Vehicles seem to get more complicated as new models emerge, and it's difficult for the average owner to keep up with this inexorable trend. Differentials are also becoming more complex, as manufacturers try to get an edge on the one hand and make them more reliable on the other. You may have just found out that your utility vehicle or four-wheel-drive truck has a locking hub, and this could be causing issues. What is this, and how can you tell if something is wrong?
Why Lock the Hubs?
Essentially, the locking hub will disconnect the two front wheels from the vehicle's drive system when they are not required for traction purposes. In doing so, wear and tear is minimised and the vehicle should return better fuel mileage. Manufacturers fit these locking differentials to many vehicles these days, because they can be very handy in any difficult off-road conditions.
However, when new components are added, the risk of failure goes up, and in this case, you may find that the locks are simply not engaging. All kinds of mud or dirt could build up around the mechanical parts, and this may prevent the equipment from moving as it should in order to lock the hub.
Signs of a Problem
You may have noticed that something was wrong when you were trying to shift into four-wheel-drive. There may have been some grinding noises, but you may not have been able to determine where they came from. Occasionally, this can indicate something is wrong with the transfer case, but more often than not it's this locking mechanism.
Alternatively, the hub may have stuck in the "on" position and would not unlock, and this would produce a rhythmical "drumming" noise from underneath the car when you're driving on the highway. The front wheels and tyres are still engaged to the drivetrain, and this will cause higher fuel consumption and greater tyre wear on the front.
Checking the Issue
You can conduct a few simple checks to find out if you have an issue here. Find a flat piece of road and put the transmission into the "P" position, before raising one side of the vehicle up. When the vehicle is in two-wheel-drive mode, you should be able to rotate the wheel and tyre quite easily, but if you can see that the axle is also moving and much more effort is required, then the hub is engaged when it shouldn't be.
If it's free, try turning the axle shaft in the reverse direction, and this should, in turn, engage it. If you then turn everything in the opposite direction, hub and wheel should move together. To double check everything, hold the shaft in one hand and turn the wheel backwards with the other one. If all is good, the mechanism should disengage with a distinct "click." You'll be able to turn the tyre and wheel freely again.
How to Proceed
If you notice during this test that the hub is not locking or unlocking as it should, then this is a job for a differential repair expert. The whole mechanism will have to be taken apart to investigate the cause.