Here is a view of a front hub being removed. 
 

 
 
   
  Comparing a failed hub with a new one, notice the ABS sensor wire on this Cadillac hub. 

 
 
 
 
 
   
 A View of a differential. Check the service manual to see if the car you are working on has a separate differential fluid chamber with its own inspection plug. This is easy to overlook. 
 
 
 
 
 

 
 
 
 
 
 
   
  Here is an example of a listening device with 4 separate leads that can be clamped to various items under the car. While the vehicle is in motion another technician riding shotgun can wear the headset and dial-in each of the four sensor leads, one at a time, to localize a noisy component. 
 

 
 
 
 
 
 
 
 
 Electric motor style four wheel drive unit that eliminates the need for a four wheel shift lever in the cabin. These motors can fail rendering the four wheel drive system inoperable. 
 
 

For technicians who do not mind getting their hands dirty and doing a little detective work, driveline service can be rewarding and profitable. While most of us are familiar with basic driveline service, it can be helpful to look at common problems and even 
common service mistakes. Driveline service is still one area where you can be successful without a large investment in tools and equipment. 

First things first.  

Often the most difficult part of driveline service is communication. Like any automotive service, good communication can be the key to a profitable job and a happy customer. The first step, as in any service, is to obtain as much information as possible from the 
customer. Don't jump to conclusions just because you have seen the same symptoms a dozen times in the past. If at all possible, take a road test with the customer and try to duplicate the complaint. For the most part, trying to repair a noise from just a customer's explanation can be asking for trouble; their definition of 'odd' sounds cannot be as reliable as that of a seasoned technician's. Once you have obtained all the information you can from the customer, it is time to see what can be learned from the vehicle itself. 

If you are seeing this vehicle for the very first time, take a few minutes to look the car over in general. Does it look like it receives regular maintenance? Does it have a tow hitch rated for more than it could ever safely tow?  Is the vehicle rustier than normal or have unusually high mileage for its age? All these things can give you valuable information on how the vehicle is used and what caused the component failure. 

After you have diagnosed the vehicle and you're ready to call the customer with your recommendations, remember anything left unsaid leaves an opening for communication problems and assumptions. If, for example, you were going to replace a leaking axle seal, this would be the time to tell the customer he has other unrelated fluid leaks that you are not diagnosing or correcting at this time. After the repair -- when the customer calls to say something is still leaking -- it's too late! Make the customer clearly aware of any obvious problems you noticed, the then separate cost for the diagnosis and the total cost of the repair with parts and labor. Of course, you also can recommend related service or repair especially if it will save the customer money or prevent a breakdown in the near future. 

Universal joints.  

Drive shaft service on rear-wheel drive and four-wheel drive vehicles is pretty much limited to universal joint (U-joint) replacement for most shops. This does not mean everything else should be ignored, however. Driveshaft problems have two distinct symptoms that can make diagnosis easy. While tire balance problems are speed sensitive, driveshaft vibrations are not. A binding U-joint or damaged driveshaft will make a vibration that will shake the floor of the vehicle. While high speed can make it worse, it will be present at most speeds. A severely damaged U-joint can make a repeated 'chirping' or squeak at very low speeds. On many four-wheel drive vehicles the front drive shaft does not spin in two-wheel drive, so a complaint when in four-wheel drive would be the only reason to suspect a front driveshaft problem. The best way to check the U-joints for binding is simply to remove the driveshaft and feel how easily they can be rotated in both directions by hand. Don't forget that a dented driveshaft or a weight knocked off balance can create the same vibrations as a binding U-joint. 

The style of the bearing cup retention must be determined before U-joint service. Besides internal and external snap ring -- or 'c' -- clips, some OEM U-joints are secured with an injected nylon ring. This type of retainer is simply sheared off when the bearing cup is pressed out of the yoke. Nylon injected U-joints can not be reassembled once removed. When installing a new U-joint with a grease fitting, it is best to locate the fitting where it will be under compression and not tension because this is the weakest part of the joint. If the slip joint is separated for service, it must be marked to keep the relationship between front and rear U-joints the same. On modified vehicles, the operating angle of both U-joints need to be equal so they can balance out the inherent speed changes when they rotate at an angle. 

Drive axles.  

While many shops do not offer complete differential service, solid drive axles still offer some opportunities for general service. Outer axle seal replacement is pretty straightforward and easy to diagnose. Just remember that brake linings contaminated with gear oil will need to be replaced. 

Diagnosing drive axle noises can be a little more challenging. For the most part, outer wheel bearing noises can be verified by steering left and right while driving, rocking the vehicle to move the weight more left and right. Bearing noise that is noticeably worse in one direction indicates the opposite outer wheel bearing is at fault. In other words, noise on a left turn indicates a problem in the right side bearing and vice versa. Inner differential bearing noises are not affected by vehicle sway; their noise is constant and varies with vehicle speed. Gear-related noises are usually more noticeable at one speed, and the noise can be changed by varying the driveline load between accelerating and coasting. Most inner differential problems are the result of a lack of lubrication. With all the attention paid to saving time during regular maintenance, some shops do not inspect the differential fluid level or may even fail to realize many front-wheel drive vehicles have separate differential fluids. 

On four-wheel drive vehicles, winter use can provide more opportunities for service. Manual hubs and manual four-wheel drive actuation are becoming less common, so diagnosis should start with a quick look at service information to 'bone up' on specifics. You can waste time trying to figure out why an automatic hub does not engage only to find out the front drive shaft is never powered. On a push-button operated transfer case it is common for the electric motor that operates the shift linkage to bind and stick. Simple lack of use and corrosion are the most likely causes. The same can be said for four-wheel drive hubs, but good cleaning, lubricating and reassembling usually can restore their operation. 

Hub service is one area where some special tools may be needed. Some systems use a fixed hub with a central axle disconnect system to free up the front drive axle in two-wheel drive. Most of these systems use a sliding collar to connect a two-piece axle that is engaged with a vacuum-operated cable or an electrically heated thermal solenoid. Some late-model Ford trucks take a different approach to front hub actuation. These vehicles use a special sealed hub and bearing assembly that creates a sealed chamber for a vacuum operated free wheeling hub. 

On the most complicated automatic four-wheel drive systems a computer-controlled transfer case works with wheel speed sensors to control power displacement. On Ford's Control Trac four-wheel drive system, the driver can select three different modes of operation including automatic four-wheel drive. The electronically shifting transfer case consists of front and rear Hall effect style drive shaft speed sensors, electric four-wheel drive shift motor and an electric clutch. The electric clutch is used to engage the front drive shaft to allow shifts into four-wheel drive at any speed -- even when the front wheels are stopped, and when the vehicle has become stuck while still in two-wheel drive. Many four-wheel 
drive systems do not allow engagement of the front drive train until the front wheels can be rotated to allow hub activation. When in automatic four-wheel drive, the generic electronic module (GEM) controls power to the transfer case electric clutch in response to wheel slippage, as seen through the front- and rear-drive shaft speed sensors. When in low range mode, the electric clutch is fully engaged. 

Constant velocity joints.  

The popularity of front-wheel drive and independent suspension require drive axles with constant velocity joints and, thankfully, more opportunities for service. The most common service is simple constant velocity (CV) joint boot replacement: age and exposure to the weather can deteriorate a rubber boot before the joint itself needs attention. Before recommending the boot be replaced, verify the joint itself is still good. 

Constant velocity joint problems can usually be verified during a road test. A snapping or clicking noise when accelerating during a tight turn usually indicates an outer CV joint problem. Sometimes outer CV joint noises will only be heard during normal driving, as when making easy turns and under mild acceleration. When you are moving straight ahead and accelerating, a similar noise that seems to change as the suspension is cycled through dips in the road indicates an inner CV joint problem. A visual inspection for damage (like a torn boot), or listening closely as the axles spin freely on a lift, can 
help verify which joint has failed. 

Once the failed joint has been verified, the second decision will be whether to install a new CV joint or replace the entire axle with a remanufactured unit. The popularity of remanufactured axle shafts has kept the price so low it is often no more costly than a simple 
new joint, especially when you consider the extra labor required to replace the joint once the axle has been removed. When installing a remanufactured axle assembly on a high mileage vehicle, recommend that the axle grease seal be replaced. Otherwise, a leak that may subsequently result could appear to be your fault. Wheel bearing service also has changed with late model front-wheel drive and four-wheel drive vehicles. Tapered roller bearings are still used in some vehicles and for the most part they are easy to clean, inspect, repack and put back into service. One caution though: 
Do not be tempted to tighten up a loose wheel bearing in a tapered bearing application unless the bearings are to be removed and serviced first. Often the free play is an early sign that some wear and damage has already taken place. If you tighten up the clearance on these worn bearings, then total failure is sure to follow. 
Unitized wheel bearings come in many different configurations, but, for the most part, their service is limited to bearing and seal replacement. Some unitized wheel bearings are available only as an entire hub assembly, especially when the hub has provisions for a wheel speed sensor. 

Other bearings that are available separately require special tools for their removal and installation, but they usually can be serviced on the vehicle. When performing any wheel bearing service it is important to keep all bearing and hub components as clean has 
possible, and remember to lubricate the grease seal before final assembly. 

Final notes.  

Because many driveline components cannot be easily separated to verify the source of a vibration or noise, locating a failed CV joint or wheel bearing can be the most difficult part of the repair. In some instances, special listening devices or electronic microphones can be affixed to different components to help locate the source of excessive noise. Familiarize yourself with these listening devices beforehand, because these devices can alter and amplify noises that are perfectly normal. 

During any driveline-related repair remember to be aware of all related systems. Suspension components, brake and anti-lock brake system components moved or disassembled during service cannot be ignored simply because they are secondary to the repair. One vehicle speed sensor damaged or one brake rotor warped from over-tightened wheel lugs can ruin your whole day, not to mention your good reputation.