4-Wheel Drive Operation

Locking Hubs

There are two types of front hubs available for 4x4 vehicles.

• Manual (or free-running) that require the driver to get out of the cab to either lock or unlock them.
• Automatic locking (which lock automatically when the axle shaft begins to turn).

Manual (free-running) hubs feature seals that prevent entry of dirt and moisture and are very simple to operate. Automatic locking hubs use the front axle shaft rotation to actuate a cam that locks and unlocks the hub. Locking hubs may be locked at any time, and can be left in the locked position all the time, if desired.

On vehicles equipped with a manual shift transfer case (7A195) and manual locking hubs, low range can be selected when the vehicle's front hubs are unlocked. This is useful for short distances if very low, creeper gearing (but not 4WD) is needed. Refer to Section 04-01B for diagnosis, testing and repair procedures.

In 4WD, the front hubs are locked (either automatically or manually). As a result, the front wheels are no longer free to turn independently. The front and rear driveshafts are also now locked together and must turn as one.

In 4WD, just as in 2WD, each front wheel travels further than the rear wheel on its side. But this time, the front wheels aren't free to turn independently of the rear wheels. They are linked to the drivetrain by the transfer case and front driveshaft.

Points of Possible Slippage, Four-Wheel Drive

Item	Part Number	Description
1	4602	Driveshaft
2	7A195	Transfer Case
3	4602	Driveshaft
4	3B366	Locking Hub
5	4026	Differential
6	4841	Driveshaft Slip Yoke
7	3010	Front Axle
8	4841	Driveshaft Slip Yoke
9	4841	Driveshaft Slip Yoke
10	4026	Differential
11	4006	Rear Axle Assembly

Even though difference between the inner and outer front wheels is compensated for by a front differential, torsional windup is still going to occur.

• Anytime one driveline component travels further than another, there will be driveline or torsional windup that must be released.

4WD in a 4x4 provides increased drive traction and performance in off-road situations and when driving on snow or ice. It is not designed for operation on dry surfaces and when traction is good. It is not suitable on dry paved roads or dry parking lots. Where 4WD should and shouldn't be used gives us the answer as to what releases the driveline or torsional windup.

When a 4x4 is driven off-road in 4WD, the wheels are in contact with dirt, sand and sometimes mud. The vehicle is also likely to bounce over bumps and ruts.

• In turns under these conditions, driveline or torsional windup will occur. If it does, the loose dirt or sand or the slippery mud will allow wheels to slip a little, limiting the windup. Tension is also relieved when the wheel bounces in the air going over a bump. Snow and ice will also allow the wheels to slip and limit the windup.

When operating in 4WD on dry hard pavement, even more torsional windup occurs which is not easily released. When the windup becomes great enough, the driver will feel the vehicle hop, skip or bounce. This is caused by the front or rear wheels slipping as the windup is released. Whichever wheels have the least traction are the ones that will slip.

• The tighter the turn, or the greater the traction, the greater the hop, skip or bounce. This torsional driveline windup is the cause of many owner complaints concerning the performance of their 4x4 vehicles.

The following illustration shows that tires of even slightly different sizes roll different distances every revolution (see A). The same effect is true for tires that are the same size but inflated or loaded differently (see B). Operating a 4x4 in 4WD with tires of different size or inflation will produce driveline or torsional windup, even when driving straight ahead. If the vehicle is being driven in 4WD on dry, hard pavement, the driver will notice drag and may experience the hop, skip, bounce effect as the front or rear wheels release the windup.

Tire Rolling Differences Due to Size and Inflation Pressure