Interview with a Viscous Coupling Expert

by Derek Drew

I located this morning an expert who works at a manufacturer of viscous couplings. As part of a lengthy interview, he said these things:

1. The viscous coupling fluid goes bad over time when it acquires small metal particles from the surrounding metal and suspends these particles.

2. The process of having the fluid go bad is called "gellation" and was caused as a reaction causes a gas to be formed.

3. You can fix a VC by replacing the fluid with fresh fluid. You should also examine the condition of the plates.

4. The percentage of air in the viscous couling is critical, and the percentage is almost always a low of 80% and a high of 86%. He said that if you went above an 86% fill then you would have the VC activating on the highway during normal driving and a dangerous situation would exist. If you went below 80%, he said you might never achieve lockup, even when stuck. He said all these numbers are subject to investigation and confirmation, which he will try to give me in August after consulting with Europe to check on our exact unit. He said the probable range for our vehicles is 84% to 86% air -- possibly closer to 86% for a vehicle intended to go off road as ours is.

5. He said a room temperature VC should *not* be under any internal pressure, and that it might be a simple fix to partly repair a VC to simply open up the bleeder holes and let the fluid under pressure in there out and then put the plugs back in and keep using the unit.

6. He said that he wanted to help us in order to keep VCs from getting a bad rap amoung OEM manufacturers. He said that even if his company did not make the SDP units themselves, his company would have an interest in helping us and making sure our VCs were not wearing out prematurely and that they were being rebuilt properly.

7. He said that if Wheedle and Oregon Transaxle were refilling the VCs without letting any air get into them (which both Wheedle and Oregon told me!!!) then the rebuilds from Wheedle and Oregon would be faulty. The symptoms would be early wear, intense heating of the VCs, and lots of premature lockup.

8. I observed that this might be the reason Wheedle and Oregon both told me they are moving to a lighter formula fluid--that they are mistakenly not adding the required 14% air and that they are not rebuilding the units correctly. The GKN official agreed that this is a logical possibility. He said the idea of a VC being 100% filled with fluid was something that would simply never be done under any circumstances. This statement tracks the statements of the engineer Wolfgang in Tim Smith's article from VW in 1979.

9. I believe it is possible that after my guy checks with SDP that someone will shut him up and they won't talk to us any more.

10. He said the proper refilling procedure is to apply pressure at one little hole while applying a vacume at the other little hole .

11. He said the key to preventing the fluid going bad over time was additives in the fluid that his company routinely uses. He said it is possible that SDP does not use these additives.

12. He said the fluid gets hot and that this is what causes the lockup.

13. I asked him why the VC doesn't appear to have any air in it and asked him why it seemed to be 100% fluid. I asked if perhaps this was a special VC that required 100% fluid. He said that no, this would not be the case. He said that in manufacturing the air is added and then the air gets mixed up in the fluid and occupies the little holes between the plates of the VC and you cannot see the air any more.

14. He said not to quote him on any of this. I guess that means I should leave his name out. :-) I think what he really meant was that he wanted to investigate and confirm what he told me.

Conclusions

1. Anyone with a bad VC should hault any work being done by Wheedle and/or Oregon until we resolve the issue of the % air.

2. Anyone with a bad VC might want to simply vent it, reseal it, and reuse it, and see if that fixes the problem.

3. Derek gets this week's "VC Award."

Additional Info

Derek Drew offers this additional information from a 'the very best sources':

1. If you refill a VC so that it is full it will burst its seals. Never refill so it is full. (The proper percent air is 84% to 86% according to an estimate from another expert familiar with Viscous Couplings but this percent cannot be confirmed yet.)

2. The plates inside the Viscous Couplings have microscopic burrs on them that are critical to the proper torque transfer of these units. This means that the plates wear out and cannot be reused!

3. Since replacement plates are not available, this means Viscous Couplings should not be rebuilt until a source of replacement plates can be identified.

4. Any viscous coupling under pressure is likely a bad viscous coupling! If you open the unit, and a lot of VC juice comes out, the VC is likely bad.

5. Viscous Couplings have a useful life of 60,000 to 90,000 miles. After that, they are worn out and should be replaced. This makes it sound like they should be replaced as often as your clutch!

6. There is no difference between the Viscous Coupling fluid in the 14" and 16" models.

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last modified April 5, 2000