Viscous Coupler info.

Starting a discussion for info specifically about how VC's work, what ours is like, and possibilities of repar/replacement/upgrades.

MJH part: 40300-SH9-000 Drw#005 Qty:001 SHAFT ASSY., PROPELLER (NO.2) May Have Been Discontinued 948.33
This is all straight from the 89 civic service manual.

Viscous Coupling
The viscous coupling unit is located on the front end of the No.2 propeller shaft. It consists of a housing that is connected to the No.1 propeller shaft by a tripod joint. Inside the houning are 79 plates, which have 0.2 mm (0.008 in.) of clearance between one another, surrounded by silicon oil. The 40 housing plates are engaged with the splines in the housing and the 39 hub plates are splined to the shaft. The plates have holes in them to aid in heat dissipation. The viscous coupling unit also contains approximately 10 % air to allow for the thermal expansion of the silicon oil.

Whenever there is any difference in the speed of rotation between the No.1 and No.2 propeller shafts, such as when the front wheels lose traction, there is friction between the housing plates (drive side) and the hub plates (driven side) .This friction is caused by the resistance of the plates sliding against the silicon oil. This resistance between the plates and the silicon oil is what begins to transmit torque from the housing plates to the hub plates and eventually to the rear wheels. This transmission of torque is proportionate to the difference in the speed of rotation of the wheels.

As the difference in propeller shaft speed continues, the temperature of the silicon oil keeps rising. Due to thermal expansion, the pressure inside the viscous coupling unit is also increasing as the temperature rises.
When the pressure becomes high enough, the housing plates begin to contact the hub plates and the engine torque to the rear wheels increases rapidly, as noted in the diagram below. To reduce plate wear, there are spacer rings to limit plate to plate contact. (VW site)
When the front and rear wheels turn at different RPMs (at a rate greater than 6%), the shear force raises the temperature and viscosity of the silicon liquid inside the VC. The silicon becomes solid like and engages plates inside the VC with the result that power is transmitted from the center drive shaft (the engine) to the front differential (the front wheels).When the front and rear wheels begin to rotate at relatively the same RPMs again, the liquid deactivates," becomes less viscous and more liquid like, and as a result the front wheels/diff. disengage, and the van is powered again only or primarily by rear wheel drive.

How stuff works
Under normal conditions, both sets of plates and the viscous fluid spin at the same speed. When one set of wheels tries to spin faster, perhaps because it is slipping, the set of plates corresponding to those wheels spins faster than the other. The viscous fluid, stuck between the plates, tries to catch up with the faster disks, dragging the slower disks along. This transfers more torque to the slower moving wheels -- the wheels that are not slipping.

When a car is turning, the difference in speed between the wheels is not as large as when one wheel is slipping. The faster the plates are spinning relative to each other, the more torque the viscous coupling transfers. The coupling does not interfere with turns because the amount of torque transferred during a turn is so small. However, this also highlights a disadvantage of the viscous coupling: No torque transfer will occur until a wheel actually starts slipping.

A simple experiment with an egg will help explain the behavior of the viscous coupling. If you set an egg on the kitchen table, the shell and the yolk are both stationary. If you suddenly spin the egg, the shell will be moving at a faster speed than the yolk for a second, but the yolk will quickly catch up. To prove that the yolk is spinning, once you have the egg spinning quickly stop it and then let go -- the egg will start to spin again (unless it is hard boiled). In this experiment, we used the friction between the shell and the yolk to apply force to the yolk, speeding it up. When we stopped the shell, that friction -- between the still-moving yolk and the shell -- applied force to the shell, causing it to speed up. In a viscous coupling, the force is applied between the fluid and the sets of plates in the same way as between the yolk and the shell.
Note that Viscous-Coupling LSD is a speed-sensing device: under no-slip condition, no torque will be sent to another axle. Whenever slip occurs, theoretically up to 100% torque can be sent to any axle, depending on the traction difference between front and rear axle. Therefore it is a part-time 4WD.

Being a part-time 4WD, it does not have the neutral steering of a permanent 4WD can obtain. For cars based on rear-wheel drive models, such as Porsche 911 Carrera 4, this is not a real problem - as normally the car runs like a RWD car thus is capable to deliver the desirable throttle oversteer . However, for other front-wheel drive-based cars like VW Golf Syncro and Volvo 850 AWD, the part-time 4WD can do nothing to correct their understeering manner. This is the first disadvantage.

The next problem is the delay before the 4WD get into effective. Since viscous liquid is not a fixed medium (unlike gear), it takes time and speed difference to be effective. The function between speed difference and torque transfer is an exponential function - that means in the early stage of slip, torque transfer remains near zero.

To cure this problem, most manufacturer varies the final drive ratio such that introduce a slightly speed difference even in normal condition. As a result, the car actually runs with 95:5 torque split between front and rear. This shorten the delay time. However, it is still impossible to match the pure mechanical Torsen LSD.

It might be less effective than Torsen system, but it is certainly the cheapest, so we can find it in many mass production 4WD cars.


  • superhatchsuperhatch Moderator
    Jaker wrote:
    Based on this photo, which is an actual count of the teeth on the gears in my transmission, and the rear diff ratio of 2.529:1 which I've also counted, we can conclude the following:


    1 revolution of the front wheels, which translates to 1 revolution of the 53 tooth gear leads to 2.3043 revolutions of the 23t gear, which leads to 2.547 revolutions of the 19t gear (2.3043 x 21 / 19). That 19t gear is attached to the driveshaft, so for every revolution of the front wheels, the driveshaft turns 2.547 times. With the rear diff being 2.529, that means the rear wheels turn 99.3% the speed of the front wheels based solely on the drivetrain. Since the rear wheels are going to be turning the same speed as the fronts (unless there's some slip), we can conclude that there is no viscous coupler pre-load in the Honda Wagon system.
  • Is this part known as the number 2 propeller shaft?

    I was looking on Majestic Honda for one of these the other day for fun, looks like it may be discontinued. The service manual tells you not to service it, just to leave it alone.
  • superhatchsuperhatch Moderator
    The_Head wrote:
    The service manual tells you not to service it, just to leave it alone.

    I'm assuming you're talking about the VC? If so, could we get a scan, or pic of it?
  • JakerJaker familEE
    On the left side of the box numbered #5: *** SEE ABOVE FOR PHOTO ***
  • superhatchsuperhatch Moderator
    ^ I updated and you posted at almost the same time. :)
  • Yeah sure, I can get you a pic in a day or so, unless someone beats me to the punch.
  • Sorry, my camera would not take clear pics of the FSM. Hope this helps..
    Viscous Coupling Stall Test

    1. Raise the car off the ground and place safety stands under the side sill reinforcement sections.
    2. Start the engine.
    3. Keep the engine speed at idle.
    4. Shift into low gear and gradually release the clutch
    5. Apply the parking brake firmly.

    Viscous coupling is OK if the engine stalls
    Viscous coupling is faulty if the engine continues running
    Viscous Coupling Precaution

    The Viscous coupling unit contains no serviceable or replaceable parts. If it is found to be faulty (fails the stall test or shows signs of leakage), it must be replaced as a unit.

    Do not remove the TORX bolts from the viscous coupling unit.
  • tshiretshire Band Wagon
    I'm not sure I agree with your conclusion. If, as you say, the rear wheels turn 99.3% the speed of the front wheels based on the drivetrain's gearing, then it seems that there is a slight preload of just under 1%.

    It seems to me that only a perfectly matched front/rear wheel speed would result in zero preload. Is it your contention that less than 1% preload is, for all intents and purposes, essentially a 'zero preload?'
  • JakerJaker familEE
    The drive to the rear wheels is slower than the front (99.3%, so 0.7% slower). This would not qualify as a "pre-load" in my opinion, unless the fact that the rear half of the VC is moving slower than the front would lead to pre-load. My expectation would be that the gears would lead to the rear half of the VC being driven slightly faster than the front half to truly pre-load the internals.
  • Great info Head...thats gonna help out a lot.

    As for pre-load. My understanding is that the fronts need to be spinning faster than the rears in order for the VC to engage. If that's the condition to induce the VC wouldn't the rear wheels spinning slower be a pre-load? The VC locks when there is a difference in speed of the plates. In common practice this is because the fronts are spinning. If we slow down the rear plates by 0.7% wouldn't the fronts already be turning 0.7% faster? Ex. The threshold of lock is 5.0% difference in speed. If the rear plates are running at -0.7% the front only needs to achieve 4.3% change to create a 5% difference.

  • JakerJaker familEE
    I could be wrong, but I think you've got it backwards. The drive to the back wheels needs to be seeing some torque load for the plates to lock. The only way that'll happen is if the front "tires" turn 5% faster than the rear "tires", so in fact with the underdrive of the rear end, the fronts would need to turn 5.7% faster than the rears before the plates would lock.
  • tshiretshire Band Wagon
    I think Superhatch has it right concerning the .7% lower speed of the rear wheels resulting in a certain amount of pre-heating of the viscous fluid, and hence a slight pre-load of the coupler.

    Of course, I am partly operating on the assumption that Jaker's math is correct. And it may be possible that a .7% speed differential is insignificant in this particular circumstance. It would be interesting to hear from a Honda engineer on the topic.

    I suspect that disengaging the rear drive shaft via the disconnect lever on the transmission would result in a slight increase in fuel economy; though it might not be enough to notice from one fill-up to the next unless the driver were superhumanly consistent in both driving routes and driving style from one tank fill to the next.
  • I know this is a little off topic and im sorry but my 4wd isnt working on my 87 i just bought i tested it and the vc is bad. you and see the front of the shaft spinning and where it comes out of the vc its not. so i need to know where i can find a vc i called honda cant get it, and all the parts stores dont even show a 4wd civic. winter is coming and i need to get it fixed thats the whole reason i bought the car so if you guys could help it would be greatly appriciated thanks.
  • This info pertains to 88-91 RT4WD.

    I thought the 84-87 had push button 4WD and it wasn't automatic?
  • My 2 cents. Because the coupler relies on friction between the plates and the fluid, it is not significant which plates are rotating faster. For example, if the 4wd is disengaged, the drive plates are stationary (not being driven by the trans) and the driven plates are spinning (being driven by the rear wheels). This condition will cause the VC to engage, causing rotation of the drive end components all the way up to the point where it is disengaged at the trans. Engaged or disengaged, all of the rear drive components will rotate, even those in front of the VC. This indicates (to me) that there should be no increase in mpg by disengaging the 4wd as all components are still rotating. You will note that I said should, I realize that some people have reported increases by disengaging the 4wd. There may be other factors at play that have yet to be revealed.

    Also, remember that torque transmission is variable based on difference in rotational speeds, torque is transmitted even below the 5% threshold, just not full torque. If a 5% difference in rotation causes full lock, then a .7% difference is more than 10% of the amount required for full lock. I would think that would be sufficient to generate a preload.

    Of course, I've been wrong before.
  • 87 was the first year for the rt 4wd
  • So what I'm getting is that if we remove the viscous coupler and install a standard driveline we will have a failure in the drivetrain (most likely the rear end) because the front and rear wheels are geared differently and with a standard driveline the engine would try to drive the front and rear wheels at different speeds...correct?
  • I just "engaged" the AWD via the two 10mm bolts on the transmission and noticed it took quite a bit of force to turn and then hold the sliding mechinism into the AWD position, it seemed to want to return to the 2 WD position. Has anyone else noticed this action.? Was also wondering if any brave soul had attempted to dismantle the viscous coupling (I know it says "no user parts, etc") but was curious as to the "why". Does it contain stored energy via spring loading or some other method.? Or did they build it intentionally to discourage any repair.? Thanks for your time....
  • I would be interested in laying my hands on a failed VC in order to dissect it and see if there is a possiblity for repair. It might also be possible to adapt a VC from another vehicle. Perhaps a VW Synchro or some other. I just found this link: ... x.php?id=9

    They claim to "recharge" VCs. Pictures of numerous models.
  • Good point, thanks for the "link". I know "Merry-can" vehicles will use the same part manufactuers, like New Process for differentials, etc. So it would not surprise me to find that Honda "couplers" were very similar to another japanese vehicle....Of course there is probably enough difference to ruin the chance of interchangability. Wonder if a junk yard would have a "cross-reference" guide for these.?
    Thanks for the response and ideas...
    Phil in Alaska
  • When you're engaging the drive to the rear wheels, be aware that you are essentially "shifting a gear". It is in fact what is happening inside the transmission when you move that lever on the outside. As far as what's happening inside the box, it's no different than moving the shifter inside the car to change from neutral to a particular gear. Now think of it in this light at the same time as truing to select reverse. I'm sure you've noticed that sometimes when you try for reverse, it simply won't go into gear and you need to let the clutch ever so slightly to let it drop into reverse. This is because there are no synchros on the reverse gear, and when it won't go in, the teeth from each gear you're trying to mesh together are hitting head on rather than one tooth into a groove on the other gear.

    To make a long story short, when you try and engage 4WD and it won't go in, this is basically what's happening All you need to do is rock the car slightly back and forth, and it should drop right in.
  • Thanks for the fast response, I thought that might be the case and did try to "rock" the wagon back and forth but must not have "rocked" enough.... It seemed to engage, I am going to try the suggested "raise the car completely off the ground and at idle apply emergency brake and see if the engine stalls", trick.
    Take care, Phil in Alaska
  • I emailed that website to see if they had any options for our wagons VC.
  • The_Head wrote:
    I emailed that website to see if they had any options for our wagons VC.

    Interested in their reply.
  • The_Head wrote:
    I emailed that website to see if they had any options for our wagons VC.

    Glad you picked up the ball. I'd been thinking about contacting them, but I don't need one (yet), so motivation was lacking. :wink:
  • I just filled in the contact form on their website. They may not reply. Some people don't check that stuff. I will let yall know if they say anything.
  • Thanks for the info, there must be quite a few people looking, and also running around with just front wheel drive. I have to try and track down my vibration problem, I don't know what, if anything a driveline shop can do as far as checking the individual components of our drive trains in terms of balance, play,etc. It is kind of amazing the little wagons worked as well as they do, don't think they could have stuffed another "whirlygig" under there if they tried....
  • Last week I tested the VC on my 91RT and found it to be bad. :( I emailed STE ... x.php?id=9

    last night and asked about rebuilding. To my surprise, I received a reply this morning.

    "Good Morning Craig: By my info. your Honda has a transfer case without a viscous coupler. I base this on the fact we have never seen one or been asked for one, and I have no cores for it so if you do have one I would need yours to even see if this is something we could reman. Other than that this is a dealer part. Sorry I don`t have better news for you but if I do find info on this one I will e-mail you
    Dave H. STE Couplers "

    I am in the market for a non-functional VC to send to them. Anybody?
  • They never replied to me. I'm glad you got something. It seems like bad news though.
  • The big question here should be this:

    Are there spider/differential gears "between" the FWD portion of the transmission and the transfer-case portion? If there are, then creating a one-piece driveshaft or somehow locking the VC together will not cause a failure when driving on pavement. The spider gears will allow the rear wheels to turn at different speeds than the front. This is how most normal 4x4s work that have "full-time 4wd". It will likely decrease MPG, however.
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