The Viscous Coupler Saga

What started as a simple two-hour task of changing a torn CV boot, quickly ballooned into a month-long ordeal. Before I knew it I had all four half-axles out and rebuilt all 8 CV joints. This, naturally, led to a test of Chimera’s viscous coupler (VC).

Viscous couplers are the devices that transfer power from the transmission, in the rear of the van, to the front wheels while allowing some slip between the front and rear wheels. This slip is what allows the van to go around corners on dry pavement. When the wheels begin to spin out, such as on ice or sand, the VC engages, coupling the front and rear wheels, thus making the van all-wheel-drive.

When I tested the VC, I found that what I thought all of this time to be an all-wheel-drive vehicle, was actually just two-wheel drive. The VC was shot, it was transferring zero power to the front wheels. I weighed my options, and decided that I would attempt to rebuild the VC. Syncros are somewhat rare and the youngest ones are still over twenty years old, so there is no such thing as a new VC, and the trick to rebuilding them seems to be some sort of coveted secret.

Viscous couplers are interesting mechanical devices. They consist of an inner shaft, connected to the front wheels, an outer shell, connected to the transmission, and a bunch of disks immersed in a viscous fluid about the consistency of honey. Half of the disks are attached to the inner shaft, and half to the outer shell. When the rear wheels slip, the disks spin in the fluid, transferring power to the front via the disks attached to the inner shaft. This isn’t a particularly complicated device, but it’s rather sensitive to the integrity of the disks, the shaft that they’re mounted on, and the amount and quality of the fluid.

Cutaway

Having no idea if it was possible to rebuild the VC, I decided to open it up to have a look.

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I discovered some very dark fluid.

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I drained out the nasty dark fluid.

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Then, out came the circlip on the inner shaft that holds all of the discs in place.

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The discs alternate between the ones attached to the inner shaft and those attached to the outer shell.  Some have slots cut in them and others have holes.

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I removed all of the disks, cleaning them and stacking them in reverse order and orientation so that they would be easy to put back in.  Once I got all of the discs out, I found the culprit for the amount of sludge in the VC… A twisted seal ring. This guy allowed silicone fluid to leak out and nasty dirt from inside the diff to get in.

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I was able to get a rebuild kit from a member of the online forum www.thesamba.com. It included new seals and new fluid to replace the old.

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I carefully weighed the fluid as I refilled the VC so that the correct amount went back in.

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Once it was refilled, I put all of the discs back in and sealed it back up.

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With no way to bench test it, I put the VC back in Chimera and tested it as I had earlier.  It was a pleasant surprise when I successfully drove around the block.  I still had the rear half-axles out, which meant that I was driving with front wheel drive only!  With all of the snow and cold weather lately, there has been ample opportunity to further test the VC under real 4wd conditions, and it has been performing great.

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