Advance Essentials: Drivetrain Basics—CV Axle Shafts and CV joints


Every vehicle’s drivetrain uses a drive shaft to transfer torque from the transmission to the wheels. The drivetrain of a rear-wheel drive (RWD) vehicles use a single drive shaft between the transmission and the differential. The drivetrain of a four-wheel drive (4WD) vehicle uses two drive shafts: one drive shaft between the transmission and rear differential, and a second drive shaft between the transfer case and the front differential.
Traditional drive shafts for RWD and 4WD vehicles use a universal “U” style joint. This U-joint allows the drive shaft to flex up to a 20-degree angle — needed for driving up and down over bumps. For the drivetrain of front-wheel drive (FWD) vehicles, more flex is needed because the front wheels need to turn left and right by as much as a 50-degree radius. That’s why car manufacturers use Constant Velocity (CV) joints at both ends of the drive shaft (called a CV axle shaft) of the drivetrain of FWD vehicles. In addition to providing greater turn angles, the CV axle shaft also reduces vibration during turns.  

The CV joints of the drivetrain are constructed with two halves that rotate around steel ball bearings in a cage. The entire assembly is filled with grease and sealed with a pleated rubber boot. The rubber boot is the most-common failure point. As it ages, the pleats crack and centrifugal force spins out all the grease. Then road grit gets inside the joint, which coat the moving metal parts, resulting in powdered metal. Don’t ignore the telltale signs of CV axle shaft boot failure, like grease stains throughout the inside edge of your tire and suspension. A cracked boot can destroy a CV axle shaft in less than a week so replace it immediately. You can also damage your drivetrain’s CV boot by driving in deep snow or getting stuck in the sand, so inspect the boot regularly, especially if you’ve been off-roading.

Naturally, even if the boot is intact, a CV joint can still wear out. The outer joint (the one next to the wheel) wears out far more often than the inner joint (located next to the transmission). When it wears, the CV axle shaft makes a metallic popping or clicking sound on hard left or right turns. Or, it can cause a “shudder/vibration” or a clunking noise when you accelerate or decelerate. Some worn CV axles can make a humming or growling noise that mimics the sound of bad wheel bearings.

What are your replacement options? If you notice a ruptured CV axle shaft boot early, you may be able to install a new boot kit. To see if your CV joint is a candidate, stick your finger into the remaining grease (with the engine off). If it feels gritty, the joint is toast. Replace the entire CV axle shaft. If the grease is smooth, replacing the entire shaft could be the better option. After all, a complete rebuilt CV axle shaft with a rebuilt inner and outer CV joint would only cost slightly more than new boot kits. Plus, to replace the boots, you have to remove the entire shaft that’ll take twice as long to do and save you only a few bucks. Our advice? Have your mechanic replace the CV axle shaft with a new CV axle shaft or rebuilt unit. But if you’re a bound and determined DIY’er, we have the extra parts you’ll need, including a special clamping tool, new boots, and a new axle shaft-retaining nut.Removing and replacing the CV axle shaft

Every vehicle has a different method for removing the CV axle shaft from the transmission. Refer to a shop manual to find the right method for yours. Once you have the new CV axle shaft in place, make sure it’s fully seated in the transmission before you torque down the retaining nut. If it isn’t properly placed, it’ll make a horrible racket as you drive and may actually pop out during a sharp turn or large bump. Don’t forget to replace the nut!

The outer end of the CV axle shaft is usually held in place with a CV axle shaft nut. Some are fixed in place with a cotter key. Those nuts can be reused. But crimped nuts and “torque prevailing” nuts (look like they’ve been “squashed” and therefore now out-of-round) cannot be reused. If your vehicle uses either of these nuts, a replacement nut should be packaged with the rebuilt CV axle shaft — check for it before you leave the store. If you decide to replace just the CV boot, make sure you buy a replacement axle shaft nut.

Now that you have the necessary parts, be sure to consult your owner’s manual and follow the manufacturer’s recommendations to ensure warranties are not voided.Recommended tools Keep each of these four by your side before you begin any replacement within the drivetrain system.

 

Torque wrench: Removes ball joint and strut fasteners, plus the axle shaft nut. It will also be used to reinstall each fastener. We advise two torque wrenches, one for smaller nuts and bolts, and one that can handle up to 200-ft/lbs for the axle shaft nut.

Pry bar: If your CV axle shaft is held in place by “C” clips, you’ll need a long-handled pry bar to pop it out. You’ll also need a pry bar to move down the lower control arm, disengaging the steering knuckle from the ball joint.

Axle shaft nut: “Torque prevailing” axle shaft nuts are slightly distorted or oval-shaped when new. During use, they are tightened down, with the axle shaft forcing the nut into a more perfect circle. Once you remove this style of nut, discard it and replace it with a new nut. Do NOT reuse these types of fasteners.

CV boot clamp pliers: If you opt to replace just the boot rather than install a rebuilt shaft, you may need a special “ear” style of crimping pliers or a banding clamp tool.