Syncro viscous coupling &
differential lock FAQ
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Syncro viscous coupling & |
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Written by Jim Davis
What is a differential lock?
A standard or "open" differential is designed to allow a pair of drive wheels to operate at different (differential) speeds. This commonly occurs whenever the vehicle goes around a curve or corner. This type of differential will only transmit as much power to the wheels as the wheel with the LEAST traction can use. On slick surfaces this means that just having one of the two wheels lose traction will cause neither wheel to be effective at applying power to the ground. To circumvent this situation, locking differentials (sometimes called "lockers") were created to be able to apply power to both wheels regardless of their traction. A locking differential creates a solid lock between each half of the axle and will not allow one wheel to spin without the other. On an off-road vehicle this even allows one wheel to completely lose contact with the ground and still apply power to the other. While a differential is locked, the vehicle's ability to turn a corner is hampered though. With both wheels turning at the same speed, the vehicle has a significant tendency to travel in a straight line. Because of this a means of engaging and disengaging the locker is needed. Most lockers (such as a "Detroit Locker") remain locked and automatically unlock while turning corners. Some differential locks (such as the "ARB Air Locker"), including the Syncro's locker, use a driver initiated control to manually engage and disengage the diff lock at the driver's will. Don't confuse a "real" locker with a mere "limited slip" differential. Limited slip differentials do not provide a complete mechanical connection. Most rely on a clutch of some sort, which will, when mildly stressed, slip, allowing one wheel to spin. Limited slip diffs have significant applications on race or sports cars, but do not create enough of a lock condition to be very useful off-road.
How does the Vanagon Syncro's differential lock work?
Pull the knob to engage the lock. Push the knob back in to disengage the lock.
Pulling the knob actually "requests" that the diff lock be engaged. It will lock when the rotation of the axles lines up for locking and they are moving at about the same speed. When the locking occurs, the light on the dash will light up. The manual says to drive straight ahead after pulling the knob, but I've found that by doing this it can take a very long time for engagement. This is because the axles are not lined up and are traveling at the same speed and don't really make any progress toward lining up. I've had better success at traveling a slight curve for quicker engagement.
How can I test to see if the lock is engaged?
Generally, the diff lock should not be engaged while on pavement. For a short test, it can't do much harm. Make a semi-sharp turn with it engaged while on pavement. You should feel sort of a "binding", causing the van to resist the turning and continue in a straight line.
What are the empty 'spaces' on the diff lock console?
Some countries (not USA or Canada) got Syncros with locking front differentials. These models used the leftmost knob position and light position for the front diff operation just like the more familiar rear diff lock. Differentials without the locking unit will not except the internal locking parts without machining. The locking parts are essentially the same as the one used in the rear differential. Some other Syncros were built without the automatically engaging viscous coupling. These have a simple mechanical coupling at the front end of the transmission, which is operated by a knob in the center position of the diff lock console.
What does the Syncro's owner's manual say about diff lock operation?
When the knob is pulled or pushed, engagement or disengagement is only initiated. The actual locking of the differential can be delayed if the wheels are rotating at substantially different speeds. When driving around a corner it could also happen that the lock will not engage or disengage. Under such conditions slow down and drive in a straight-ahead direction so that the lock may engage or disengage.
The warning light above the knob indicates the operational condition of the lock:
The light comes on only when the lock has engaged properly.
The light goes out again once the knob has been pushed in and the lock has actually disengaged. If the warning light does not light up, after selecting the differential lock, and after an extended drive, the electrical system and the lock should be checked by your VW dealer.
WARNING! Never lock the differential when...
driving on paved roads. 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 tires wear and also to "jerking motion" when the vehicle is being steered. The differential lock for the rear axle must only be used when the vehicle is stuck or is in immediate danger of becoming stuck.
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.
How does the Vanagon Syncro's viscous coupling work?
The viscous coupling is constructed of a series of metal plates. Half the plates, (plates 1, 3, 5, 7, etc.) are connected to the input drive shaft coming froward from the transmission. The other half of the plates (plates 2, 4, 6, 8, etc.) are connected to the front differential. These plates are enclosed in a chamber with a very unusual silicon fluid. When the rear wheels and front wheels are moving at the same speed, all the plates move at the same speed, nothing special happens, and little power is transmitted through the coupling. But, when the rear wheels begin to lose traction and spin, the plates connected to the driveshaft begin to spin faster than the plates connected to the front differential. This causes a shearing action in the silicon fluid, which almost instantly heats and thickens, becoming much more viscous. This now highly viscous fluid locks the two sets of plates together and power is transmitted to the front differential and thus the front wheels. VW says this happens as quickly as 1/4 turn of the spinning rear tire.
How can I check to see if my viscous coupling is engaging?
The Syncro all-wheel-drive system's viscous coupling is the device that automatically engaged to send power to the front wheels. While driving on something slick like ice (or a steep dirt hill), accelerate as quickly as possible to try to get the rear tires to break loose, about as soon as they do, the front wheels should engage and move you forward or at least spin along with the rears. All of this is completely independent of the operation of the rear diff lock - don't confuse them.
Another crude way to check is as follows:
Find a flat, open surface like a parking lot and jack up one rear wheel with a floor jack from behind so that the floor jack's wheels are parallel to the van's. Jack it only enough to get one wheel barely of the ground.
Make certain that the rear diff lock (if applicable) is not engaged.
Start the van and ease it forward slowly. With a properly functioning VC, power should be transferred to the front wheels and the van will move forward dragging the floor jack with it, or will pull itself off the floor jack. Be careful.
Not exactly a 'nice' method, but it works!
How long does it take the syncro mechanism to engage?
The Syncro all-wheel-drive system's viscous coupling engages in a fraction of a second. If it's working properly, you won't 'feel' it engage. It does so smoothly and automatically.
How much torque is displaced to the front wheels?
Under normal driving circumstances, the front wheels get less than 5% power from the engine. Under extreme circumstances, the front wheels can receive 100%, but the rear wheels would need to be airborne or on a frictionless surface.
