is selectable, while AWD has constant torque at every wheel. Although the AOC engages quickly on even the oldest units, the car chooses when the torque split is necessary. That isn’t strictly a negative, though. On top of the packaging benefits, it has less weight to rotate under normal conditions, so the average MPG return is higher than a traditional system. Plus, that comes without a lack of traction when friction fades. It’s a quirky but practical design⁠—what’s more Swedish than that?

The Swedes continued to develop their design until 2011, when BorgWarner bought the Traction division. From there, BW continued to revise the Haldex design into what it is today. Now it’s as capable as ever and helps performance vehicles like the Mk8 Golf R be the best version of themselves.

 

Haldex Generations

The Haldex system appeared in the late nineties, remaining a reliable and effective all-wheel-drive form. Its continued success stems from an effective design, but its many updates over the years have kept it competitive and ever-improving. 

 

Haldex 1 - Mechanical Pump

The first generation Haldex system reached consumers for the first time in the 1998 Volkswagen Golf 4Motion with the same essential components still in use today. The bevel gear attached to the transmission at the front of the car rotates and sends engine power through a driveshaft and into the AOC mounted just ahead of the rear differential. Where this system shows its age is in the activation of the clutch plates. 

Producing the hydraulic pressure required to lock up the clutches is a mechanical differential pump. Unlike traditional pumps, the mechanical differential pump is built into the clutch packs for maximum packaging efficiency. Two pistons are attached to the input side clutch pack housing, while roller bearings and the “lifting plate” are fitted to the output side. Slip-free driving has each side of the shafts rotating at the same speed at all times, which produces little to no pressure from the pump. As soon as the front wheels lose traction, the input shaft clutch-set on the driveshaft side begins to spin faster than the lifting plate on the differential side. The speed differential inside the housing uses the bearings against the undulating lifting plate to work the pistons back and forth to create significant hydraulic pressure, which then locks the clutches with the help of several valves. 

Under normal driving conditions, only 10% of the engine’s torque runs through the rear wheels. At 90° of wheel slip, the AOC engages, and up to 50% of available torque is transferred to the rear. While highly competent for the day, the slip required to begin operation makes the first generation a reactive setup. Where Audi’s quattro AWD found in cars like the S4 has constant power at each wheel, the Haldex-based system only engages after lost traction. Better than nothing, but not exactly ideal. 

Resting Torque Split:

  • 90/10

 

Haldex 2 - Electric Improvements

The differential pump built into the Gen 1 AOC was a simple but effective way of creating the required hydraulic pressure, but the series of valves controlling the pressure was a bit crude. Drivers were at the mercy of the mechanical limitations of the valves, so Haldex engineers were quick to devise a much more accurate solution. Debuting in 2002 was the second generation of Haldex’s design, with a new and improved, electronically-controlled solenoid pack.   

FCP Euro Haldex DEM module

The second generation remained a reactive system, although the torque application improved thanks to new tech. A solenoid-controlled proportional throttle valve replaced the linear throttle valve, and a pressure sensor was introduced and used actively for closed-loop pressure control. Depending on the application, the DEM, Differential Electronic Module, or ECU controls the solenoids and uses the pressure sensor data. Its inclusion made the system more accurate and variable, allowing for more or less rear-wheel torque when deemed necessary. In turn, that reduces the fuel consumption and wear on the clutch packs inside the AOC. It also allowed the unit to simplify itself in shape and size thanks to the solenoids’ much smaller relative size over the mechanical valves. 

 

Resting Torque Split:

  • 95/5

 

Haldex 3 - PreX AWD

Just two years later, Haldex was ready with their next revision. The chief complaint against the previous generations was in the system’s response time, requiring around 90° of wheel slip before the AOC had enough hydraulic pressure to lock up. New for the third generation was simply a check valve. Placed in line with the small, electric hydraulic feeder pump, it allowed the system to pressurize immediately upon startup for an instant torque transfer when called upon. Haldex referred to the new operation as the “PreX” solution. 

Somewhere during production, the feeder pump was enlarged to better meet the system’s demands. Regardless of size, once the initial pressure provided by the pump was exhausted, the mechanical differential pump continued to supply the necessary pressure. The larger electric pump was a sign of the future, though, as the outdated mechanical pump was becoming unnecessary. 

Resting Torque Split:

  • 95/5

 

Haldex 4 

2007’s introduction of the fourth-generation Haldex system saw another significant shift in the way the system operated. A single electric pump took over from the specially designed mechanical differential pump, in use since Haldex’s introduction, as the primary pressure provider. Rather than relying on whatever was generated mechanically, the electric pump was commanded by the DEM/ECU and could vary its pressure based on the situation determined by the system’s sensors.

FCP_Euro_Haldex_Explained_Volvo_Haldex_Unit_XC90

The change from mechanical to electric pump came with a few changes as the hydraulic system was nearly all new. In place of the proportional throttle valve was a pressure-reducing valve, while the revised hydraulic routing and lack of a mechanical pump also necessitated the inclusion of a hydraulic accumulator. The accumulator was fed by the feeder pump at all times to ensure a quick lock-up when needed, bolstering the “PreX” capabilities. 

Torque Split:

  • 100/0

 

Haldex 5 - BorgWarner

The Gen 4 Haldex design was the best yet seen from the Swedes, but the BorgWarner acquisition of Haldex Traction meant a change of engineers. Thankfully, the American company continued development without altering the existing performance. What they did in their first generation was simplify the AOC. Although the electric pump was a leap forward, there were still a handful of other components that the system relied on to operate the clutches, and BorgWarner was able to strip them all away.

The simplification’s major change was the introduction of the Centrifugal Electro-Hydraulic Actuator. The actuator contains a pump and overflow valve in one unit, which allowed BorgWarner to remove the individual accumulator, solenoids, and fluid filter. The all-in-one pump pulls hydraulic fluid from the AOC’s small sump and uses its internal overflow valve to ensure the proper amount of pressure reaches the clutch packs. The fluid is routed through a pressure relief valve when pressure needs to bleed from the clutch packs.

The DEM/ECU controls all system functions. Various sensors have been added since its introduction, and technology has advanced quite a bit, so now, more than ever, the Haldex system is at its most accurate.

Resting Torque Split:

  • 100/0

 

Haldex Model Breakdown (American market)

Haldex Gen 1 Models:

  • 2002 Volvo S60 AWD
  • 1998-2006 Audi TT quattro (8N)
  • 2004 Volkswagen Golf R32 (Mk4)
  • 2002-2005 Volkswagen Passat 4Motion (B5)

Haldex Gen 2 Models:

  • 2003-2005 Volvo S60
  • 2003-2005 Volvo V70
  • 2003-2005 Volvo XC70
  • 2004-2005 Volvo S80 (5-cylinder only)
  • 2003-2004 Volvo XC90
  • 2006-2008 Audi A3 (8P)
  • 2007-2009 Audi TT/TTS (8J Pre-Facelift)
  • 2008 Volkswagen Golf R32 (Mk5)
  • 2006-2010 Volkswagen Passat 4Motion (B6)

Haldex Gen 3 Models:

  • 2005-2008 XC90
  • 2006-2009 S60
  • 2006-2009 V70
  • 2006-2009 XC70
  • 2006-2009 S80

Haldex Gen 4 Models:

  • 2009-2014 XC90
  • 2009-2012 XC70
  • 2009-2012 S80
  • 2010-2012 XC60
  • 2011-2012 S60
  • 2009-2012 Audi A3 (8P)
  • 2009-2015 Audi TT/TTS/TTRS (8J Post-Facelift)
  • 2012 Volkswagen Golf R (Mk6)
  • 2009-2014 Volkswagen Tiguan
  • 2009-2017 Volkswagen CC

Haldex Gen 5 Models:

  • 2013-2018 Volvo V60
  • 2013-2016 Volvo V70
  • 2013-2016 Volvo S80
  • 2017-2018 Volvo S90
  • 2013-2018 Volvo V40 Cross Country
  • 2014-2018 Volvo S60
  • 2017-2018 Volvo V90
  • 2013-2018 Volvo XC60
  • 2013-2016 Volvo XC70
  • 2016-2018 Volvo XC90
  • 2015-2020 Audi A3/S3/RS3 (8V)
  • 2016-Present Audi TT/TT S/TT RS (8S)
  • 2015-2019 Volkswagen Golf R (Mk7)
  • 2018-2021 Volkswagen Tiguan 
  • 2019-Present Volkswagen Arteon

 

Haldex Common Issues & Failures

The Haldex design has, indeed, been a smashing success for a handful of manufacturers over the last two decades. However, it hasn’t all been sunshine and rainbows for the specialized system. Their nature made them very compact, which brought about several potential strength and longevity issues you’ll have to look out for in Haldex ownership.

 

Clogged Filters

The hydraulic system and the wet clutch pack are responsible for transferring torque from the front wheels to the rear. Although the clutches are wet and surrounded by hydraulic fluid, they still wear against each other and create dust-like debris. However, that debris is trapped within the pump’s hydraulic fluid with nowhere to go. 

Every generation has a filter responsible for straining that clutch debris and preventing it from entering the pump. The filters are very good at their jobs as pumps never die from ingesting contaminants. Unfortunately, the clutches can wear much faster than VW, Ford, or Volvo would expect them to, causing debris to build up a sludge-like blockage. The frequency and severity of blockages will differ between the fifth generation and all before it as the former’s revisions shrunk the screen and changed it from a replaceable cartridge to a fixed screen.

Whichever generation you have, the result will be the same. Partially or fully clogged filters block fluid flow to the pump, causing reduced engagement effectiveness. You may also get hit with a few diagnostic codes depending on vehicle manufacturer and generation. 

Like an engine, the only way to avoid contaminated oil and clogged filters is to service the unit on time. The filter replacement process varies as Haldex changed the filter location, but the process is always simple; remove the filter and replace it, then drain the fluid and refill. The BorgWarner-developed, fifth-generation system is the only one to have a reusable filter screen attached directly to its primary hydraulic pump so it has to come out to clear any blockage.  

Common Fault Codes:

  • VW/Audi - Gen 2/4
    • 02248 – Valve for Controlling Clutch Operating Angle (N373)
    • 01155 – Clutch – 003 – Mechanical Failure – Intermittent
  • VW/Audi - Gen 5
    • 16670 – All-Wheel Drive – C111307 [008]
    • C111307 – Mechanical Malfunction – Passive / Sporadic
  • Volvo

 

Electric Pump Failures (Gen 4 & 5)

Although classified as two separate issues, you’d be hard-pressed to find a wrecked hydraulic pump that wasn’t caused by a clogged filter. Most commonly, a wrecked pump is going to affect a Gen 5 system for Audi S3 and Mk7 Golf R owners as their aggressive driving and small mesh filter aren’t conducive to extended service intervals; however, the Gen 4 system is no exception to the problem. Pump failure is typically characterized by wheelspin and axle bounce/tramp off the line, but hardly ever a warning message on the dashboard. There may be a flash from the traction light, but it’s more likely there’s a stored fault code without any warning. 

For older Volvos and Land Rover systems, general age is likely the culprit. Pumps run about every 20 seconds to keep the system primed while the ECU powers up the motor for 2 seconds and turns it off again. The excessive use wears out the brushes within the pump, causing them to cease operation. Once the pump has died completely, the driveline will function solely in a FWD state. 

This failure is generally reserved for the fourth and fifth generations, though every generation can be affected. The “PreX” pumps especially can fall to this same burnt-out fate, so tread lightly when it comes to the pumps. Replacement is the only option here, though it’s certainly a DIY job. 

Common Fault Codes:

  • VW/Audi - Gen 4
    • 00448 – Haldex Clutch Pump (V181) 011 – Open Circuit – Intermittent
    • 00448 – Haldex Clutch Pump (V181) 011 – Open Circuit – Defective
    • 00448 – Haldex Clutch Pump (V181) – 011 – Open Circuit – Intermittent
    • 00448 – Haldex Clutch Pump (V181) – 002 – Lower Limit Exceeded – Intermittent
  • VW/Audi - Gen 5
    • C111204 Haldex Clutch Pump, Defective
    • 16671 – Pump for Haldex Clutch – C111204 [008]
    • 16668 – Pump for Haldex Clutch C111207 [008]
    • 16666 – Pump for Haldex Clutch C111213 [008]
  • Volvo
    • P1889-12 
    • P1889-14 
    • P1889-18 
    • P1889-19 
    • P1889-49 
    • P1889-7A 
    • P1889-71 
    • P1889-74 

 

ECU Faults

The AOC controller is a fairly common trouble spot on older Haldex units. Nearly every controller sits to the side of the AOC to keep the system compact and as simple as possible. However, their exposure to heat and the elements under normal driving conditions can work against their longevity. 

FCP_Euro_Haldex_Explained_Differential_ECU_Fault_Warning

Corrosion and burnt boards are possible in every generation of Haldex there is and make for a potentially tricky repair. As they control the function and record the functionality of every component, corrosion or melted ECU parts can pop diagnostic codes that point to other places in the system depending on which part of the circuit board is damaged. If you’re lucky, you’ll receive a communication fault which is specifically an ECU/DEM issue.  

On the other hand, the most advanced Gen 5 controllers can malfunction without any internal damage. Software bugs can arise following damaged components elsewhere in the system, adding insult to an already injured wallet. Replacing a damaged pump and resetting the system is supposed to eliminate the fault code and have you back to carefree driving, but that’s not always the case. Many have found that the Haldex ECU retains the previous fault after replacement and reset. The cause is incorrect numerical values within the firmware that controls the AOC. The fix is either a replacement or a removal and rewrite of the data chip. 

Common Fault Codes:

  • VW/Audi - Gen 2/4
    • 02248
  • VW/Audi - Gen 5
    • 131599 – All Wheel Drive Clutch – U0114 00 [009]
    • 131599 – Control Module for All Wheel Drive Clutch – C111307
    •  
  • Volvo
    • CEM-1A64
    • BCM-0095
    • BCM-0094
    • DEM-0003

 

Angle Gear/Bevel Box Failures

Volvo and Volkswagen owners love to use different terms to describe their components, but they’re essentially the same, differing only in their mating to their respective transmissions. No matter what it’s called, reliability hasn’t always been perfect. Owners of every generation have reported some kind of problem with them ranging from fluid leaks to complete breakage of internal mechanical components. Issues typically remain small on unmodified vehicles, but power increases can have some catastrophic effects.

 

Angle Gear Collar Failure

Earlier Volvo systems used a toothed collar to connect the front differential to the angle gear. The sleeve is relatively compact and identical for all models, regardless of vehicle weight and torque. It has become a common source of breakage within the angle gear through poor metallurgy or general wear. Luckily, the collar is a replaceable component, saving owners the cost of replacing the complete transfer box. Failures are found more with manual transmissions due to the increased stress of spirited driving and hard launches, but automatic transmissions aren’t immune. All of them will eventually break with age and mileage, around 100k, according to owners. Volvo has since released an updated design that features a nitrite coating for increased wear resistance. 

Replacing the collar can be DIY, depending on the model. Certain XC90s require subframe removal to get the gear, making the replacement a significantly more involved job than other models. If you find that you lack drive to the rear wheels from a broken collar, you don’t necessarily have to replace it immediately. At that point, the DEM will sense the fault and operate in its failsafe FWD mode. You’ll be able to keep on driving until you’re ready to repair.

 

Angle Gear Leaks

The most common issue on the older Volvo angle gears are fluid leaks. The angle gear housing has   that can all leak, so it’s best to replace them all at once. Removing the angle gear for seal replacement is the same process as replacing the collar, so those two jobs could and likely should be done at the same time if no service history is present. 

Running the angle gear with a leak for too long can begin to prematurely wear the bearing and other components within the transfer box. With less than a liter of fluid within the box, it’ll run out quickly, too. If you can’t get in there to replace the seals immediately, ensure you keep it topped up. If you’re handy with a wrench, then replacing the seals might just be a job for you, too. Removing the angle gear assembly requires only a few more steps than an axle replacement for most models, and you won’t need any special tools during repairs. 

 

Bevel Box Ring Gear Failures

Volvos aren’t regularly the high-powered, hard-launching sport compacts and sedans that find their way to track days, but the Volkswagen Golf R and Audi S3 are. Both VAG models were some of the quickest street cars you could buy a few years ago and are easily made faster with modifications. The positives associated with the AWD system, hard launches, and grip have also increased strain on the drivetrain components seen regularly by the rate at which those models clog their pump screens. But more than that, their pressure on the bevel box has led to more severe issues. 

Inside the bevel box is a ring gear pressed onto the shaft that leads out of the transmission. Over time and from repeated aggressive launches, the ring gear will spin on the shaft. Once spun, AWD traction will suffer, and the metal-on-metal friction will tear up the gear and shaft, sending metallic debris through the box. Unfortunately, the only way to replicate the issue is while driving, as the force required to spin the gear is fairly substantial, and the only fix is a complete replacement. 

 

Haldex Controller Upgrade

The Haldex-designed AWD isn't nearly as adaptable as the sports sedans and hatches it's equipped to. Stronger mechanical components and beefier units from a larger vehicle aren't available. Software flashes are the only way forward to cope with more torque; luckily, they've been pretty effective.

HPA Motorsports and United Motorsports are the two companies that offer remapped control over the AOC's lock-up capabilities for Volkswagen and Audi products. Through hours of engineering and testing, both have pushed the hydraulic clutch system beyond VAG's limits to keep up with the modifications owners put their vehicles through. Testing has shown that both tunes can hold double the torque of the OE mapping without any major changes to the rest of the driveline while also improving fuel economy. They've achieved that by giving the Haldex ECU three distinct maps with varying aggression. 

Although they both offer the same service, United Motorsports is a bit more accessible. Their available tunes cover every generation found in VAG products, from the Gen 1 in the Mk4 R32 to the latest Gen 5 system in the Mk7 Golf R and 8S TTRS, and all cost the same: $599. HPA, while still a very respected company, offers less compatibility than United. Tunes are available for the Gen 2 and 4 systems, covering the Mk5 R32 and Mk6 Golf R only, and cost a few bucks more. They also offer the "TOUCHMOTION AWD PROGRAMMER" for Gen 4 systems. A standalone touchscreen interface is connected with the included control unit, allowing owners to monitor and adjust their AOC clutch engagement as they see fit. It's not for the faint of heart and shouldn't be messed with without experience, but it can make for an incredible tool for adjusting to race track conditions.

 

Haldex Service Recommendations

The Haldex AOC unit, like any other fluid-filled component in the car, needs regular servicing. As we’ve seen, the fluid can get contaminated, and the AOC filters can get clogged. Servicing your Haldex unit is the only way to prevent potential disaster, and it shouldn’t take a substantial amount of work. To assist you in your DIY effort, we’ve put together a handful of service kits containing the proper fluids, filters, seals, and plugs so you don’t have to worry about the right parts. 

FCP_Euro_Haldex_Explained_Differential_Pump_Service

If you’ve just bought a Haldex-equipped vehicle, do yourself a favor and service the system as soon as possible. Standard service intervals fall between 20,000-30,000 miles depending on application and generation, but anywhere within that range should be fine. Modified cars and those running the Gen 5 system may want to check the pump screen earlier than that interval, as other owners have reported blockages well under 10,000 miles. When replacing the fluid, only use the OE Febi Bilstein fluid. It’s explicitly specialized for the AOC and contains a complex mix of additives to ensure every component is functioning properly. The parts connected to the AOC, the Bevel Box or Angle Gear, and the rear differential all use non-specialized 75W-90 gear oil. Their service intervals are about double the AOC, coming in around 60,000 miles.


author image
Written by :
Christian Schaefer

Car and motorsports-obsessed writer/editor for FCP Euro's DIY Blog. Constantly dreaming of competing behind the wheel or searching for another project. Owner of a turbo Subaru Forester and a ratty Porsche 914, neither of which are running.


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