Installation Guide for vans with difflock

Note: Read entire guide before starting installation.

Use common sense! Don't do something if it doesn't seem safe or doesn't make sense to you. If in doubt, stop and seek advice from a qualified mechanic or call for advice.

Do not disassemble decoupler

The decoupler has been carefully assembled and calibrated, don't screw it up!

Required tools and oil

A 17mm combination box and open end wrench.

Two 13mm-combination open and box end wrenches.

Ratchet with 13& 17mm sockets, short and medium extensions.

Phillips screwdriver.


Drill or Dremel tool.

Soft hammer.

Two large jack stands (six ton works well) and wheel chocks.

Floor jack.

Work light.

Transmission oil, see draining (complete refill is 5 qt)

Fluid pump or squeeze bottle with spout and short hose.

Oil drain pan.

Transmission drain and fill plug tool (17mm male allen socket).

Solvent, contact cleaner, rags.

Bently service manual. Not required but helpful.

Clean transmission

Now is the time to check under the van to see how dirty, oily or muddy things are. A do it yourself carwash or powerwasher can really help here!

Positioning the van for the installation

With van on a level surface, place wheel chocks under rear tires and firmly set emergency brake.

Undo the two bolts from the rear of the engine skid frame.

With jack under the front cross member (just aft of the spare tire pan) jack up the front of the van as high as your jack stands will reach.

A good place to set the jack stands is on the round subframe cross member just aft of the front wheels.

Think safety! Make sure van is well positioned and stable.

Pull skid bars and shift rod

Undo the skid bar bolts (17mm) and drop the engine skid frame and the long skid bars. Pull the skid assembly out to the rear.

With the transmission in neutral, (this is a safety measure), undo the two 13mm shift rod bracket bolts on the transmission and tie the shift rod off to the passenger side.

Drain oil, Stop!

Loosen the filler plug before you drain the oil (it helps to be able to refill)!

You can either drain the trans. oil completely, or just be prepared to lose about a quart of oil (depending on how high the nose of the van is raised) when you pull the output housing off the trans. Have a pan ready as you will have some oil leak out no matter what you do.

Lower, don't remove, front transmission mounts,

Using a 17mm wrench on top and a 17mm socket on bottom, feel for the bolt shaft protruding above the nut and unscrew bolt until bolt is level with the top edge of the nut.

Detach drive shaft from transmission

Remove the four bolts and nuts securing the drive shaft to the output flange of the trans. (13mm open end wrenches).

According to the Bentley manual you have to loosen the front differential mounting bolts. This is not required when you lower the transmission first.

Tie up the drive shaft towards the passenger side of the van with wire

Clean area around output housing

Use solvent or spray cleaner to remove any dirt or grease from joint area.

Remove existing output housing

Using a 13mm socket and ratchet undo the seven bolts from the output housing including the ground strap bolt. Do not remove the ground strap from the body of the van

Tap the housing and the flange with a soft hammer to loosen from trans. Position drain pan to catch oil. Remove housing and coupling sleeve by pulling towards front Send both the housing and the coupling sleeve back for the core deposit

Prepare Decoupler for installation

Important: Check existing housing for two locating sleeves, one on each side (sometimes they are stuck on the trans.) and transfer to decoupler housing or leave in trans. Making sure that there are two of them and that they are on either side.

Apply sealer to mating flange surface. Note: more is not better, a thin continuous film is best. Thin, as in paper-thin. Apply only to the perimeter of the housing; don't cover the entire wide area as this is in contact with the large bearing. Look at the trans. to see.

Clean up exposed flange of trans. with solvent or contact cleaner.

Verify that there is grease or oil in the recess of the decoupler shaft

Install Decoupler

Install in reverse order of removal.

Don't be too concerned about oil leaking across sealing face of trans. Mine have always been wet with oil and the sealer has sealed every time.

Verify that the new housing is lining up and seating on the locating sleeves before tightening the bolts.

Torque the seven bolts (remember to reinstall the ground wire) to 15 ft lb. as per Bentley 35.103.

Test decoupling action by hooking a vacuum line to the black nipple on the vacuum solenoid and sucking strongly. Now, by hand, turn the red driveflange. It should turn freely.

Now hook vacuum line to the blue nipple and suck again while turning the driveflange. The decoupler should engage and the red flange should not turn any more.

Reinstall driveshaft

Torque the four bolts to 26 ft lbs. as per Bentley. Use blue threadlock on these.

Tighten transmission mount bolts

No torque given in Bently, just good and tight.

Refill transmission

Measure amount of oil drained plus what leaked out and add same amount of fresh oil to trans. now. Complete refill is 5 qt. It's easier to fill with the skid bar off.

Hook up switch

The pigtail with the plug for the indicator light switch can be found laid into the large harness bundle along the frame behind the charcoal canister in the driver's side wheel well. Remove the hose clamp holding the canister to better view the area. Pry out the large red tie wrap holding the harness bundle at the very front of the wheel well. Find the plug to the fuel tank sender (plugged into the side of the fuel tank behind the spring tower) and follow the wire forward about a foot. Separate the individual harnesses until you find the one with two wires coming out of the 18 inch sheathe: one brown and one brown with a black trace. This one has a rubber hood for the plug on the end. You will have to clean it up and probably dig out the dirt with a wire and compressed air or water. Using a stiff wire and taping the harness plug to it, thread the wire through the frame opening following the path taken by the fuel line going from the fuel pump to the fuel filter. Plug the harness into the electrical switch on the decoupler. Slide the rubber hood over the contacts.

Install vacuum switch and lines

Lower spare tire pan and remove spare tire

Inside van, on center console, remove the knob on the existing difflock switch by holding the shaft with smooth pliers and unscrewing the knob

Remove the four screws securing the syncro difflock panel onto the center console

Now undo the gold metal bracket and pull out a ways to give some working room

Remove lower plastic shroud by pulling straight back; check clearance on top edge

Warm the plastic indicator panel and drill a hole for the new switch shaft in the center position. Be careful - this is a brittle piece of plastic!

Install light bulb into center position of PC board with a quarter twist

Thread two vacuum lines along harness bundle on driver's side towards the front of the van. Identify one line with a marker or tape. Follow the same path as the lines to the difflock and difflock switch. Zip tie lines to bundle where needed. Under the front center tunnel, tape the two vacuum lines to a straightened coat hanger and feed up beside the existing vacuum harness. A little soapy lube helps here.

On the existing difflock vacuum switch locate the middle vacuum line, cut a 2-inch piece out and install the triangular tee. Cut as close to the switch as practical to leave enough length on the harness end to hook up the tee or just plug tee directly into switch.

Install the new vacuum switch onto the gold bracket in the center position with the provided nut.

Connect the two lines to the outer nipples on the switch using rubber elbows. Identified line to front nipple.

Connect the two lines to the vacuum solenoid on the decoupler. Identified line to the blue nipple

Connect a short line to the center nipple on new switch and suck while activating the switch. With the ignition on, the center position light should come on when the decoupler is engaged with the switch shaft pulled out. If not, have someone turn the driveshaft back and forth to facilitate engagement. If the light doesn't come on, reverse the two outer lines on the switch and try again. The light should be out when the driveshaft turns freely and should come on when the shaft won't turn.

When proper operation is verified, connect a short line from the new tee on existing switch to the center nipple on the new switch, or if space permits, just connect the tee to the center nipple

Replace bracket and panel being careful to not kink vacuum lines.

Replace spare tire and secure pan making sure vacuum lines are not pinched.

Replace shift rod and skid bar assembly

Reverse of removal, lube shift ball on trans. with grease. CAUTION, don’t break off decoupler vacuum solenoid hose nipple when installing the upper middle bolt and nut The blue nipple is in direct line with the bolt and nut and is very fragile.

Remove jack stands

Lower van with floor jack. Check and fill oil level in transmission, level should be at bottom of filler opening. Remove wheel chocks.

Test drive

With decoupler switch in decouple mode (knob pushed in all the way) listen for any unusual noise while driving in a straight line. Now make some slow sharp turns listening again for unusual sounds. With wheels straight ahead, stop and pull out on decoupler switch and gently start to engage clutch. The indicator light should light and the van is in syncro mode (all wheel drive). Next, while driving in a straight line, push the decoupler switch in and lightly let up on the gas two times. This removes torque from the coupler and lets it shift. The indicator light should go out and the van is now in rear wheel drive mode. If any unusual noise or operation is observed, call or email for tech support.


Decoupler won't shift or shifts slowly: This condition is usually caused by lack of sufficient vacuum. The system has a check valve with a very fine filter in it. The valve is located by the vacuum reservoir in the middle under the van on the driver's side. Over years, the filter can clog up causing slow engagement of both the difflock and the decoupler. Remove and try to suck trough the valve. It should allow air to flow in one direction. If there is a restriction try bypassing the valve. If the decoupler now operates properly replace the valve. Another reason for low vacuum is a split rubber connector in the system, usually on the transmission vacuum solenoid. Check these for softness, splits or rot. Look for kinks or pinched lines. Test by first blowing through the line with the other end open, it should flow freely. Close the end and suck on it to test for sealing. If these are OK, check the switch on the dash, these often leak air, reducing vacuum. Test by blowing through the middle nipple with the front nipple blocked and the shaft pulled out. No air should leak out past the silver shaft. Replace switch if it leaks. Check the difflock vacuum actuator. Test by sucking on each side with a tube to test for vacuum tightness. Replace if it leaks. The tee on the large vacuum line from where the difflock system gets it's vacuum has a restriction in it that has been known to clog up if the line ever gets disconnected and sucks dust. Poke a fine wire through the tee with the engine running to clear it out.


Do Not engage or disengage the decoupler while making a turn or while spinning a wheel. Doing so may damage the pinion shaft and is not covered under warranty.

Only engage or disengage the decoupler while stopped or while all wheels are turning at the same speed, i.e. traveling in a straight line. To engage or disengage at speed, lightly let up on the gas pedal two times after activating the switch to ensure full coupling.

Do Not tow the van on a wheel dolly where one set of wheels is immobilized and the other set is turning. The decoupler was not designed for this and damage may occur to the transmission. If you need to tow your van this way remove the driveshaft. It is ok to flat tow the van with all wheels turning on the pavement and the decoupler disengaged.


When the decoupler is disengaged the van drives and behaves essentially as a rear wheel drive van, so drive accordingly. In a safe area, test braking, turning and acceleration both in 2wd and syncro (4wd) mode. Become familiar with the different dynamics of the van in the two different modes. If you have a grassy area to test on, this will accentuate the differences in the two modes. Keep safety in mind and learn how your van handles,

When conditions warrant, operate your van in syncro mode (4wd).


You have successfully installed the Syncro Decoupler.


CNC precision milled parts

Precision ball bearing construction

Corrosion resistant stainless steel actuating shaft for smooth dependable operation

Hard coating on critical parts for corrosion resistance and long life

Spring loaded indexing of shaft to hold unit in chosen mode

All new bearings, seals, and control parts

Proven VW controls integrate perfectly with the syncro panel

Factory look with all working parts internal in transaxle

Technical assistance:

Tom Lengyel