Adapted from an article written by Larry Carley for Brake & Front End magazine
Four-wheel drive (4WD) locking hubs are one of the most neglected items on 4x4 trucks and SUVs. Automatic or manual locking hubs are used on many part-time, and even some full-time 4WD vehicles, to reduce friction when 4WD isn't needed. If the front wheel hubs are disengaged from their driveshafts when the vehicle is in 2WD mode, it can reduce friction improving fuel economy and the longevity of the front axles, differential and tires.
Manual hubs allow the axles to be disconnected from the wheels. They come in various designs and varieties, but essentially have two settings: locked or unlocked.
On a vehicle quipped with manual hubs, the vehicle has to be stopped so the driver can get out and twist a dial on each hub to switch them from 2WD to 4WD. This locks the hubs to their axles so the axles can turn the front wheels.
It is up to the driver to unlock them. He'll have to stop, get out and reset the dials to the unlocked (free) position. Some manual hubs can be locked by stopping, shifting into 4WD, and then backing up a short distance.
With automatic hubs, there is no need to stop or get out of the vehicle. The spring-loaded hub automatically engages on the fly when the transfer case is shifted from 2WD into 4WD. The application of torque to the axle causes the mechanism inside the hub to slide in and lock. This is a much more convenient setup for occasional off-roading or when road conditions are rapidly changing.
Automatic hubs should remain in the locked position as long as the transfer case is in 4WD, even if the vehicle is being driven in reverse or coasting down hill. But some automatic hubs have to be manually locked to prevent slipping when engine braking is being used on a downgrade. Most automatic hubs will only unlock when the transfer case is shifted back to 2WD. Some types of hubs also require the vehicle to come to a halt and be backed up about six feet before they will disengage. As long as the transfer case remains in the 2WD mode, the front wheels are undriven. When shifted into 4WD, the unlocked hub automatically slides into position and locks the axle so both front wheels become drive wheels.
Some hubs use engine vacuum to engage the front hubs. It is critical to look for possible vacuum leaks at the hubs. In some cases, like KIA Sportage models, leaking seals at the hubs can cause the engine vacuum to suck in contaminates into the wheels bearings. Also, vacuum lines may leak or become blocked. Some aftermarket suppliers offer a kit that can replace failure prone OEM vacuum and electric front axle engagement systems. The cable system works like hubs with full control from inside the cab without stopping and reduces front differential wear and right axle breakage. For serious off-roaders, the kit has the advantage of not disengaging when ignition is turned off or the differential is submerged.
If a vehicle has been equipped with a locking front differential, like a Detroit Locker, some type of manual or automatic locking hub is usually required. A locked differential is great for serious off-roading because it keeps both front wheels turning regardless of traction. But it can make a vehicle difficult to handle or steer when driving on dry or wet pavement, or even in snow, loose dirt or sand. Unlocking one or both hubs allows the front wheels to turn at different rates for easier steering and turning when maximum traction isn't needed.
Locking hubs are often the weak link in a 4WD drivetrain. It's not unusual for hubs to fail and remain in the unlocked position. If a locking hub fails to engage, the front wheels won't pull when the transfer case is shifted in 4WD and there may be some grinding or whizzing noises indicating slippage inside the hub. Sometimes the hub making the noise is actually the "good" hub because the other hub is not engaging.
Ford Rangers, in particular, seem to have a lot of problems with their automatic locking hubs.
Some experts say the best way to prevent hub problems is to clean, inspect and lubricate them every two years or 24,000 to 30,000 miles. If a vehicle is being driven extensively off-road or through hub-deep mud or water, the hubs should be serviced more often.
Servicing a locking hub requires some disassembly. The outer hub cover is usually attached to the hub with five or six Torx screws. Once the cover has been removed, you can loosen and remove the rest of the clips and/or fasteners that attach the locking hub to the wheel hub. Follow the disassembly procedure in the manual and pay close attention to the order these parts come out and their alignment. If the wheel bearings need to be cleaned and repacked, this would be a good time to do that job, too.
Inspect the condition of the splines inside the hub as well as those on the axle, also the drag sleeve and locking hub bearings. Clean and lightly coat with the recommended type of lubricant (ATF, wheel bearing grease, multipurpose grease, white lithium, etc.). Do not pack the locking hub full of grease.
For reassembly, check the preload on the inner wheel bearing lock nut, and adjust to specifications (typically .003 to .010 inch of end play). Then reassemble everything in reverse order.
If the vehicle has Warner-style automatic locking hubs, the drag sleeve retainer washer is installed between the inner and outer lock nuts. The tang on the inside diameter of the washer is assembled on the keyway of the axle shaft. When assembled, one of the holes in the drag sleeve washer must align with the pin protruding from the bearing adjustment nut.
Once this has been done, the outer bearing lock nut can be torqued to specifications (typically 150 to 200 ft. lbs.). Then the splined spacer can be slid onto the axle shaft against the end of the spindle. The automatic locking hub is then slid into place using a clockwise rotation to align the splines and the notches in the drag sleeve and washer. When these parts are aligned, push in on the hub housing and then pull it out so the wire retaining ring seats into the internal groove in the wheel hub (listen for a snap).
Next, rotate the wheel counterclockwise to unlock the automatic hub. Install the spacer and bolt on the end of the axle shaft and tighten to specifications (typically 90 ft. lbs.). The purpose of this bolt is not to hold the hub in place, but to prevent the axle shaft from moving inboard allowing moisture to get inside the hub.
Check to make sure the rubber sealing ring is properly positioned on the outer housing and against the end of the wheel hub. Then install the bearing in the end of the hub sleeve, followed by the cover and spring assembly. To help keep out moisture, the joint between the hub cover and wheel hub can be sealed with a small bead of RTV silicone.
To check the operation of the automatic hub after it has been installed, shift the transfer case into 2WD. The wheel should turn freely, but not the axle if the hub is disengaged. Reach behind the wheel and turn the axle shaft counterclockwise at the U-joint to lock the hub. Now the wheel and axle should turn together when either is turned clockwise. Reverse the direction of rotation of the wheel. The hub should make a snapping noise as it disengages and once again allows the wheel to turn freely.
4WD LOCKING HUB REPLACEMENT OPTIONS
If an automatic hub has failed, the only fix is to replace it. The question is, with what? Some off-roaders say automatic hubs are too weak and can't take the punishment. So they replace their stock automatic hubs with manual locking hubs that are available from various aftermarket suppliers.
Manual hubs are more reliable and less apt to fail, they say, but manual hubs can't be engaged or disengaged on the fly. So if you want to change the type of hubs that are on your truck, you should understand the differences and be willing to trade one set of advantages for another.
Trucks that are factory equipped with manual hubs can also be converted to automatic hubs. In some cases, the end of the axle may be drilled for the hub attachment bolt. But the conversion is relatively easy.
SLIP OR NO SLIP?
One thing you may not know is that your wheels can still slip even when driving in 4WD. Unless a differential is a limited-slip type or has a viscous clutch pack, one wheel can break traction and spin on a slippery surface. This can happen to either the front or rear wheels (or both). But usually your truck will have at least one front and one rear wheel turning to pull it along. Even so, you may think a slipping wheel means you have a bad hub or transfer case. But actually this is normal and does not indicate a problem.