The following info was taken from the owner's manual:
The Differential Lock of Syncro 1991
When driving around a curve, the outside wheels turn more than the inside
wheels. A rear gear set, part of the transmission, called the differential,
compensates for the difference. If, however, one wheel gets on a slippery
surface and starts to spin, the differential will not transmit power to the
other wheel. The car will be stuck. The differential lock will allow power
to be transmitted to both wheels to help get the vehicle moving again.
Because the differential is then unable to compensate for cornering, vehicle
handling and maneuverability will be reduced. Therefore the differential
lock may only be used under limited conditions.
The differential lock for the rear axle must only be used when the vehicle
is stuck or is in immediate danger of becoming stuck.
A locked differential cannot compensate for cornering and therefore increases
tire wear, reduces vehicle maneuverability, driveability and can impair
vehicle handling performance. To reduce the potential for loss of vehicle
control, only use the rear differential lock under the specific and limited
Never lock the differential when driving on paved roads, or
normal dry roads as it will not provide any advantage. Under these
circumstances, an engaged differential lock will impair the vehicle’s
maneuverability. When the differential is locked, the rigid connection
between the rear wheels has a rubbing effect on the tire when cornering.
This leads to increased tire wear and also to a “jerking motion” when the
vehicle is being steered.
Never lock the differential when one wheel is spinning. First
bring the spinning wheel to a standstill, and only then engage the
differential. This prevents damage to the drive train or uncontrollable
acceleration leading to loss of control and personal injury.
Never lock the differential when at excessive speed on very
Never lock the differential when the vehicle is being towed.
Never lock the differential when testing the vehicle on a
dynamometer (engine output analysis and brake test) used for performance
testing. (the transaxle drive shaft must also be disconnected for this test)
Using the Differential Lock
Start from a standstill or slow way down and drive in a straight line.
Engage or disengage the differential with the knob in the center of the
instrument panel. When the knob is pulled (or pushed) engagement (or
disengagement) is only initiated. The warning light above the knob only
comes on when the lock has engaged properly. The actual locking of the
differential can be delayed if the wheels are rotating at substantially
different speeds. The lock will not engage (or disengage) when driving
around a corner.
When the road surface has improved the lock should be disengaged.
The light goes out again once the knob has been pushed in and the lock has
All-Wheel Drive Operation and the VC (viscous coupling)
In contrast to conventional all-wheel drive systems that require
transmissions to be manually engaged when needed, the Vanagon Syncro
all-wheel drive system is always permanently engaged. Only the amount of
power being transmitted to the front and rear wheels changes automatically
as the road surface demands.
As the driving force applied to the road goes to four wheels instead of two,
the amount of slip is considerably reduced. This improves the traction
between tire and road surface and thereby, the driveability on slippery road
surfaces. On vehicles with two-wheel drive, the driving wheels can spin on
slippery surfaces if too much acceleration is applied. This greatly reduces
tire grip and can result in loss of vehicle control.
By distributing the driving forces to four instead of two wheels, the wheels
of the Syncro are less likely to spin under the same conditions.
Driving power is greatly increased by the continual all-wheel drive
especially on snow and slippery surfaces, when starting off from a
standstill, and when driving on rough roads. Within reason the Vanagon
Syncro can go places where other vehicles may likely get stuck. It is easier
to stop on slippery hills and get going again.
The engine power is transmitted evenly to a moderately slippery road
How It Works
The automatic distribution of power is accomplished through a fluid shear
coupling (viscous coupling) that is part of the front axel. The design of
the viscous coupling keeps driving force losses to a minimum while
distributing the forward thrust to all four wheels when needed.
When the going gets rough, especially when wet, slippery or snow-covered
roads cause the rear or front wheels to slip, the viscous coupling
automatically transfers driving force to the other wheels to help you keep
The distribution of drive forces allows the Syncro to adapt to changing road
surfaces and provide increased traction automatically.
While it has the ability to handle poor weather and road conditions and has
limited off-road capabilities, the Syncro is not an off-road or all-terrain
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