GRX Drivetrain

Transmissions, Transfer Boxes, Axles, Drive Shafts
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GRX Drivetrain

Post by Clem » 20 Jun 2013 07:56

Hi guys,

I just want to check if I have gathered correctly from what I read on this forum as to the GRX drivetrain. From what I read I understand that:
  • There is no differential in the transfer case and so when four-wheel-drive is selected, the two propeller shafts are connected together in such a way that they are forced to rotate at exactly the same speed.
  • Moving at speed on a straight road, assuming that the manual hubs are locked, four-wheel-drive high can be selected on-the-fly as the two propeller shafts will be moving at the same speed and can therefore be engaged on the move.
  • One would not be able to engage four-wheel-drive high on-the-fly at speed if the manual hubs are not locked as that would imply that the front propeller shaft is stationary (the assumption is that the auto locking hubs have not come into operation).
  • Engaging four-wheel-drive low requires a dead stop – at least in principle.
  • Engaging rear differential lock requires a dead stop – at least in principle.
  • When getting a very heavy load moving it should be possible in principle to shift from four-wheel-drive low to four-wheel-drive high or from two wheel drive low to two wheel drive high at low speed, provided care is used.
Have I got the picture correctly?

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Re: GRX Drivetrain

Post by ricster » 20 Jun 2013 09:51

There is no differential in the transfer case and so when four-wheel-drive is selected, the two propeller shafts are connected together in such a way that they are forced to rotate at exactly the same speed.
As far as I know the transfer case is locked, so front and rear propshafts will spin at the same speeds

Moving at speed on a straight road, assuming that the manual hubs are locked, four-wheel-drive high can be selected on-the-fly as the two propeller shafts will be moving at the same speed and can therefore be engaged on the move.
Before doing this pull down the sun-visor on the drivers side. There is a speed limit at which you shouldn't engage 4H from 2H. Your Patrol will have auto locking hubs ( unless you have manually clicked it over to "lock")

One would not be able to engage four-wheel-drive high on-the-fly at speed if the manual hubs are not locked as that would imply that the front propeller shaft is stationary (the assumption is that the auto locking hubs have not come into operation).
You would be able to engage 4H at low speed with Auto hubs on the "Auto" setting. With manual hubs unlocked, the same can be done but only the prop and side shafts will turn, without any power to the front wheels.

Engaging four-wheel-drive low requires a dead stop – at least in principle.
Not only in principle.... unless you want a lot of gearbox spare parts... :lol: .... so yes dead stop before moving from 4H to neutral to 4L

Engaging rear differential lock requires a dead stop – at least in principle.
Ideally yes. I do engage mine when moving SLOWLY forward or backward. As you flick the switch the diff-lock light flickers ( trying to engage ). Sometimes it needs a little movement to get it locked ( diff-lock light stays permanently lit), especially if on a bend. I VERY GENTLY move forward till it clicks into locked position. The Diff-lock on the Patrols is probably the one "weak point", but with care it should be ok. Mine is still original and it has been put through its paces a few times!!

When getting a very heavy load moving it should be possible in principle to shift from four-wheel-drive low to four-wheel-drive high or from two wheel drive low to two wheel drive high at low speed, provided care is used.
As far as I know..... nope. You will have to stop dead to shift through from 4L, to neutral, to 4H to 2H. If that 4.8 with tiptronic g/box cannot move that heavy load, you need to phone Grant ( forum and Nissan 4x4 Club member) from Catapillar, cause that is all that's going to move that weight... :rolling: :rolling:
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Re: GRX Drivetrain

Post by Clem » 20 Jun 2013 10:54

Many thanks Cedric. That helps clarify things a good deal. Let's hope I don't break it!

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Re: GRX Drivetrain

Post by ricster » 20 Jun 2013 11:06

ha ha.... The Patrol has one of the strongest drive trains available.... dare I say it .... even stronger than the cruisers. The Patrol diffs and gearboxes are renowned for their robust construction. The motors too. There is a reason why the Patrols are used by the UN. They have a high specifications requirement, and Patrols clearly meet these requirements!!
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Re: GRX Drivetrain

Post by Clem » 20 Jun 2013 11:38

Cedric, that's very good to know and will make for some peace of mind after the years of driving a Land Rover. It never really did give much trouble, except for (a) having to treat the 5th gear like cut glass, (b) breaking a diff between somewhere and nowhere (in fairness though the vehicle was loaded waaaay beyond spec and we were doing some incredibly rough "wheel-in-the-air-low-range-wide-open-throttle" stuff), (c) that crappy rubber coupling on the back prop that I eventually replaced with a heavy duty UJ (d) the fuel pump that left me on the side of the road (e) the dry joint on the spider immobiliser behind the dash that made the engine keep cutting out in Botswana (f) the power steering pump hose that started pouring oil in the Caprivi on the way to Liuwa (g) .....ek raak nou moeg vir tik.

You'll understand why my spares and tool kits were the first thing I always packed and which had total priority over everything else. Still, I have lost of good memories but I'm going to have greater peace of mind when I leave home now. I guess I'll still be carrying a few spares and spanners though.

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Re: GRX Drivetrain

Post by ricster » 20 Jun 2013 11:49

ha ha.... yip a spanner or two is never a bad idea to have on a trip.... you never know when a Landy will be in need of help when you are passing in your Patrol... :rolling:

Your wife will be sooooooo happy that the spanner space has decreased, and that she now has extra space for shoes etc.
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Re: GRX Drivetrain

Post by Peter Connan » 20 Jun 2013 16:51

Clem, I agree with everything you've said xcept for the diff lock.

I usually engage that while moving, with the proviso that I will never engage it when a wheel is spinning. I then use a very gentle throttle until I am sure it has engaged properly.

You will find with the Patrol that you can drive far more gently, and need resort to momentum far less often precisely because you have that rear diff lock waiting in the wings (although you probably learnt that already after you installed the lockers in your Disco). I would also recommend that you look at a good aftermarket suspension, for the following reasons:

1) It seems to improve articulation considerably.
2) It improves the vehicle's ability to carry the kind of loads overlanding tends to entail.
3) The patrol has a very long wheelbase (at least compared to the disco), and while this is a major contributing factor to the vehicle's wonderful stability at speed, it does mean that it touches more easily on crests, and a bit of height helps a lot here.
Mag ons ons kenniskry met lekkerkry aanhoukry.

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Re: GRX Drivetrain

Post by Clem » 20 Jun 2013 17:34

Peter Connan wrote:Clem, I agree with everything you've said xcept for the diff lock.

I usually engage that while moving, with the proviso that I will never engage it when a wheel is spinning. I then use a very gentle throttle until I am sure it has engaged properly.

You will find with the Patrol that you can drive far more gently, and need resort to momentum far less often precisely because you have that rear diff lock waiting in the wings (although you probably learnt that already after you installed the lockers in your Disco). I would also recommend that you look at a good aftermarket suspension, for the following reasons:

1) It seems to improve articulation considerably.
2) It improves the vehicle's ability to carry the kind of loads overlanding tends to entail.
3) The patrol has a very long wheelbase (at least compared to the disco), and while this is a major contributing factor to the vehicle's wonderful stability at speed, it does mean that it touches more easily on crests, and a bit of height helps a lot here.
Thank you once again for the advice Peter. I will then engage differential lock and move slowly to make sure it is properly engaged before applying power.

The differentials on my Discovery are gear driven torque biasing differentials. In my view, one of the most wonderful inventions ever made. They add huge stability (huge stability) to the vehicle on slippery gravel roads and the like when moving at speed. It is not even necessary to lock the centre differential. Honestly, it is the only four-wheel-drive I have ever been able to happily, safely and comfortably drift at speed through corners like a rally car. The things are real lifesavers as well from the perspective of blow-outs as they keep the car absolutely stable and I can testify that on at least one occasion they have added greatly to my personal safety (I cannot say with absolute certainty that we would have rolled anyway but the probabilities are high) when a sharp rock penetrated a tyre and caused a massive hole and sudden air loss. If you park with the car with the right-hand wheels on a hard surface (for example tar) and the other two wheels on a loose slippery surface, you can engage low range first and absolutely floor the throttle and you will get no wheel slip at all. The only downside is that you need a minimum of 5 foot-pounds of force on the opposing wheel in order to make them work and direct the power to the wheel that has the traction. If the opposing wheel has absolutely no force on it at all, then it acts like an open differential. You can still redirect the power by lightly applying the brakes but you risk breaking something by doing that.

Relatively early on my Discovery taught me something about momentum (this was in the days before the torque biasing differentials had been fitted). We were travelling solo on the Wild Coast and I had just forded a river and was trying to climb out of the bed on the other side up a very steep embankment with ruts and the like that really tested the wheel articulation. I had learned much of my basic driving skills at the old Allendale quarry (now sadly part of history) and had to some extent been led to believe that momentum was the answer to many problems. So, I went at the thing like a bull in a china shop and despite a number of repeated tries, kept getting cross axled and sliding backwards. As there was no other route but through, I then decided that I would simply have to move the vehicle as far up the embankment as I could and then winch it further through the obstacle. With that in mind I then decided to move the vehicle gently up the embankment as far as I could in preparation for winching. Lo and behold, going slowly and evenly she climbed through with no fuss at all. Momentum is not the answer to everything. In my view, it is far better and to sit a little bit and contemplate the terrain and then decide how best to place your wheels and go at it slowly or with the minimum amount of momentum that you think will do the job. That is probably 90% of the game for me. My old Series III 2.25 diesel 88 inch station wagon with leaf springs and all has on occasion done what fancy Range Rovers could not do, simply because I went slowly and took the time to walk through and read the terrain properly first before attempting it in the vehicle.

I will take the hint on the suspension lift in due course. First I am going to take time to get used to the vehicle and understand its characteristics. I can appreciate though that the breakover angle may be less than absolutely ideal.

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Re: GRX Drivetrain

Post by Stefan » 20 Jun 2013 21:19

Clem wrote: [*] When getting a very heavy load moving it should be possible in principle to shift from four-wheel-drive low to four-wheel-drive high or from two wheel drive low to two wheel drive high at low speed, provided care is used.[/list]
Hi Clem,

I don't know how you would do this using the auto-box, but I also assumed it couldn't be done, but after reading about it on the 4x4community site and trying it out, it works as follows on a manual:
Start in 4L, going up through the gears to about 4th, put the gear in neutral, shift to 4H, and then 2nd in the gearbox. I haven't tried it towing a heavy load, just want to see if it could be done.

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Re: GRX Drivetrain

Post by Clem » 21 Jun 2013 04:19

Thanks Stefan. That's more or less how I recollect doing it. The Land Rover manual at the time stated that one would start in low range and pull away just to get the load moving. Then you would throw the primary transmission into neutral and shift the transfer box into high range and then re-engage the primary transmission. However, all this had to be done at a maximum speed of 8km/h. So it really was just to get the load moving. I have done it and the transfer box, being non- synchromesh, did not like it very much at all. It's a good way to damage the gears if you ask me and not a great idea at all. I think I will follow the philosophy that if the GRX cannot move it normally, then it should be left alone. Towing massively heavy loads is very hard on the drivetrain anyway and I hope to make this machine last and then some.

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