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http://www.vwsyncro.co.uk/data/forums/topic.asp?TOPIC_ID=2365
The Syncro Transmission System.
Fitted in a variety of vehicles, the system fitted to the Mk2 golf Syncro will be described. First used in approx. 1986, a modified version of this design is still used today (Haldex)
Essentially an extension to the front wheel drive system, (without any form of differential locking) the gearbox is modified to accept a 90 degree take off (Transfer Box) from the differential cage to provide power to the rear wheels.From the Transfer box a propshaft is connected to a Viscous Coupling on the front of the Rear Differential with shafts driving the rear wheels.Transfer Box
This splits the available power off the gearbox via a rubber bushing to the propshaft and again via a second rubber bushing to the Viscous Coupling.
The rubber bushings offer a degree of flexibility when the engine moves as well as reducing transmission snatchViscous Coupling
This is the “magic” bit of the Syncro 4wd system and is the part that controls the amount of drive sent to the back wheels.
The VC consists of a series of circular plates immersed in a silicon fluid. The construction is of “in” and “out” plates interleaved together, the “in plates are connected to the main casing and form the input part of the VC unit. The “out” plates are connected to the central output shaft connecting to the rear differential.
When all 4 road wheels are turning at the same speed, the “in” and “out” plates in the VC turn together at the same speed but are isolated from each other and no power is transmitted through the VC unit.
If one or both front wheels begin to slip the car begins to slow but the drive through the transfer box, the propshaft and the VC unit continue to turn at the same speed.
The “in” discs in the VC unit are now effectively turning faster than the “out” discs and this difference causes the silicon fluid in the VC unit to “sheer “and change from a thin fluid to a thick gel. This causes the VC to “lock” the discs together allowing power to be supplied to the rear wheels. (The greater the difference in speed of the discs the faster the fluid reacts and the harder it locks the discs together).
The vehicle is now in 4WD or more realistically 3 or even rear wheel drive!!The final part of the VC story is now the discs are locked together, there is no speed difference between them and the silicon fluid begins to thin, as it does the discs start to slip and the fluid reacts and locks the VC again. (This happens so quickly that drive is supplied continuously to the rear wheels until the front wheels stop slipping)
Rear Differential
This operates as a conventional differential with one exception it allows drive in one direction and freewheels in the other. This can be reversed by a mechanism that is controlled by an electrical solenoid and vacuum actuator to enable 4WD when reverse gear is selected.
(The operation of this device gives the characteristic “Clunk” occasionally heard when the car is moved in reverse)The 2 deciding factors that determine how much power is sent to the rear wheels is the difference in speed between the front wheels and the difference in speed of the VC plates (or more correctly speed difference between the front and rear wheels)
This system simply but cleverly provides a fully variable 4WD system that theoretically can vary between 100:0 to 0: 100 i.e. front wheel drive, through 50:50 split 4WD to rear wheel drive.
Realistically rear wheel drive (0:100) is not possible as both front wheels would have no contact at all…. I guess the front wheels hanging over a cliff just might count!
Normally its the gearbox switch that fails, or maybe possibly the actuating arm on the vacuum control in the rear diff.
The only way to really check is by substitution, does yours still make the clonk when you reverse? thats usually a good indicator.
Its you Syncro from years ago! I thought it was, but then I thought it cant be!
Here it is in all its former glory.
