The who, what, when, where and how of the Volkswagen Syncro.

What is a Syncro? When was it made?
Syncro is Volkswagen's name for their four wheel drive system that incorporates a viscous coupler to automatically engages the wheels that are not constantly driven by the transmission.  There is one exception to this rule and that is the Quantum Syncro from the late 1980s which is really an Audi quattro system that used a differential instead of a viscous coupler.  Syncro was found on the Vanagon / T3, Golf, Jetta and Passat models.  Syncro started in early 1985 for the T3 and 1986 for the Golf with the Jetta and Passat following a few years later.  Porsche also used this system on their all wheel drive models.  The Syncro all wheel drive systems was still being produced until the end of the Eurovan / T4 production.  The Syncro system now has been superseded by the 4Motion system that uses a Haldex clutch instead of a VC.

How does the Syncro system work?
On the Golf / Jetta / Passat models, the standard 020 transmission is fitted with a power take off that has 10:11 gearing.  This PTO goes between the transmission and passenger CV joint.  Attached to this PTO is a drive shaft that goes to the rear of the vehicle.  At the rear of the vehicle is a VC (Viscous Coupler) and then another 11:10 differential that distributes the power to the rear wheels.  The Eurovan / T4 Syncro operates in much the same way.

Rear differential and Viscous Coupling from a 1987 Golf Syncro 1987 Golf Syncro transmission showing the PTO

The Vanagon Syncro works in quite the same way as the Golf / Jetta / Passat with the exception of the transmission.  The transmission was redesigned to incorporate the PTO internally.  The drive shaft is driven at the pinion speed so there is matching final drive ring and pinions front and rear of the Vanagon Syncro.  The Vanagon Syncro can also incorporate manually engaged differential locks on the front and rear axles.  On the front differential the VC is also an internal device.  

Who built the Syncro?
Volkswagen and Steyr-Daimler-Puch traded technologies in the 1980s.  Puch designed and build the Vanagon Syncro in Austria in exchange for the VW six cylinder Turbo Diesel engine that it was using in the LT so that it could be incorporated in the redesign of the Puch Pinzgauer military vehicle.  The Golf / Jetta / Passat Syncro were all built in Germany.

How does a Viscous Coupler work?
The viscous coupler is constructed out of a series of plates.  The even number plates are attached to the input of the device, the odd numbered plates are attached to the output of the device.  Inside of this sealed coupling is a silicone based fluid / gel.  In the simplest of terms, when the fluid is agitated it turns into a solid and joins the odd and even plates together forming a solid connection.  This generally occurs when there is a difference in speed between the wheels driven by the transmission and the wheels on the other side of the VC.  Minor differences in speed generally do not cause the VC to lock up - this enables you to have 2WD operation when parking the vehicle.
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