Thread: Please Explain !!!

Your question made me think. I know that when I change a tyre, I chock the other wheels and upon completion of the wheel change, when I go to retrieve the chocks, I usually have to kick out one side as the vehicle has tended to want to roll either forward or backward.

While I have not actually tired it, I seem to recall that if I for instance, jack both rear wheels off the ground, that if I manually rotate one wheel, the other on the opposite side rotates in the reverse direction at the same rpm.

Also with a rear wheel drive, with both rear wheels off the ground, if I were to have the engine running and in gear, say at idle speeds, if I were to hold one wheel so it would not rotate, then the other wheel would rotate at twice the speed of when both wheels were rotating. I also recall doing this with one wheel off the ground and the other still on the ground, the engine in gear and at idle - that is when you find out if you have a functioning limited slip or locking differential - not good if you do.

That could be the situation with your 3 and a locking electronic diff. All would be fine until the computers kicked in and assumed the 3 was "off road", had a wheel hanging in mid air and was for the moment stationery, but wanting to proceed. That would not be the time to be under it or have a wheel off.

These actions are a result of the internal design of an unlocked or non locking differential that allows one wheel to rotate in the same direction for a short time slightly faster than the other side, as when going around a corner. I say for a short time, as if one had a smaller wheel on one side, (wrong size spare tyre), eventually any differential would go bad.

I am not really answering the question, but I think that is just the way a differential works and hence what you are experiencing is normal. I think that is why it is called a differential rather than gear box.

Also do not be too certain that leaving a door, (any or all), open will keep the air suspension from dropping or doing the self level thing. About the only fairly certain way I know to keep the air compressor exhaust valve closed is to disconnect the battery so as to depower the computer that could tell the exhaust valve to open. It has a mind of its own and it really can drop fast. Alternatively, there are some fuses one could pull, the disconnecting the battery is for certain.

From a Luddite Defender owner... assuming you have the vehicle on the flat and NOT chocked (and not braked in any way), you should be able to move the vehicle by rotating the lifted wheel. But you do have to drive all three wheels on the ground with your one. I would assume this would take a fair effort. Go check the wheel cranking start thread. Something has to give: either engine turning or wheels. If there is enough resistance in both, then your lifted wheel won't turn either.

You cannot turn the one wheel that is off the ground because it is attached the diffs and drive line.

If you have the rear jacked up with both wheels off the ground and turn one wheel, the opposite will wheel turn in the opposing direction. This is because it is attached through the diff.

You would have noticed that vehicles that are driving and lose traction on one wheel, then lose momentum, end up spinning the wheel that lost traction uselessly. What you witness is the reverse of the phenomenon you have here. That is, the engine is rotating the wheel, but in your case you are trying to rotate the wheel by hand while the engine is not running. You could turn the wheel but in doing so you would also be revolving the drive shaft, transmission, etc. through to the engine. To do that might take some effort.

To test the theory, try starting the engine while jacked up. Put it into gear and see what happens to the raised wheel. Stop the engine and try turning the wheel by hand. Takes a lot of effort

If the transmission is in neutral you should be able to turn the wheel and drive shaft.

Then again, do the test on someone else's car :P

The clunking is the backlash in the drive line. That is supposed to be that way.

Disco2HSE is absolutely correct about the differential theory and the reason the off-the-ground wheel wont turn, but I would be very careful about testing the theory by starting the engine with one wheel jacked, especially with a D3/D4. As suggested, if you have to try it, do it on someone elses car!

There are some very sophisticated systems on D3's and D4's and if either the EDiff or the traction control kick in, you might (will probably) find the vehicle driving itself off the jack.



Not a good look!

Cheers

Al

With Tr in onroad, handbrake off, gearbox in neutral, door open, other wheels chocked there is no reason why the wheel should not turn.

Garry

Exactly Garrycol, Im afraid some people dont know how a open centre crown and Pinion type diff works . On both my RRC and my Disco1 anywheel jacked off the ground without the handbrake and the center diff not engaged will rotate . It Must !
If one looks carefully , if only one wheels is off the road then the drive goes out the tailshaft , thats if the tailshaft is free to turn ( No tailshaft hanbrake, no center diff locked up ) Whether the Transfer case or Gearbox or clutch or Sprag brake on the auto or torque convertor is engaged is irreleavent if the center diff is allowed to differentiate.
Therefore , I believe the Center diff in A D3 or D4 is placed in the fully locked position when the ignition is removed !
Add to Gary's comment
With Tr in onroad, handbrake off, gearbox in neutral, door open, other wheels chocked there is no reason why the wheel should not turn and
Engine and ignition turned off

Now ,will someone go out and jack any wheel off the ground, and the try to rotate it in either direction .With the engine turned off and the Key pulled out of the ignition ???????

no need...

on your old school vehicle to get one wheel to turn you must have it off the ground (obviously) the park brake off (if its the rear your trying to turn) and either the Tcase in neutral or the main gearbox in Neutral (or any of the drive gears for an old school auto)


IF you have the park brake on, the gearbox in gear (or an auto in park, assuming its locking pawl still works) the tcase in gear and you only jack up one wheel then its not going to turn.

IF you jack up one rear wheel it too is not going to turn, (more to do with the hand brake acting on the rear propshaft than the mechanics of the Tcase)

heres what you're missing

from what youve written

Originally Posted by 400HPONGAS

when placed in "Park" will at least lock the box to the motor so you would have to transfer torque back through the center diff . Please Explain !!!

and

Originally Posted by 400HPONGAS

Whether the Transfer case or Gearbox or clutch or Sprag brake on the auto or torque convertor is engaged is irreleavent if the center diff is allowed to differentiate.
Therefore , I believe the Center diff in A D3 or D4 is placed in the fully locked position when the ignition is removed !
Add to Gary's comment
With Tr in onroad, handbrake off, gearbox in neutral, door open, other wheels chocked there is no reason why the wheel should not turn and
Engine and ignition turned off

you've got a fundamental misinterpretation of where some bits are in the drive line...

the park brake pawl (which Im assuming is what you mean by the term Sprag brake) engages on the output shaft of the automatic which is the same as the input gear on the Tcase. In a modern electronic managed auto if you put it in park (or for some other reason it engages the parking pawl) it stays there Until a bunch of pre-requisites are met.

The Center Diff in the Tcase works the same as an open diff in the axles (with the exception of torque limiting units or that wierdo VC limited one that appeared in the rangie). Lets, just for the moment, say that theres only 3 parts to a diff and lets just call them inputs. In order for any one of the inputs to be able to turn at least one other input must be able to turn if you stop 2 inputs from turning then the 3rd one must also (excluding lash) stop turning.

now in your case, what I suspect is happening is this.

youve got one front wheel off of the ground and your trying to turn it. essentially your trying to turn the front propshaft through the front diff so your trying to turn the front of the Tcase. Cool.

now lets look at the 3 inputs of your Center Diff

We know that the rear shafts not going to turn by virtue of the fact that both rear wheels are on the ground and (in theory) chocked (preferably with the parkbrake on as well) so its not going to turn. Thats one input on your Tcase diff stopped from turning.

Youve got the front wheel off of the ground so in theory if the front propshaft turns the wheel will turn. on your Tcase diff thats the second input free to turn.

If youve put the vehicle in park the parkbrake Pawl on the auto has engaged, its now holding the input to the tcase, If the Tcase input is held assuming that the Tcase is in gear then the output is also held and the output of the Tcase is the input to the Center diff. Thats the 3rd input of the Tcase Diff stopped from turning.

cool?

Try this.

pull your disco up, leave it in gear, have someone chock the rear wheels, turn the engine off and without removing the key leave a door open and then jack up the wheel and see what happens.

If I recall correctly the following conditions need to be met for the auto to disengage the parking pawl once its been put in park. (excluding the emergency tow conditions)

There must be battery power, The security system must be in the disarmed condition, the key must be in the IGN on position (or for keyless start the computer must be in the engine run/start condition) with the steering unlocked the foot brake applied AND the auto must not be in the park position. In the case of Some autos (and I dont remember if this applies to the landies) the engine must also be running, the Auto must have oil pressure
and the park brake needs to be engaged

Of all of those conditions only one of them relates to protecting the Auto from damage and thats the auto must have oil pressure, the rest are to protect the operator and the vehicle from having an accident by way of operator stupidity thus protecting the manufacturer from being sued because you cant prosecute or defend against prosecution with "that person is an idiot"

which is why I decided to keep it brief