Understanding diff lock in the P38

[LavisLane]

Maybe i should have labelled this "not understanding..." as i'm trying to get my head around the big picture and hoping someone can clarify for me. :eek:

The questions started recently when my uncle from Vic, an LR collector and know all, asked me wether i had considered upgrading to after market diff locks on my P38 (which is about the only thing he doesn't have much experience in, mainly early series...).

I've never really understood how the diff lock works, when it works and what the strength/weaknesses are. This has been in the "must learn this!" pile for some time now.

Reading the workshop manual i've downloaded, it advises as below;

Quoted from P38 Manual
A differential is fitted between the front and rear output
shafts to allow the propeller shafts to rotate at different
speeds when the vehicle is cornering. Drive to the
front propeller shaft is through a viscous coupling,
which eliminates the requirement for a differential
lock.

This suggests that there isn't technically a diff lock and it works between front and rear. Is there a left and right? I'm guessing that being dependant on the viscous coupling could have responsiveness and over heating draw backs.

Next question is - do i need to get after market lockers for medium 4wd activity, no rock climbing or anything, mainly sand and the occasional bush bash.

Thanks in advance for any enlightening!

D

[BigJon]

Your Rangie has front and rear diffs between the axles.

It also has a viscous coupling between the front and rear propshafts.

The diffs between the axles allow the wheels on either side to turn at different speeds.
The viscous coupling allows the front and rear propshafts to turn at different speeds.
This is essential for on road driving (going around corners all wheels travel a different distance and therefore rotate at different speeds).

For normal offroading that setup is fine.

If you wanted to venture into harder terrain where you were likely to get stuck through lifting wheels, then crossaxle difflocks will be your friend.

These work by forcing the left and right wheels on the locked end to rotate at the same speed.

For example a rear difflock will join the rear axles together making the rear wheels turn at the same speed, regardless of lifted wheels, lost traction, etc.

I would think that for your intended use, the standard setup will be OK.

[LavisLane]

Thanks BigJohn,

Since i couldn't see any evidence of a diff lock, I assumed that the P38 must have one that engaged automatically when it sensed that it needed both left and right to turn at the same speed through a lifted wheel etc.

Planning a big trip out west Vic later in the year so better get an upgrade i guess....

D

[BigJon]

What sort of big trip are you planning?

The P38 is very capable straight out of the box provided everything is in good condition and working correctly.

I am pretty sure most if not all P38s had traction control of some sort, be it rear wheels only or all four wheels.

[LavisLane]

Still unsure of all the details. I was invited over beers (many beers) so a little hazy. I know it's from Albury to Gippsland area though.

Maybe getting diff lockers for the P38 is a bit of an overkill then...

[BigJon]

See how it performs standard before you worry about expensive modifications like difflocks.

A decent set of offroad rubber gives a much better increase in offroad performance for less $$.

[PaulP38a]

Agree with BigJon - spending $1500 on a set of 5 good tyres is a better investment than a diff lock. The P38A is an immensely capable vehicle in standard form.

I'm a big fan of Cooper ST's. Had a set of these on my previous P38A and never had a problem keeping up with any Jap or Yank 4WD's off-road.

Pre-99 models had traction control on the rear wheels at less then 80Kph, or thereabouts. 99-on had 4 wheel traction control up to 100Kph.

If you do decide on a diff-lock, consider the front rather than the rear. I have a rear air-locker on mine and it didn't quite cut the mustard on the weekend when I needed it. I'm pretty sure it would have be ok if I had a front locker instead. Then again I made a couple of mistakes on my approach up the slope so I shoudn't be blaming the vehicle :o see the video YouTube - Failed attempt at bowl climb

On a related subject that may also be useful to LavisLane, does anyone know how the TC works in low range? In the video above, I seem to be getting some excessive spinning of opposite wheels.

Cheers, Paul.

[Remy]

Lower your tyre pressures as well 20-25 PSI. This will increase traction immensely - more tyre surface in contact with gravel. I recently attempted a fairly steep incline (my p38 1996 TC in rear only). With full tyre pressure no chance but get a bit of air out of the tyres and up she goes.

Traction control isn't like a diff lock - the diff lock as the name suggests will lock the diff so that both wheels drive at the same rate/speed - that is why axles break when one tyre is wedged and doesn't want to turn but also why they perform better than a standard 4wd in many hard offroad conditions (steep muddy wet slopes).

Traction control is like ABS but in reverse. When the sensors sense a wheel is spinning they retard power to that wheel (either by applying the brake to that spinning wheel or through some sort of engine management which reduces the power and thus the spin).

Paul the clip didn't seem to work but i think i have seen it before. The TC won't help the wheel that isn't turning turn (diff lock in front would) but should reduce the wheel spin of the wheel that is spinning. You will notice the TC light flicker on and off as it applies some breaking force to try and slow that wheel down - it probably won't stop the wheel though. The feeling through the steering is much like hitting the breaks hard in a ABS car in the wet. You can feel the break be applied every few milliseconds.

Hope my crude explanation helps!

[PaulP38a]

Thanks Remy, that all makes sense.

There were 3 things that I did wrong on that video...


tyre pressures were too high (28psi - should have been around 20)
not enough momentum - I let the car stop
bad line - I slipped in to the rut about half way up.

On the second attempt, I chose a slightly better line and kept the momentum up. Up and over no problem without the locker.

Couldn't be bothered to let the tyre pressures down... after all, I was out driving with a Pajero and Triton who had already let their tyres down to 20psi :D

[PhilipA]

Just remember that going up a steep hill, about 80% of the tractive effort is by the rear wheels.

I recall breaking a rear axle in my old 2door at Mt Walker.
I had a detroit locker so had 3 wheel drive. BUT even on a non lumpy hill I had no chance of going forward. This illustrated to me that most of the drive is from the rear.

You may have been better off by lowering the suspension and thus softening the spring/air? rate to get better articulation and keep both fronts on the ground.

If your rear traction control does not overheat then I guess that a front locker could help but on a gnarly climb you still have the possibility of an overheat , and then the front locker will not help much.
In a non TC car IMHO it is very clear cut that it should be rear.
In a TC car it is not so clear cut but I personally would still go for back, as these fancy traction aids always seem to give up when you most need them. I have seen it happen on a club trip on Mt Airlie when a member got the 38A version of three amigos just before the most difficult step.

Regards Philip A

[Remy]

Diff Lock Article (http://www.pps.net.au/4wdencounter/articles/difflocks.html)

[PaulP38a]

If your rear traction control does not overheat then I guess that a front locker could help but on a gnarly climb you still have the possibility of an overheat , and then the front locker will not help much.
In a non TC car IMHO it is very clear cut that it should be rear.
In a TC car it is not so clear cut but I personally would still go for back, as these fancy traction aids always seem to give up when you most need them. I have seen it happen on a club trip on Mt Airlie when a member got the 38A version of three amigos just before the most difficult step.

Excuse my thickness Philip, what's the "38A version of three amigos"?


Diff Lock Article (http://www.pps.net.au/4wdencounter/articles/difflocks.html)
Good read - and quite enlightening.

ARB have quoted me approx $1600 to supply and fit a front air-locker to my Rangie.

In terms of cost and priority, it is now right behind the transmission replacement and purchase of a Faultmate.

cheers, Paul.

[PhilipA]

Excuse my thickness Philip, what's the "38A version of three amigos"?

Discovery 2 has 3 lights that light up when the ABS and traction control fails.

I do not know the light show variant for 38s .

The car in question simultaneously blew a heater hose (known early 38 problem) and coincidentally (ie not connected) had the traction control failure lights come on. LOL. Luckily we had a Rover dealer mechanic on hand, but he had to be snatched up the obstacle.

It can be caused by a sensor getting wet, wheel bearings failing/slop or just plain cussedness.
Regards Philip A

[adm333]

I do.....

The message centre says "TRACTION CONTROL FAILURE" and you hear a warning beep.

[PaulP38a]

I do.....

The message centre says "TRACTION CONTROL FAILURE" and you hear a warning beep.

I'll add that to the list of things I don't ever want to see on the Message Centre of my P38A. Thanks for the heads-up.

Cheers, Paul.

[LavisLane]

I do.....

The message centre says "TRACTION CONTROL FAILURE" and you hear a warning beep.


Well that's a nice one to add to the list.. :( This has recently come up on mine after some medium rocky action out west.

It originally came up as "TRACTION CONTROL OVERHEAT" and i guessed that it just had a big work out and might cool down and sort itself out. Guess not :eek:

Booking in with Graeme Cooper next week for a service and a smoothing of my new Haltech (very happy with that!)

Cheers,

Dave

[2door grover]

the vc center in the transfercase is like sort of like a open diff center not a locker.

if you lose traction to the front wheels ( both or one) it will send nearly all of the power to them as its the easy way for it to go, like a open center. think of it as your car just went from 4wd to 2wd. with a center diff lock (tcase) it makes both front and back drive shafts turn at the same speed no matter what.

then diff locks on in the axles come into play and keep the wheels turning at the same time.

if you have axle lockers and a vc t/case you will still only get drive to the front or back wheels when one has less grip, you will just be turning both wheels insted of the one thatsin the air.

[Scouse]

the vc center in the transfercase is like sort of like a open diff center not a locker.

if you lose traction to the front wheels ( both or one) it will send nearly all of the power to them as its the easy way for it to go, like a open center. think of it as your car just went from 4wd to 2wd. with a center diff lock (tcase) it makes both front and back drive shafts turn at the same speed no matter what.

then diff locks on in the axles come into play and keep the wheels turning at the same time.

if you have axle lockers and a vc t/case you will still only get drive to the front or back wheels when one has less grip, you will just be turning both wheels insted of the one thatsin the air.But the VC locks up when it detects slippage, resulting in a locked centre diff.

There are no locks as such on the axles, just traction control on the rear or front + rear (depending on model).

[2door grover]

But the VC locks up when it detects slippage, resulting in a locked centre diff.

There are no locks as such on the axles, just traction control on the rear or front + rear (depending on model).

ok i didn't know that, i was told that a vc will never lock up and will slip sort of like a toque converter and that is why they are replaced with a locking center diff in rrc and disco's

is that right?

[adm333]

The viscous coupling is designed to be an "automatic" locking centre diff, I guess to alleviate the Range Rover driver from having to make those decisions.

A viscous gel substance floats between fins of the front and rear output shafts.

When the speed differential becomes two great the gel heats up and causes the fins to lock together. It is all meant to happen quite quickly, such that front or rear wheel spin should "activate" the locking mechanism.

In my experience with my RRC '94 and my current P38 both of which have the VC, it doesn't quite work fast enough and if I had a choice, I think I would rather be able to choose myself whne I locked in the centre.

http://www.aulro.com/afvb/attachments/general-chat/475d1155773992-off-road-ability-d2-vs-p38-viscous-coupling.jpg

[Bilbo]

I can only comment as an RRC and D2 owner but will start by 100% endorsing what Philip A said - if you only buy 1 diff lock, it goes at the back. A quick home test for the viscous centre diff is to jack up one corner (just a little) and try to drive off - if it is working normally it will lock almost instantaneously and the 2 wheels on the ground will drive you off the jack. If it is buggered, the wheel in the air will spin. I reckon my viscous centre diff locks up off road damn near as good as my old FJ55 (which had no centre diff). The traction control on the D2 is really impressive (when it works) but not nearly as good as a locked or viscous centre diff because it can only work after a wheel starts to slip; and even then it only guarantees you 1 wheel drive (if you have no CDL).