Need help understanding the 4x4 functions / selections

[bfrench]

Hi, Folks,

Did the search thing and found some useful information but I'm still left with some blank areas such as what is happening when in the 2WD selection.

I'll go ahead with what I think is correct.

1. In 2WD the front axle hubs are electrically controlled and are disengaged allowing the front wheels to freewheel.

The front axles and pumpkin containing the front differential and gears are not turning.

The front drive shaft from the transfer case also does not turn because it is disconnected in the transfer case which only allows the rear drive shaft and connected axle components to turn.

2. In A4WD the front hubs are electrically engaged causing the front axle shafts and differential gears to turn causing extra drag decreasing gas mileage.

The front axle drive shaft is disengaged from the front differential until the drive system detects a differential between the front and rear wheel rotations - spin - then engages the front drive shaft completing a 4 x 4 system until the spin is no longer detected and then drops back to the front axle no longer being engaged.

3. In 4WDHI or 4WDLO - the front hubs are always engaged, the front drive shaft is always engaged and is supposed to be used in 4 wheel drive conditions only.

4. The lockable hubs are only on the front and the use of wheel spacers / wheels with other than standard offset will somewhat reduce the hub life
- how much is still open for debate until we have a bunch of operators with some serious mileage on our trucks.

Ok, guys - how well do I understand the correct workings of the 4 x 4 basic system?

Thanks,

Bill French

[RayVoy]

Bill, let's keep you after school :p and see if I can help

I'll describe the front assembly. Because of the independent front suspension we have a combination of parts to create a front axle assembly. There is a front diff that is mounted to the frame of the truck. On each side, of the diff, we have an intermediate axle, this is connected to the wheel hub with an half axle. This half axle has constant velocity (CV) joints at either end. These joints provide the flex required by the moving suspension.
The left wheel (driver's side) is always connected to the diff. When the wheel turns, the 1/2 axle turns and the intermediate axle turns, this turns a gear in the diff.
The right wheel has a disconnect on the intermediate shaft, which is disconnected in 2 wd. When disconnected, only the 1/2 axle turns with the wheel.
When the selector is placed in any of the 4 wd positions, this disconnect operates and the right front axle becomes whole. The wheel is now connected to the intermediate shaft and the diff.
The front diff is an "open" diff (the rear can be open or locked), meaning that torque will fall toward the wheel with least resistance. This is why a rear drive car will get stuck when one wheel is on ice.

Ok, let's leave the front diff for a minute and talk about the transfer case (TC). Without going into a lot of detail, select 4 hi or 4 low and the TC will connect the front drive-shaft to the rear drive-shaft. The engine turns the drive-shafts.
Select 4awd and the TC will hold off with locking the front drive-shaft to the rear one. It waits until there is detectable slippage at a rear wheel before internal clutches connect the front and rear drive-shafts.
So, when in 4awd with no slippage, the front drive-shaft turns, but because it is driven from the front wheels.

So, the purpose of the axle disconnect is, as you said, to provide a torque leak for the front open diff. By having the disconnect, the right axle has the least resistance and the torque from the turning left wheel is leaked out the right axle and the front drive-shaft doesn't turn.

You also asked about locking hubs. We don't use these, they are sold hubs and spacers should not reduce the life expectancy

Hope this has helped

[bfrench]

Bill, let's keep you after school :p and see if I can help

Hope this has helped

Thanks, Ray,

A considerable help.

Now I just need a little time to absorb it all.

Bill