• Nov 30, 2007
All-wheel drive system supplier Haldex has introduced its latest iteration in the new Saab Turbo X and 9-3 Aero XWD vehicles. XWD strands for Cross-Wheel-Drive and is technically called Haldex version 4.0. Its introduction has generated some excitement amongst general consumers and tuning enthusiasts. Haldex has been riding the high wave with their XWD release as it was recently reported that they won a contract to develop a system for Hyundai. We have yet to see the new all-wheel-drive offering hit consumers in order to better understand its potential under extensive abuse though. However, the wonder that is YouTube has provided us with video of the Saab Aero XWD in action during a Saab comparison test drive event. Haldex has also supplied a wealth of information detailing the elements of their 4.0 system.

The main components of the new Haldex system are the Power Take-Off Unit (PTU), Limited Slip Coupling (LSC) and eLSD (Electronic Limited Slip Differential). The PTU is the final drive unit at the front of the vehicle that transmits power to the front wheels and sends power down the driveshaft to the rear wheels. It is not a Haldex design, but is required to adapt the system to a front-wheel drive vehicle. The LSC sits at the rear of the vehicle in-line with the driveshaft. It controls the torque split between the front and rear wheels of the vehicle. The LSC sends torque to the eLSD that sits between the rear wheels. The eLSD transfers torque to the two rear wheels. Follow the jump to continue reading more about the system.



Like with XWD, previous generation Haldex systems also included an LSC and an LSD. However, with the new system Haldex significantly redesigned the workings of their LSC. The LSC is still a clutch pack that adjusts torque split depending on hydraulic pressure. It is the method of fluid flow through the device that has changed. A large complaint about the old system was its lagging response time. LSC versions 3.0 and earlier used a built-in pump to create hydraulic pressure on the clutch pack to increase the torque drive to the rear wheels. While efforts were made on Haldex's part to create pre-emptive torque by adding a check valve and feeder pump to provide some instant pressure when triggered by wheel slippage, it was still limited in capacity. That is why for version 4.0 Haldex made an effort to improve response time by eliminating the hydraulic pump built in to the LSC, which also reduced its overall packaging size. Instead they have added a proportional pressure release valve with an accumulator that is kept filled by a detached feeder pump. This provides more instant response by holding the valve open to limit the torque drive to the rear wheels and keeping the hydraulic fluid flowing through the system. That way when rear torque is demanded, the valve closes and hydraulic pressure is already there.


The above image shows the LSC unit on the left side. The component on the right side is the eLSD. Hydraulic lines can be seen running between them. Click the image for a high-res view.

The LSD used by Haldex 4.0 is also not the mechanical limited-slip differential of old. They swapped the old system LSD for an electronic unit. The eLSD works in much the same way as the LSC, a feeder pump and pressure relief valve are used to control hydraulic pressure on the differential clutch pack. This allows for complete control of the rear differential lock-up without the need to wait for wheel slippage to occur. The system has its own control unit contained in the LSC. This control unit communicates between the vehicle systems to get sensor input for data such as wheel speed, rpms, throttle position, steering wheel input, etc. It also works with anti-lock brake and traction control systems.


The above image shows a close up view of the LSC unit and eLSD. The eLSD sits between the rear axles and the LSC is mated to the driveshaft. The electronic control unit can be seen as a rectangle mounted on the nearside of the LSC. Click the image for a high-res view.

The XWD system can transmit 100 percent of available torque to either the front or rear wheels. However, for those conditions to occur one end of the vehicle would have to lose all traction, like driving on ice for instance. During a standing start the rear wheels are put to use, without the need for any slip to occur. Then under straight-line cruising conditions, to conserve fuel and driveline wear, the torque split to the rear wheels is reduced to a level between 5 and 10 percent. Also up to 40 percent of torque can be transferred by the eLSD between the two rear wheels to the one with more grip. The system can adjust torque splits based on calculated conditions, such as those that indicate an aggressive lane change maneuver, to effectively reduce overstreer or understeer without any wheel slip occurring. In the event that some wheel slip does actually get to occur, the system can react more timely and efficiently than in the past.


The above picture shows the XWD driveline. At the forefront of the picture is the eLSD coupled to the LSC unit. The PTU can be see at the opposing end of the driveshaft. Click the image for a high-res view.

The new Haldex system is an improvement on older iterations. Its characteristics are more like those of some of the Japanese manufactures such as Nissan or Mitsubishi than it is like the European Audi Quattro system. While the Haldex 4.0's integration into the Saab vehicles puts it into a front-wheel drive based platform it will be interesting to see if that won't be too much of a hindrance on performance. The Mitsubishi Lancer Evolution has been very successful despite its front-wheel drive base. It would be wonderful to see a Saab Turbo X get thrown in the X-Games Rally ring with all the Subraru WRX STIs or hit the tarmac in Time Attack against all the usual Evos. Until then, the most XWD action we get to see comes from a Saab hosted test drive event. Check out the embedded video below.


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    • 1 Second Ago
  • 17 Comments
      • 7 Years Ago
      i wonder if Volvo will get to use this system
        • 7 Years Ago
        I hope so, maybe 'eLSD' standard too.
        Hopefully when the S80 gets refreshed.
        Volvo should not offer front drive at 40 large.
        Use haldex with the 3.2 I6 with the 3.749 axle ratio.
        Use the 3.464 T6
        Use the 3.333 with the HO version of the T6, say 330hp.
      • 7 Years Ago
      Holy mother of increasing complexity, Batman.

      I heart Torsens.
      • 7 Years Ago
      Hehe I remember a friend that lifted the front wheels of his old Huyndai SUV to try out the AWD system, what he got was a gentle push or nudge from the back wheels that he could stop by just leaning at the car. This sure looks more promising, but does the driver reach for the handbrake during the first long corner on the wet section? Then it's cheating :P
      • 7 Years Ago
      Is SAAb still in business?
      • 7 Years Ago
      remember the days when you could fix your car with a couple of tools from Sears?
      • 7 Years Ago
      Didn't read the blurp, but that's some good driving!
      • 7 Years Ago
      the Saab XWD Haldex 4.0 all-wheel-drive system looks simple but it's very complex. For a small driveshaft unit to transfer that much forward motion is amazing.

      The XWD system can transmit 100 percent of available torque to either the front or rear wheels.

      Very interesting all wheel drive design...
      • 7 Years Ago
      i'd like to drive this back to back with a turbo Legacy, just to see how the new system stacks up to an established player in the field.
        • 7 Years Ago
        Turbo legacy automatic, right?
        The planetary center differential of the auto is something like 44.6/55.4, and there is a gear mesh to move the power down to the front axle, so the split is probably closer to 44/56 than the marketed 45/55.

        The stick has a 50/50 center differential, but there is a gear mesh to get power to the rear axle, so the stick cars are technically front biased, even if it is something like 51/49.
        I wish subaru would offer the 35/65 planetary, at least in some cars, also the '45/55' center differential would be great in the low powered cars (the non turbo/ non boxing6 legacy)
        • 7 Years Ago
        I'd be interested to read it...

        but with a winter storm forcasted for tomorrow in my location, I think I am going to stick with my turbo Legacy. :D

        Haldex FTL
        Subaru Symmetric AWD FTW.
        Subaru Variable Torque Distribution/Driver Controlled Center Diff, Limited slip rear diff (STI drivetrain) For the shutout. The S204 even put front and rear torsen LSDs into that mix.

        Even Audi's longitudinal Quattro has gone to electronic brake force distribution to shuttle torque to different wheels, instead of torque-biasing diffs. Using brakes for forward traction at other wheels is dumb, when mechanical drive systems work better. The whole idea is to get as much of the vehicle's contact with the ground to participate as possible.

        Think about what it would be like with the glove on the other hand... if brakes only worked on one or two of the wheels at any given time, based on traction slippage. Other wheels brake only after one or two start to slide the tires...

        If four brakes, properly balanced work best to stop the vehicle, then why wouldn't all four wheels providing forward motion be just as good for acceleration traction?

        Even without a front LSD, most subarus with rear LSDs, and a real center differential, at least 3 of the tires are pushing at once, and the 4th wants to to help. Better than good-ole 1-wheel drive on FWD or RWD without limited slip differential.

        After driving the real thing, the pretenders seem a bit overrated. Useful, probably, but not equal to the best.
          Ken Gilbert
          • 2 Years Ago
          for the last 10 years i've had an '03 subaru wrx 5mt, so VC center diff, VC rear diff. i've recently picked up an '08 saab 9-3 aero xwd for my wife. had it out in the snow for the first time last night. the xwd system is no joke.
      • 7 Years Ago
      I was also happy to see the mention of what conditions 100% torque transfer occurs.

      Also glad they cleared up the rear diff issue. Earlier info. hinted at a Honda and Mitsu-like twin clutch system that could actively reduce torque to one wheel.

      This article is stating it is merely an electronic limited slip.
      • 7 Years Ago
      "However, for those conditions to occur one end of the vehicle would have to lose all traction, like driving on ice for instance."

      Thank you for cutting through most of the marketing hype and providing the only coverage of this system that has properly described the torque-transfer between axles.
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