Decoupler - Lengyl

From VanagonWiki

Tom Lengyel designed and makes a decoupler which is probably the most popular version used in the United States.  It works somewhat differently than the original German version.  It is sold by many of the popular Vanagon/Transporter parts vendors in the US. 

The decoupler disconnects the Syncro transmission output shaft from the drive shaft....and thereby from providing power to your front wheels, thus converting your Syncro from full time AWD (by way of its viscous coupling) into, a 2WD vehicle.  It is controlled by the same vacuum system which controls your rear differential lock and has exactly the same type of vacuum powered switch and indicator light as you have on your dashboard for your differential lock, with the switch installed in the center position of the switch panel.

When the dashboard switch is pulled the decoupler is engaged and the green light in the switch turns on, indicating you are in 2WD mode.  One point to note is that if, for any reason, your vacuum system fails, the actuator on the decoupler defaults to 2WD mode.  (I believe the original, German made, decoupler defaults to 4WD wth a vacuum failure).  This action is a matter of preference with each Syncro owner.  Some active offroaders prefer that a vacuum failure leave them in 4WD and a better chance to get home if they are out rock crawling somewhere.  Others may prefer that, if their vacuum system fails, their Syncro be left in 2WD mode for a long freeway drive home.  This preference is likely dependent on how aggressive your viscous coupling has become, or if you have replaced it with a solid shaft.