Hopefully this is a simple set of questions that won't make me look too stupid!!
I've just bought a defender 90 and I'm interested in learning a bit more about how the four wheel drive works. Can any one explain or point me in the direction of some info?
Specifically:
Is it permanent fwd?
The small "gear lever" in the cab is marked thus:
__________L | | DIFF | | | |----------------| N | | LOCK | | | |_________| H
Does this control both diff locks and the high and low gear ratios (if there is such a thing)?
How many positions are there? Five - diff locked, diff unlocked, H, L, N?
If so how do you select say diff unlocked and low ratio?
When would you use neutral?
I don'rt want to muck about with it in case I muck it up, eg leave it in a setting not suitable for road driving etc - otherwise I'd experiment :-)
It's in 4WD all the time, but there's a centre differential that can be locked so that power will always go to at least one wheel on the front and one wheel on the rear. Due to the way differentials work, if the centre diff is not locked and you're on slippery ground, it's possible for one wheel to lose grip and start spinning, the differentials will then route almost all power through to that wheel and the other wheels won't get as much. Technically they will get as much torque as the spinning wheel but that's often not enough to keep you moving. If you then lock the centre diff, power will once again go through to the other axle and chances are you'll start moving again unless a wheel on that axle also starts spinning.
Basically leave the diff lock off unless you're on loose ground, otherwise you get transmission wind-up as all four wheels travel different distances when cornering, the purpose of the differentials is to prevent this from being a problem, so if you lock the middle one then you'll get wind-up between the two axles and that can cause damage to the transmission. It's best to only use the diff lock if the ground ahead looks really bad as the wind-up can actually cause a wheel to slip and lose grip, but that's only really a problem if you're cornering too, so it's generally safe to put it on if the ground is loose (e.g. mud or earth). I'd suggest you take it to an off-road site and play around, with a Defender you can get away without diff lock quite often so you'll get the feel for it better if you only put it on when things look really sticky. Note that driving on tarmac even through water or slushy snow doesn't count as loose ground as the wheels will still make contact with the tarmac. Diff lock doesn't give you more grip, it just makes better use of what's available but tarmac gives you enough to mean diff lock isn't going to help much and risks you winding up the transmission. If the ground is loose enough to allow you to skid at jogging speeds with medium braking then that's "loose".
The gear level is a four-position one, moving it forwards and backwards controls high and low gear ranges, moving it side to side controls diff lock, so you can have any combination of high and low or diff lock on or off by moving it forwards and backwards or side to side. Neutral isn't much use when driving as you don't go anywhere!
For road driving, the crucial thing is to pull the lever all the way across to the right, so the diff lock light goes out (sometimes this takes a while and you might need to spin the wheels, weave left to right or drop the clutch a few times), leave it in high range on the road otherwise you won't go very quick.
Don't try to change from high to low or back again when moving at more than a walking pace, the transfer box is not syncromesh so you'll get a horrible bang and potentially wreck the box, you can get away with it and Landrover even have a procedure for changing from low to high when towing a heavy load but I've tried it and it doesn't sound nice.
Finally, it's a good idea to use low range and diff lock from time to time, otherwise you won't spot problems with them and the linkages can sieze, I use low range when reversing and pop diff lock on once a week when doing some regular nearby lanes even though it's not needed.
BTW I'm assuming you've got an old defender and not a new one with traction control, things get a bit more complicated with that. Also I'd suggest you get a Defender owner's manual for your vehicle, they don't cost much, available from landy shops and worth getting for a beginner. Finally a cheap book on off-road driving would be a good idea, ideally one that's got a Defender on the front, I've got a specific book in mind but can't remember the name, it's a slim one but gives you the info you need, it's not massively complicated, you're not planning an overland trip to Africa after all!
Blimey, I forgot to say the most important reason why I recommend getting an owner's manual!
Jacking up a 4WD vehicle with a transmission handbrake (the handbrake is on the rear propshaft) is hazardous if you don't take the centre differential into account. If you jack up a rear wheel with the centre diff unlocked, then the handbrake will no longer be effective as the rear wheel in the air allows the rear wheel on the ground to turn, and with the gearbox in neutral allows the front wheels to turn as the centre diff will instead rotate the gearbox output shaft. So when jacking up a rear wheel, always put the diff lock on. This will make sure the handbrake still stops at least the front wheels from turning. When jacking up the front, the handbrake will act on the rears so no need for diff lock then. Chocks under the wheels too just to make sure!
Basically, to keep it simple, when changing a tyre, always put diff lock on first to make sure the handbrake works on at least one axle.
To add a couple of things: there are central 'neutrals' on both the difflock and ornery side of the box, but as Ian says they can't really be used in the normal way.
I sometimes use low ratio when crawling in traffic, as Ian does just to keep the linkages used and usable.
So the vehicles have three differentials, one on each axle and one in the middle, between front and back. It's the middle one you can lock with the smaller lever, and that's normally quite enough for all but the most extreme conditions. If you fit extra lockers they need careful use:
Transmission wind-up is not a nice thing. As you go round corners, one wheel on an axle has to go further than the other. This is compensated for mechanically by the diff on that axle. Lock that diff, and one wheel _must_ skid as you go round corners. It applies to both front and back axles, and the fact the front wheels turn isn't very relevant.
There's a lesser effect _between_ the front and back axles, as roads aren't smooth. If you drive in permanent centre difflock (that's the diff that the standard extra lever locks), torque forces can build up in the propshafts and halfshafts, as one prop will have been asked to turn more than it should have needed to. Result: extra tyre wear, UJ and/or shaft damage, etc. all depending on what you're doing and how long for.
Difflock assumes wheels _will_ slip, thus releasing this stress as you move along. You shouldn't use it on grippy surfaces for that reason.
I very occasionally go between high and low ratio (back-forward on the lever) when rolling VERY slowly, e.g. crawling in traffic, and I de-clutch when I do it.
I only engage/disengage difflock when stationary, and with the main box in neutral. This usually takes several attempts, and a fair bit of pointless revving. :-(
I bet there's a classier way to do it, which I'll be reading about any second now...
Hmm, I just whack the lever across and it pops in, I don't even bother stopping, ditto turning it off, I'm never travelling fast though. Why do you stop? I'd have thought that would hinder rather than help!
To the OP, as for when to do it, no real need to do it every week or anything like that, just do it on occasion, I don't think more than say 6 times per year would be really necessary. I just do it every week because I happen to be driving on rough surfaces every week. I wouldn't do it on tarmac even in a straight line, it ought to be OK but might as well wait until you're on loose ground.
Difflock should engage at normal engine speeds as it's not related to the engine revs at all. It may be sticky to disengage but that can generally be cured by reversing a short distance. If you've got a pesky auto box shunting forwards and backwards a few yards a few times generally works. Whatever happens, more revs - however nice they feel - should not help the situation.
If the wheels aren't spinning relative to one another it can be engaged at any time - however don't just smack it into low box at 30mph and expect to still have bits of transmission left ... That said, ideally diff lock would be engaged before traction is lost which is generally before you tackle the slippery stuff and if you do your basic research that would be when you start off on a slippery section ...
Many thanks for all the replies (especially Ian who I think almost wrote the equivalent of the owners manual :-). I've had a go now with the difflock/ratio changes (wouldn't have felt confident mucking about with it before) and it's very simple when you know how!!
I'll invest in the owners manual and I think a basic 4WD drive course before going too mad.
No need to use the clutch when putting diff lock on or off, you can still be moving in fact, but if you're spinning your wheels don't throw diff lock on in a panic as it'll make a dreadful bang and may wreck the transfer box. As you get used to the truck you'll be able to guess when you're likely to need diff lock and engage it beforehand. If you spin your wheels and stop to engage diff lock then it'll be harder to get going again than if you had diff lock engaged in the first place, but never engage diff lock when the wheels are spinning, if you decide you need it then you have to stop, throw the lever, then apply power slowly until it engages. The diff lock won't always come on or off as soon as the lever is thrown as the locking mechanism may be pinched by the drivetrain, once that pressure goes, the lock will engage or disengage as requested.
As for changing from high to low and back, I dip the clutch to do it. Sometimes it's hard to get out of high or low gear, if that happens, then I find that moving forward, then coming off the accelerator so the drivetrain relaxes a bit allows the transfer box to be moved into neutral, then clutch down to go from neutral to the other range. The transfer box is a bit agricultural so sometimes you do need to jog the gears a bit like this to move the lever, I've never needed to strain at the lever, just apply some drive, then unapply it and move the lever to neutral, that normally does the trick.
IIRC the locking mechanisium is a dog clutch if the dogs are on top of each other they can't engaged and lock the diff. When it comes to disengage if there is much torque on them they can't slide apart.
A double de-clutch with the main box in gear should enable you to change the transfer box without really horrible graunching noises. Just like every one had to drive before they invented syncromesh. B-)
L to H isn't to bad, instructions for that are in the User Manual and is required when hauling a heavy trailer. L to get you up the hill or moving then into H once over the hill or moving and want to go faster than a snail.
No. On tarmac you will get transmission wind up regardless. This is because the front and rear wheels will in practice have marginally different diameters. The mechanical coupling ensures they are rotated at the same speed but they will attempt to cover different distances - nasty! Either the tyres slip (off the tarmac) or the transmission breaks.
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