Driving my Volvo XC90 in Colorado had me thinking about AWD systems and how they relate to vehicle performance. This led me further down the rabbit hole, and before I knew it I was neck-deep in learning about how all of Volvo's AWD systems worked. Here's what I found:
There are six variations of Volvo's all-wheel-drive:
- Pre-Haldex - GKN Viscous Coupler
- 1st Generation Haldex
- 2nd Generation Haldex
- 3rd Generation Haldex
- 4th Generation Haldex
- 5th Generation Haldex
Pre-Haldex - GKN Viscous Coupler
Used On: 1999-2006 Volvo S80 T6
When the vehicle is traveling with relatively equal traction on the front and rear axles, the two halves of the coupler will be rotating at nearly the same speed. When the car loses traction and the front wheels begin to spin, the side of the coupler attached to the front axles will begin to spin faster than the side attached to the rear axles. This+ difference in speed causes a shearing effect on the dilatant fluid that the coupling is immersed in. When the fluid heats up, it quickly transforms from a liquid to a state resembling a solid, locking the plates together and forcing the two sides of the coupler to be locked together. This type of AWD system was inexpensive to produce, required almost no maintenance, and was fairly reliable. However, in practice on older Volvo AWD models, this system proved to be troublesome and prone to failure. Volvo has used Haldex Traction's AWD system for nearly all of their models since the introduction of their FWD/AWD platform. However, before the switchover to the Haldex
system in 2003, Volvo used a viscous coupling designed by GKN Driveline to transfer torque to the rear axles. A viscous coupling is an assembly of closely spaced plates bathed in a dilatant fluid (essentially just a thickening fluid). This viscous coupling links the front and rear drivetrains.
Haldex First Generation
The first-generation Haldex AWD system is the first reactive hydraulic AWD system produced by Haldex Traction. This means the AWD system relies on wheelspin to activate the torque transfer from front to rear wheels. It uses a mechanical pump connected to the input and output shaft of the Haldex clutch assembly.
Normal driving with ample traction means the car is nearly entirely front wheel drive. When wheel slip occurs, the difference in rotation speed between the input and output shaft drives the hydraulic pump. Operation of the clutch pack is almost entirely mechanical—a mechanically driven pump and a mechanical linear throttle valve were used to determine wheel slip and lock up of the clutch pack, converting the system from 90% / 10% front/rear torque distribution to 50% / 50% torque distribution within 90 degrees of wheel slip.
Haldex Second Generation
Haldex's second generation is also a reactive AWD system, just with the addition of computer-controlled solenoids to control the clutch plates. This system reacts within 90-degrees of wheel slip.
When wheel slip occurs, the difference in rotation speed from front to rear axles activates a mechanically-driven hydraulic pump. The pump forces hydraulic fluid through a computer controlled solenoid, which then engages a set of clutch plates to transfer torque to the rear axles.
Haldex Third Generation
The chief complaint about performance on the first and second generation systems was the reaction time required to trigger the torque transfer to the rear axles. And so, Haldex's third generation system is the first proactive AWD system deployed on a Volvo.
As soon as the engine is started, an electric pump pre-pressurizes the AWD transfer clutch. When the traction control computer senses wheelspin, the pre-pressurized clutch pack is engaged instantaneously and torque is transferred to the rear axles. Once engine torque is transmitted through the Haldex unit, the mechanical oil pump takes over from the electric pump and continues to supply pressure to the hydraulic clutch pack. Haldex claims a reaction time of just 15 degrees of wheelspin. This AWD system is marketed as "Instant Traction" by Volvo, and "PreX" by Haldex. You can view a slow-motion video of my XC90 equipped with third-generation Haldex Instant Traction below.
Haldex Fourth Generation
Haldex fourth generation is virtually identical to the third generation system, but does away with the mechanical hydraulic pump.
Instead of relying on the electronic pump to pre-charge the AWD system, the fourth iteration of Haldex's AWD coupler uses the electric pump to supply hydraulic pressure for the entire operation of the torque transfer from the front to the rear axles. Haldex fourth generation is also a "pro-active" AWD system. Volvo also continues to market the fourth generation system as "Instant Traction".
Haldex Fifth Generation
The fifth generation of Haldex's AWD system focused on tighter integration of components and a simplified construction. Previous versions of Haldex's AWD system used a set of pumps (mechanic and electronic), accumulators, and solenoids to precisely control hydraulic pressure applied to the clutch pack.
The fifth generation system simplified the design and uses only a computer controlled high-pressure hydraulic pump to replace the accumulators and solenoids used in previous versions. Volvo also markets the fifth generation system as "Instant Traction".
