Decoupler and Solid Shaft vs. Viscous Coupler, Rear "Air Locker"

All Syncro Vanagons were originally fitted with a device called a viscous coupler (VC) which allows them to be in full-time "kinda" 4WD. The purpose of the viscous coupler is to allow slippage between the front and rear axles when the vehicle is driven in high traction situations. For example, when doing tight maneuvering in a dry parking lot. Anyone who has driven a Vanagon Syncro has experienced some level of "binding" when performing such maneuvers. If there were no VC at all, just a solid shaft for example, the binding would be 100 times worse, and in fact after a few maneuvers something would probably break. Part-time 4WD vehicles, like a Toyota truck for example, are 100% fully locked in 4WD when selected, and 2WD otherwise. Vehicles like that are REAL 4WD vehicles. The Syncro Vanagon, like any all-wheel-drive Subaru or Audi Quattro, are considered "all wheel drive" vehicles because they all are full-time "kinda" 4WD, and have some sort of device that allows for slippage. Thus my use of the word "kinda". The problem with a VC system is that, when in an extreme off-road stuck situation like soft sand, if your rear wheels completely lose traction not all of the engine's torque is transmitted to the front wheels. Rather, only the amount of torque the VC is designed to transmit gets there. And in soft sand with the rear wheels completely buried, it ain't enough to get you out. Last time we were in Baja, only one of five pop-top Syncro Vanagons we took got stuck in the sand. All five had a decoupler and solid shaft instead of the VC, except for one. I will let you guess which one of the five got stuck in the sand...

Many Vanagons are equipped with an optional lockable rear differential, often referred to as a "rear locker", or "air locker", or "diff lock". You can tell a Syncro with this option because it has an extra console on the dash board, just under the radio and directly in front of the shift knob. On this console is a place for three knobs. Vanagons equipped with this option have only one of the three knobs, the right most.

On this console is a diagram of the Vanagon's drive system and wheels: Rear axle and wheels, drive shaft, front axle and wheels. There are places for three green indicators on this console, only one of which is actually hooked up. When the diff lock knob is pulled out and the green indicator illuminates, the two rear wheels are locked up together 100%. This device is commonly referred to as an "air locker" because the means by which the locking takes place is pneumatic. The knob is attached to a pneumatic switch that is actually plumbed via plastic tubing all the way back to the transaxle, on which there is a servo device that creates the force that moves the components within the rear differential that locks the two wheels together. The indicator is controlled by a completely different electrical system. That is why when the knob is pulled, the indicator does not always come on instantly. Rather, it comes on when the rear axle is actually locked up, tripping a switch on the transaxle, that in turn sends the electrical signal up to the green indicator so it lights up. This rear locker system has NOTHING to do with 4WD. In fact, we have added this system to 2WD Vanagons for added off-road traction.

Not all Syncro Vanagons were equipped with the pneumatic system and console, only those equipped with the optional rear locker. However, ALL Syncro Vanagons have the indicator electrical wiring system. And this electrical system contains not only a plug for the optional rear locker electric "ON/OFF" indicator switch, but two others as well. One is located at the front differential (for the addition of a front locker), and one is located at the forward end of the transaxle located at the rear end of the vehicle. So, what about these guys going around and saying, "If the engineers intended the Syncro to have a decoupler, they would have designed it that way"? Precisely correct. The existence of this electrical system on all Syncro Vanagons, and the space for two more pneumatic switches in vehicles equipped with rear locker is clear evidence that VW intended all along for the Syncro Vanagon to have a means by which the operator could lock or unlock not only the front and rear axles for improved traction, but also to provide a means by which the vehicle could be switched into and out of 4WD at will. The reason they did not follow through and outfit every Syncro with a decoupler, and front AND rear lockers was probably a decision made by the accountants, not the engineers. It is almost as if the engineers threw up their hands and said, "OK, fine, we can't afford to put triple knobs in every Vanagon, so let's just go ahead with this stupid VC idea instead". But just to spite the bean counters, they snuck the wiring harness in anyway! Nice job! Because, indeed, it would have been harder to add the electrical system than it is to add the pneumatic and mechanical systems...

In the case of the Syncro Vanagon, the VC is located at the front end of the drive shaft, inside the front differential assembly. It is a cylindrical device about 6 inches in diameter, and about six inches long. It is not practical to simply replace the VC with a solid shaft to achieve 100% 4WD. That would make for a very unwieldy beast indeed, with axles and/or transaxle breaking again and again. In fact, what happens with the VC is that, over time, it begins to seize up and act like a solid shaft. That usually results in your wallet getting a rather solid shaft.

What is really needed is a solid shaft in place of the VC, and a means by which 4WD can be turned on and off on demand. Some sort of decoupling device. Hmmmm. What to do, what to do... Well, it turns out VW originally planned for all Vanagons to have such a device.

As it turns out, the front-most housing that is present on all Syncro transaxles can be retrofitted with the components necessary for the drive shaft to be coupled/de-coupled remotely. With these modifications and a solid shaft substitute for the VC... now we're talking! GoWesty sells these two components, together with the extra pneumatic switch, knob, plumbing, and even the additional illumination bulb needed for the console. When installed, this system turns a Syncro Vanagon into a completely 2WD Vanagon when the middle knob is in, and into a REAL 100% 4WD vehicle when the knob is pulled and indicator comes on. 

What's even better you don’t have to give up any of the benefits of an AWD vehicle (stability and maybe fuel efficiency**). For one thing, you don’t have to remove the VC—you can leave it in place if all you want is to bypass it when making sharp turns. It’s just that, with a solid shaft in place, when you pull the knob—you get REAL 4WD when you need it. And, unlike many other 4WDs—like a Toyota truck, for example—the running gear in a Syncro Vanagon is always spinning in unison.  That means you can switch into and out of 4WD on the fly, at any speed.  You don't have to come to complete stop, slow down, or let off the throttle to engage 4WD.  Even on totally dry days I use mine. When driving my '87 Wolfsburg Syncro Westy weekender on the highway at 75 mph, and I hit a stiff cross wind, I just reach down and pull the middle knob, and viola! It settles the van right down.  is simply the best of both worlds. But, just like with the use of the factory rear locker (when so equipped), caution must be exercised when negotiating tight maneuvers on dry pavement to keep from breaking an axle, CV joint, or drive shaft U-joint since there is no longer a device (viscous coupler) that allows for slippage.

Basically, it comes down to common sense. I mean, why would anyone want to pull any of the knobs on dry pavement in a situation where you needed to perform tight maneuvers? But, then again, what constitutes a "tight maneuver"? The only thing you HAVE TO remember when driving a Syncro with solid shaft and decoupler in a high-traction (dry pavement) situation is:


Just stick to that golden rule, and you can't go wrong. 

GoWesty is so convinced that the decoupler is the only way to go that we will not warranty our rebuilt Syncro transaxles unless the customer has a decoupler, or agrees to add a decoupler at the time of install. We and many other shops learned this lesson the hard way: Warranty repairs over and over and over and over again on Syncro transaxles until, finally, the VC was replaced. And since a new VC costs about the same as our decoupler/solid shaft combo (which never wear out)... well, you get the picture—everyone goes with the solid shaft and decoupler. Furthermore, back when were restoring and selling Syncros, they all got a decoupler. Period.

Check out the YouTube video link to further your education on this topic. Please note that the video discusses ONLY vehicles WITH a viscous coupler, but with and without differential locks (no decoupler or solid shaft). Keep in mind that this is an "official VW sales" type of video, and it refers to NEW vehicles with the viscous coupler working in a "theoretically perfect" way. The comments about the viscous coupler—and all it is said to accomplish—are somewhere between "wishful thinking" and "nonsense." But, there is much in the video that is dead-on, in particular the discussion and illustrations regarding what happens when a vehicle negotiates a turn and the resulting relative wheel speeds: all good stuff. 

**It has been claimed by the original designers of the Syncro, Steyr-Daimler-Puch (SDP), that the Syncro is actually more fuel efficient in 4WD compared to 2WD, this is what they claimed (translated to English):

“As part of the development, the deactivation of the front-axle drive was also examined. Figure 17 shows the difference in the constant consumption curve of the VW Syncro with 205 R 14 tires and final drive ratio of 38: 7 as a four-wheel versus two-wheel drive vehicle. The advantage of permanent all-wheel drive is particularly evident at higher speeds. The main reason for this is the lower rolling resistance and lower slip loss of the four tires driven for propulsion instead of the usual power transmission through two tires. In practical customer use [...] a consumption reduction of 3% was measured with all-wheel drive via viscous coupling compared to a comparatively traveling VW Syncro with only rear-wheel drive."


This does somewhat defy logic—that loading twice as many wheels, axles, cv joints, ring and pinions could actually result in lower overall drag. That is, even if that is actually what is happening with a viscous coupler at high speed, in a straight line on dry pavement—which further defies logic-and physics. The VC is not a hard, direct connection, after all. It really only comes into play when the front and rear axle are spinning at different rates. How is that happening in this scenario?

Needless to say, we have never been able to verify this ourselves. It could very well be this data was cherry picked by the marketing guys over at VW from a much larger set of data the engineers actually had over at SDP? Regardless, nobody can possibly argue that being able to disconnect the driveshaft for tight maneuvers, VC or no VC, can possibly be a bad thing!