Viscous Coupling Failure


If you are wondering about whether you need a new viscous coupling, here is how to tell:

    1. According to the manufacturer of the viscous couplings, the VC is only built to last 100,000kms (about 65,000 miles). It would be silly to replace it until it shows actual signs of failure, however.

    2. The actual sign of failure is usually that the VC gets too aggressive and starts coming on too much and too hard. In this case it is an emergency because then your transmission components are fighting each other and you can destroy them quickly. You can still drive around on a too-aggressive VC without destroying your drivetrain if you remove the center driveshaft from the vehicle–but then, of course, you will not have four wheel drive. If you decide to do this, you can drive around in two wheel drive for years without harming your vehicle and without replacing your VC. The symptom that would cause you to know that your VC is too aggressive is usually that when you enter a paved parking lot and you make tight turns after a period of highway driving that the vehicle seems to want to stop as though you had put the brakes on–the vehicle simply hates tight turns. Sometimes there is a binding and bucking sensation while driving with the steering wheel turned all the way to its limits. Some on the Syncro list, notably Rainer, think there should be *no* scrubbing of tires whatsoever in tight turns, but I feel that a slight scrubbing or resistance to tight turning is normal and will not hurt your car. When the scrubbing is enough, after a period of highway driving, to actually cause your van to want to stop rather than simply slow down then it is time to change the VC, particularly if you have very high miles. (In support of Rainer’s position, by the way, Mick Kalber of Hawaii reports that after changing his VC the scrubbing he felt went away completely. Others report the same thing. Others report that after changing the VC, their car still fights tight turns, but the effect is not increased after highway driving, and the effect is extremely mild in effect.)

    3. Less often, the VC fails by simply not coming on at all and you do not have traction to the front wheels.

    4. The official factory test is well documented in the archives, but basically, the test is to put a two by four in front of the front wheels (the factory leaves out the two by four but in testing list members have found that even the factory VCs are stiff enough to require the 2×4 to hold the vehicle back) and then jack the rear of the vehicle up in the air with a rolling jack under the car and then very slowly and carefully let the clutch out in first or low gear. After a few tries you will find that you can let the clutch out with the engine running and the VC will allow enough slop to let the wheels stay still at one end of the vehicle without climbing over the two by four while the engine turns the wheels at the other end of the vehicle. If you cannot pass this test, then you should consider buying a viscous coupling immediately in order to avoid rapidly destroying your transmission. Be sure and perform this test soon after highway driving for 20 miles or so or it may not uncover a problem. Note that if you can pass test #4 ONCE out of several tries, then you have passed test #4. That is, you can fail test #4 four times in a row but if you can get the vehicle to pass once then you are OK.

    5. Experience has shown that a failure on *either* test #4 or test #2 is sufficient cause to change your VC. That is, it is possible for you to have some nasty scrubbing in tight turns after highway driving as described above in test #2 but that your vehicle can still survive a challenge of test #4. Nevertheless, in this case, you should probably change your VC anyway. Sam Walters’ vehicle was a case in point on this issue. He passed point #4 above but failed point #2 above. After changing his VC, point #2 above was restored to proper operation and he is glad he did the change.

Note that installing larger than stock motors will typically reveal a marginal VC, that seemed OK before, to now become a very nasty VC, which scrubs too early and is too aggressive. So it is not uncommon after changing the motor to one of larger size to have to also replace the VC. The same goes for weight. If you have a VC that appears marginal try loading 2,000 lbs. of magazines or newspapers into your van, driving for half an hour on the highway, and then replicating the tests above. You will then certainly find out if that VC is on the way out in that case. If it can pass all the tests in this email with 2,000 lbs of old magazines in it, then your VC is probably still OK.

My VC scrubs slightly – is it OK?

Most VCs *do* fight tight turns very slightly whether they are new or not. You may see emphatic posts from people who have gotten brand new VCs from me that say the VCs do not bind at all, but you will see just as many that say there is still a slight binding after installing a new one they got from me. To some extent, I think the issue is one of nomenclature and degree– that one person interprets the words ‘slight binding’ differently from another person or one person is more sensitive to the effect than another person. Case in point: I drove Tom Forhan’s vehicle after he had installed a brand new OEM SDP VC that he had gotten from me and I *did* detect a slight scrubbing of tires in a tight turn on pavement (as I expected), but as Tom said, the scrubbing was less than it was before. Thus, it seems that it is a matter of degree, and we have to rely on a subjective feel for how much binding is ‘normal’ and how much is too much. I am very sensitive, so I suspect that what I call ‘slight binding’ may to another list member not appear to be binding at all.

One distinguishing indicator of ill health is when the VC becomes more aggressive after highway driving. That is generally a very bad sign, and one has to consider replacing any VC that exhibits that symptom. After eliminating a difference in size between the four tires as the probable cause, if you experience this symptom only ONCE then it is cause for some concern and you should begin thinking about a VC change.

Generally speaking one can calibrate your feeling about your VC by the feelings of the driver when in tight turns on pavement after highway driving. If the driver says, ‘Hummm….it seems like the vehicle does not really want to go when I turn the wheel all the way like this,’ or some such mild comment then possibly the VC is OK. But if the driver says, ‘Hey, from the sounds I am hearing I really don’t think this is good for my car’ and speaks with some element of alarm in his voice, then the VC is more suspect. I know this is subjective but I am afraid that is where we are on this particular test.

The ‘No-Engagement’ Test:

Here is how to test whether your VC has failed in the more rare no-engagement mode:

Basically, you jack up the rear wheels so they are off the ground. Then you put the car in, say, low gear and let the clutch out. After a couple of times, you will find with a good VC that you can let the clutch out and the front wheels will still stay on the ground still without the vehicle being propelled forward even with the motor idling. This is with the rear wheels off the ground and the front two wheels on the ground.

That is the test that is the standard VC test.

The no engagement test begins at that point. At that point, with the rear wheels off the ground and the front wheels on the ground, if you hit the accelerator the vehicle will climb up over the 2×4 (or try 4×4 for the no engagement test) and move forward. If it fails to move forward at, say, 2,000-2,500 rpm in this condition — rear wheels off the ground and front wheels on the ground) –then the more rare condition of a no-engagement failure is clearly present.

Another way to do a rear engagement test is in the winter time. Park the van so that the rear two wheels are on sheet ice but the front two wheels are not. Put a 4×4 in front of the front two wheels, or some obstacle of similar proportions. Put the car in drive and let the clutch out. The rear wheels will spin on the ice. But one or the other of the front two wheels (but not necessarily both!) will engage and pull the vehicle forward over the obstacle. If neither of the front two wheels spins (you have to watch both front wheels at the same time, so you need a helper or two), then you have a no-engagement failure. Sometimes, a driver in this scenario will look out his window and see that his left front wheel is not spinning and conclude that his VC is bad. But in actual fact the right front wheel is spinning at that time and so in actual fact the VC is working fine .)

A no engagement failure is usually the result of a seal breaking inside the VC, and the consequent leakage of the VC fluid out of the VC and into the area of the front differential where transmission oil is kept. Thus, no-engagement failures typically result in contaminated front differential oil, which therefore should be changed along with the VC.

Typically, a seal breaking in the VC is a result of severe stress on the VC, possibly caused by the more common binding type of failure covered earlier.

More Background Stuff on VCs:

Clive Smith – Club 80-90 – 2002


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