VCU
The VCU automatically controls the transfer of drive to the rear wheels by limiting the speed differential between the
front and rear propeller shafts. The unit is supported in two propeller shaft bearings attached to the floor cross
member.
The VCU comprises a short cylinder which contains an input shaft supported in a roller bearing race at the front and
a ball bearing race at the rear. Within the cylinder, slotted discs are alternately attached to the outer surface of the
input shaft and the inner surface of the cylinder. An output shaft is welded onto the rear of the cylinder. The input shaft is attached to the front propeller shaft and the output shaft is attached to the rear propeller shaft.
On models with 1.8 K Series engines, a torsional damper is bolted to a flange on the output shaft.
The cylinder is a sealed unit filled with a silicon jelly. The viscosity of the silicon jelly increases when subjected to
shear. When there is a speed differential between the front and rear propeller shafts, adjacent slotted discs in the VCU rotate in relation to each other. The shearing action of the rotating slotted discs increases the viscosity and resistance to rotation of the silicon jelly.
Section Through VCU
for pic - see attached
1 Cylinder
2 Slotted discs
3 Input shaft
4 Roller bearing
5 Ball bearing
6 Torsional damper flange (K1.8 models only)
7 Output shaft
The rear wheels are 0.8% under driven, so in most conditions the vehicle is effectively front wheel drive, with the rear
wheels turning the rear propeller shaft slightly faster than the IRD drives the front propeller shaft. Since the speed
differential is low, the increase in viscosity of the silicon jelly is marginal and there is little resistance to relative rotation
of the slotted discs.
When there is a significant speed differential between the front and rear propeller shafts , e.g. the front wheels lose
traction or traversing rough terrain, the viscosity and resistance to rotation of the silicon jelly increases to a level that
slows or stops relative rotation of the slotted discs. With the front and rear propeller shafts locked together, drive is
thus transferred from the IRD to the rear wheels.
VCU bearing
If you've got a whineing noise from your drive-train that you can't easily identify then it could be the VCU support bearings. You can remove the whole prop assembly and still use the vehicle as front wheel drive only. To do this you need to jack up one front and one rear wheel on the same side and remove all of the bolts at the ends of the prop-shaft. These have hex headed bolts for which you need something like an E14 socket, the jacked wheels will allow you to turn the shafts by hand and get access to all of the bolts. Support the ends of the shaft. Now remove the 2 bolts in each of the support bearing housings whilst supporing the VCU - it's *very* heavy ! drag it all out carefully from under the truck and take to the bench to work on.
<A href="http://www.thesysadmin.co.uk/hippo/fixes/front%20prop%20spline.jpg">
The prop-shafts are on splines onto the VCU, with a single bolt into the end to hold it on. If you undo this bolt a little then you should be able to use a wedge between the UJ and the bolt in order to force the joint apart. One of mine came off with a good hit on the wedge, but the other I had to actually dismantle the UJ and get it in a hydraulic press.
When this is all apart you can press the bearing out of the race and read the part number off the side. I bought new ones from rswww.com for about nine pounds each. Fitting them was relatively easy, and putting it all back together and lifting back onto the car similarly so.
