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Thread: The mystery of the viscous coupling

  1. #1
    Join Date
    Jan 2018
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    Beechworth
    Posts
    4

    The mystery of the viscous coupling

    Hi All

    I've never had a vehicle with a viscous coupling before so I need some help to figure out if mine is good or if it needs some tlc.

    So some questions:
    • Do I test it when it's hot or cold
    • What sort of weight should I be using to test it, and how long should it take to rotate 90degrees? For example I had a 4foot bar with 10kg on the end and it took 50secs to rotate 90 degrees, is this good or bad.


    Self evidently it's a 95 Range Rover 4.0L, with the auto, and it appears to have not been on the road for the last 10 years, looks in really good condition, but I need to know more about the viscous coupling so I can avoid it going bang in the middle of no where.
    Lyn

    P.S the series 3 was so much more simple

  2. #2
    Join Date
    Jan 1970
    Location
    Douglas Park, NSW
    Posts
    9,261
    The transfer case one I assume.
    I don't know of an official test but I have a P38 with a locked viscous at the moment. If I coast at a slow speed on full lock, it will soon come to a stop. If I use another car (several to choose from unfortunately as I have P38itis), it doesn't come to a stop.

    The 'locked' car also scrubs it's tyres, inside rear usually, very easily on anything but perfect bitumen.


    As for going bang in the middle of nowhere, I think they'll be locked up way before they go bang.
    Scott

  3. #3
    Join Date
    Aug 2010
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    Sunbury, VIC
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    13,729
    As mentioned, if it's not locked completely up, it's working. Lockup is noticable and this is what will lead to a bang at some point after - can be quite a long time too depending on hiw and where you drive it once it has locked up.

    Sounds like yours is fine - go drive and enjoy it. 😊


    1977 101 FC - 'Chucky'
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    1976 Series III Tray
    1997 Honda CBR1000F
    2003 L322 - Gone to a new home.

    I don't know how to act my age - I've never been this old before...

  4. #4
    Join Date
    Dec 2008
    Location
    Victoria
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    1,435
    Quote Originally Posted by Scouse View Post
    . If I use another car (several to choose from unfortunately as I have P38itis)
    ......and a shining example to us all you are.

  5. #5
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    Jan 1970
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    Careful, I might send one or two down to you!!
    Scott

  6. #6
    Join Date
    Dec 2008
    Location
    Victoria
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    That'd be handy.
    I haven't had my Rangie for the last 3 weeks and have separation anxiety. But can a P38 hold a candle to a Classic.....hmmmmm.

  7. #7
    Join Date
    Jan 2018
    Location
    Beechworth
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    Thanks for the feedback. Sadly I can't go and drive it yet as I need to do a few things to get a rwc first.

    @scouse how many km and what sort of km has your one that has locked up done.
    Were there any signs prior to it locking up?

    Im a details person, so if anyone has a spec sheet, that would be amazing, I've seen the one for the Freelander, but I'd assume this would be quite different.

    My concern or understanding is that if my viscous coupling isn't working correctly, it will put unnecessary load through the auto and therefore shorten its life. I don't like autos, but I fancy replacing it less so.
    Im a fan of just in time maintenance, not over the top, but just enough at the right times, hence my desire to understand more.

  8. #8
    Join Date
    Feb 2010
    Location
    Townsville, QLD
    Posts
    2,504
    Itís hard to say what causes a viscous coupling to die. Thereís no evidence anywhere to suggest any particular activity does it. Rest assured, driving it as a 4wd will not destroy it - itís a well designed, solid 4wd, that will do everything asked of it. They are not fragile 4wdís that need to be babied or take the chicken tracks at every chance. Mine regularly sees the type of terrain that you generally see on 4wd Action, albeit driven more sensibly. Wheel lifts, lockers, lots of cross-axle terrain, deep holes and big muddy rutted hills.

    Water crossings - absolutely! With my snorkel of course! Itís getting stuck in deep water that will hurt you as mentioned before, the ECUís are under the seats. The door seals are very good in P38ís, so you have time to ford long crossings without worry (think the Jardine River or the many crossings on the Tele Track).

    Ive said it to many a P38 owner - maintain it, know it, enjoy it. Donít live in fear that itíll break down, because they are not lemons like theyíve been made out to be. My D2 owning mates can attest to this.

    Cheers
    Keithy

  9. #9
    Join Date
    Jan 1970
    Location
    Douglas Park, NSW
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    9,261
    Quote Originally Posted by Gsmack View Post
    @scouse how many km and what sort of km has your one that has locked up done.
    Were there any signs prior to it locking up?
    Most of my cars are high milers, around 375k on the locked up one but a couple of OK cars are approaching that mileage too.
    There was no definite signs, I just started noticed it scrubbing tyres in spots where the others didn't. It's not a daily driver & it's only been in my possession for 12 months so it could well have been locked prior to me driving it.
    Scott

  10. #10
    Join Date
    Jan 2016
    Location
    Netherlands
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    213
    There are a few threads on here about the VC.

    In short: when you can turn it, it's ok, simple as that. VC's do NOT like shock loading they can seize up almost immediately that way. The only way to do that though is to drive like a complete and utter tool and I take it you know better I've had the unit lock up on my P38 with "only" 150K on the clock. I suspect it might have to do with the fact that I was learning how to drive a 4x4 and may have taken on some tracks that; a. were to tough for me and the tyres, b. not knowing when to quit and winch (not that I owned one back then).

    Having said that, there is a limited lifespan to the viscous fluid inside the VC unit and temperature does play a big factor here! Makes sure your transfer case get's nice and clean oil and has enough of it and they will last a lot longer. Ashcroft now sell refurbished units for a decent price (since they are no longer available new) so you can rest easy knowing there is always a replacement available.

    Not replacing them in time can be catastrophic though. Your U joints get a lot of tension on them and it has been known that the splines are ripped of the VC unit in more extreme cases.

    I too have been looking for a "how to drive with a VC unit" manual but never found any. A bit of logic should suffice here: the vc slips ever so slightly under normal circumstances. The moment your front wheels are stuck (say in a rock ledge) and the rear is on dust then the slippage will increase for a moment before the unit will lock up completely. if your rear wheels (in this example) should spin and the front is not moving, you will very quickly ruin the VC unit. Also (and this is never a good idea) having a different size front tyres from the rear will cause the vc to always have to slip a bit, potentially more then it was designed for.

    Btw, the vc unit is the same as for the RRC.

    Last but not least, the VC units can go wrong in only one way and that is to lock up. That is actually a great thing since you can always get home or to a shop to replace it.

    Cheers,
    -P

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