How do Honda Viscous Couplers work and How do you know if they are bad?

How do Honda Viscous Couplers work and How do you know if they are bad?

Posted by AKDBuilt Performance on Dec 19th 2020

.Taken from the 89 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 housing 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.

How do we know when they go bad?

again taken from the 89 Service Manual

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

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.

What does all this mean? It means they go bad and you are not supposed to rebuild them because there are NO Parts for them, all you can do is clean them and change the fluid. 

How about Freelander VC's? 

Taken from

Some of the most expensive parts to replace on a Land Rover Freelander are in the drive train – the IRD unit (transfer box), rear differential and gearbox. So why would you risk damaging any, or all of these parts? If your viscous coupling unit (VCU) has past its useable life then instead of paying out for a replacement viscous coupling unit (VCU) you could be spending thousands on ALSO replacing the IRD unit (transfer box), the rear differential and possibly even the gearbox.

The viscous coupling unit of a Land Rover Freelander is a sealed unit positioned in the center of the prop shaft. Inside the unit is a viscous fluid. Over time this fluid gets thick – much as your engine oil would – and eventually causes the prop shaft to rotate at a slower speed than is required. You can tell when you have reached this point as your Freelander will feel as though it is holding back on you, especially when turning on full lock – but don’t wait this long, by then you may have already done expensive damage!

The problem with the viscous coupling unit (VCU) is that it is a sealed unit, so you cannot check the condition of the viscous fluid inside it. There are a number of tests that people say will test if your fluid has had it, but none of these are really reliable.

The viscous coupling unit (VCU) has a life span of about 70,000 miles – after this time you are dicing with, not death, but very large bills!

So why should you not buy a used viscous coupling unit (VCU)? Well, simply because you have no idea as to the condition of the viscous fluid inside it, because generally you have no idea of the mileage it has done. So how will you know when to change it? How will you protect yourself from those big bills? You can’t!

If you have been unfortunate enough to suffer from a damaged IRD unit (transfer box) or rear differential was your mileage over 70,000 miles? Had your viscous coupling unit (VCU) been replaced previously? If your mileage was over 70,000 miles and you never replaced your viscous coupling unit (VCU) then this will generally have been the cause of your empty wallet! And if you still don’t replace your viscous coupling unit (VCU) and fit a new IRD unit (transfer box) or rear differential, then it is almost certain the same thing will soon happen again!

The key is to always buy a new or reconditioned viscous coupling unit (VCU) which has had the viscous fluid replaced. This way you know you have another 70,000 miles of carefree motoring without having to worry about damaging your expensive drive train.

So, whatever you do, DO NOT buy a used viscous coupling unit (VCU) for your Land Rover Freelander just to save a few pounds – it may turn out to be the most expensive saving you’ve ever made!

Now if you cant reach the conclusion that buying a used VC is not the BEST idea and buying an overpriced old as F Freelander one is really not a good idea then by all means go ahead. Does this mean they are all bad? No, Does this mean that people have NOT had success in refurbishing them? nope, but do yourself a favor and try and load test yours before you gamble your other expensive parts or potentially your life on it.

However for those that are skeptical we have an option coming soon here: Billet VC Stay tuned for more info!