The mystery of the viscous coupling

[Gsmack]

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[bigsmile]

[Scouse]

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.

[Homestar]

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. 😊

[Grumbles]

. If I use another car (several to choose from unfortunately as I have P38itis)

......and a shining example to us all you are. [biggrin]

[Scouse]

Careful, I might send one or two down to you!!

[Grumbles]

That'd be handy.[thumbsupbig]
I haven't had my Rangie for the last 3 weeks[bawl] and have separation anxiety. But can a P38 hold a candle to a Classic.....hmmmmm.

[Gsmack]

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.

[Keithy P38]

It’s hard to say what causes a viscous coupling to die. There’s no evidence anywhere to suggest any particular activity does it. Rest assured, driving it as a 4wd will not destroy it - it’s a well designed, solid 4wd, that will do everything asked of it. They are not fragile 4wd’s 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! It’s getting stuck in deep water that will hurt you as mentioned before, the ECU’s are under the seats. The door seals are very good in P38’s, 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. Don’t live in fear that it’ll break down, because they are not lemons like they’ve been made out to be. My D2 owning mates can attest to this.

Cheers
Keithy

[Scouse]

@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.

[prelude]

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

[Scouse]

The VC in my daily driver & 'off roader' actually slips more than I would like. I was caught out climbing a rock face earlier this year with the front left wheel spinning.
I'm still yet to work out if this is normal slippage or if I have a VC about to let go. I've tried replicating the situation in other spots but haven't managed to get stuck yet.

[callmedazz]

hi guys i am in the same situation at the moment .turning on full lock you can feel it grabbing especially on tar with loose stone .I actually removed front shaft irarely offroad,although car seemed to roll ok in drive and not coming to ahalt on slight down hill grade ,when you say lock-up to me that means as if brakes on ,am i understanding correctly .normal h/way driving seems fine even with shaft in,am i being overly concerned and refit shaft as minimal offroad is done , looking for replacement vc in good condition but to yet no avail [bawl]

[PhilipA]

The VC in my daily driver & 'off roader' actually slips more than I would like. I was caught out climbing a rock face earlier this year with the front left wheel spinning.

From RRC experience I think this is quite normal and related to the open front diff more than the VC.
However having said that , the feedback I have had over many years is that it is sitting at a rock with a front wheel spinning is the most damaging to the VC, as it has to "hump" ie rise in internal temperature until it locks.

My VC was fine at 240Kk when I sold the car, ( and stupidly gave away a 80KK VC and another transfer with stripped splines with the car). I could have been RICH!

I think the best help you can give a VC is to fit an Ashcroft , Quaife or other torque biasing diff to the front axle to minimize wheelspin and therefore front to rear speed difference.
Regards Philip A

[Gsmack]

Thanks all for the info, feeling more comfortable now. I do like Quaife LSDs, however I don't think I'll be working it sufficiently hard to justify such an enhancement in the immediate future.

I'm sure I'll be posting more questions as I get it back on the road and reliable.
All the boots on the ball joints and tie-rods are torn, so they all need replacing.
Any tips for getting the ball joints off or fitting them, my reading implies they are a press fit[bigsad] I've got good hammering skills, but sounds likes they won't help me here.

Thanks

[zzsteve]

Best guide that I found was Michael Murphy's YouTube series:
Swearing and Skinned Knuckles Episode #3: Land Rover Discovery Ball Joint Replacement Part 1 - YouTube (https://www.youtube.com/watch?v=yodDswj5qIU&t=323s)
Practically no difference between D2 and P38 for this task.
You will have a hard time finding just the boots, although MR Automotive in Redcliffe, Qld may have them.

Cheers,
Steve

[prelude]

When researching to rebuild my VCU I came across some info that was dormant in the back of my head but I forgot to share with you. ie. I'm getting old and forgetful [bigwhistle]

The VCU works by heating up the fluid inside. When warm it forces all the plates inside the vcu to stick together tightly and hence lockup your diff. This process is somewhat between digital and analog, ie. the system works by beeing on/off/on/off etc. often in a short time. The silicone oil heats up and locks, cools down and allows slippage and thus heats up and locks etc.

This process wears the oil out. Also, silicone oil is not very good at lubrication, but then again that's not what we want, we want the darn thing to lock up :) Anyway, what happens is that abrasive particles from the discs mix with the oil and the oil "burns" over time which makes the VCU turn more difficult over time. From what I have read it is not quite linear but let's just assume for the sake of argument that it is.

Putting all this info together: the VCU is a (like so many things on the P38 :P) maintenance part. It wears out. The silicone fluid can be replaced and when in time, thus before the unit is completely cooked, with new fluid can operate for a certain amount of time again. More usage of the unit; ie. in conditions where there is a fair difference between the front and rear axle, wears the fluid and thus the VCU out faster.

I have started a thread with the question if others have rebuild theirs. Let's see if we have some people with experience over here. If not, we'll pave the way with info from the interwebs :) From what I can tell right now, replacing the fluid (in time) will not wear the plates out too much and we should be able to replace the fluid in our units multiple times before they give up and considerably lengthen the lifespan of the unit and thus the car :)

-P