Constant velocity joints (CV joints) are found on many cars and even some truck today. Though vehicle manufacturers are reintroducing rear-wheel drive on more new cars, over 80 percent of all the cars on the road today are front-wheel drive, and every one has four CV joints. Most minivans also have FWD, and most all-wheel drive (AWD) cars such as Subaru, Audi, Mitsubishi Eclipse and others have halfshafts (axle shafts) with CV joints both front and rear. The same goes for the rear axle shafts on a growing number of SUVs.
The CV shaft replacement market is estimated to be around 10 million shafts a year. According to Babcox Market Research, 92 percent of repair shops service CV joints and shafts, and do an average of 8.3 jobs per month. The average job ticket is $190.65, which implies one one axle shaft is being replaced, not both as if often recommended when a CV joint fails. Replacing only one shaft is like a dentist replacing only half of a set of dentures. If a CV joint on a high-mileage vehicle has failed, chances are its twin on the opposite side is nearing the end of its service life, too, so both axle shafts should probably be replaced. The outer CV joints are usually the ones that most often need to be replaced for two reasons. One is that the outer CV joints wear more than the inner CV joints because of the steering angles they experience. The other is that the boots on the outer joints are more apt to fail than the ones on the inner joints.
CV JOINT BOOTSA boot failure is bad news for any CV joint because it dooms the joint to premature failure. A split, cracked, loose or torn boot will throw grease, draining the joint of its vital supply of lubricant. Sooner or later the joint will run dry which is not a good thing for metal-to-metal surfaces that must withstand high pressure loads and constant friction. A boot that doesn't seal can also allow outside contaminants, such as road splash and dirt, to enter the joint and wreak havoc on its precision machined and polished surfaces. If the boot problem isn't discovered almost immediately, joint failure will usually follow within a few thousand miles. As long as a CV joint remains sealed inside its protective environment, it will do its job until it wears out. But real world driving creates conditions that can cause bad things to happen to good boots. Age, heat, cold and road hazards can all conspire to breech the protective barrier provided by the boot around the joint. And once the seal is breached, trouble quickly follows. This is why you should always inspect the boots around both the inner and outer CV joints anytime you are under a vehicle for other maintenance or repairs. Checking the CV joint boots should be part of every oil change, every brake job, every alignment job, every steering/suspension repair and every exhaust repair.
CV JOINT INSPECTIONUnder normal operating conditions, CV joints and boots are engineered to last upwards of 150,000 miles. Some go the distance, but a lot reach the end of the road far short of their design life. According to one major aftermarket supplier of replacement axle shafts, CV joint shafts are typically being replaced at anywhere from 70,000 miles to 130,000 miles. What's more, some makes and models of vehicles are notorious for eating shafts. Subaru, according to this source, is one vehicle that has a higher-than-normal replacement rate. The aftermarket supplier we talked to blamed the problem on the relatively thin case hardening that Subaru uses in their CV joints.
CV JOINT BOOT FAILUREIf you're lucky and catch a bad CV joint boot before any contamination or damage to the CV joint has occurred, you may be able to save the joint. The first thing you need to do is to check the grease inside for contamination. If it feels gritty, the CV joint will have to be cleaned and inspected before the boot is replaced. If the CV joint has lost its grease and is making noise, it is too late. The CV joint has failed and must be replaced. Cleaning a CV joint while it is still on the vehicle is difficult. There are aerosol solvents and similar products for this purpose, and cleaning in place obviously saves the labor of pulling the shaft. But the shaft will have to come out anyway if you are replacing the damaged boot with a new one-piece boot. Split-boots are an option here, and save time because you do not have to remove the shaft or CV joint to replace the boot. But the seam must be glued carefully so it forms a leak-free seal. A premium quality one-piece boot is the best alternative for replacing a damaged OEM boot. Premium CV joint boots made of materials other than neoprene or hard plastic typically retain greater flexibility at cold temperatures (making them less apt to crack), and can also withstand higher temperatures, too.
REPLACE CV JOINTIf you opt to replace a damaged boot, the CV joint should be removed from the shaft, disassembled and inspected for wear or damage. On most applications, the outer CV joint is held on the shaft by a snap ring or a lock ring, but some, such as Honda and Toyota can be tricky to remove. And if you run into a tripod outer CV joint on an old Toyota Tercel or Nissan Sentra, disassembly is not possible. The entire shaft assembly must be replaced. Rzeppa-style CV joints can be disassembled by tilting the inner race to one side and inserting a dowel or similar tool into the splines of the inner shaft. Tilt the race as far as it will go to one side to expose one of the balls. Remove the ball from its cage window with a small screwdriver. The inner race can then be tilted to the opposite side so the next ball can be removed, and so on until all the balls have been removed. The cage can now be rotated sideways to remove it and the inner race. Look for nicks, gouges, cracks, spalling, roughness, flaking, etc. on the surface of the balls or tracks in the inner and outer races. The cage windows should also be inspected for dimples, wear or cracks. Each ball should fit snugly in its respective cage window because looseness here is what often causes the clicking or popping noises associated with a worn CV joint. NOTE: CV joints are precision fit assemblies. The balls should be kept in order so they can be reassembled in the same grooves and cage windows as before. Each ball and track develop a unique wear pattern, so don't mix them up. If the CV joint shows no unusual wear or damage, it is OK to reassemble and repack with grease. Use the special CV grease provided with the replacement boot (never use any other type of grease!), and pack 1/3rd into the joint and place the remainder in the boot. To install the boot, slip it onto the shaft (large end out). Then push the CV joint onto the shaft until it clicks in place or until the snap ring can be locked in place. Pull the outer lip of the boot over the CV joint housing so it lines up with the recess in the housing. Make sure the boot is not crimped, twisted or collapsed, then install the clamps. Some types of clamps require special tightening/crimping tools, while others do not.
SYMPTOMS OF CV JOINT FAILUREBad boots are not the only thing you need to look for. You also need to listen for noise or complaints that might indicate a CV joint problem. These include:
CV JOINT REPLACEMENT TIPSSince most technicians today opt to replace the entire shaft rather than individual CV joints, here are some suggestions that can help avoid problems later:
More Front-Wheel Drive & CV Joint Articles:CV Joints & FWD shafts, part 1: Basics
CV Joints, part 2: Service & Repair
How CV Joints are Remachined
Front-Wheel Drive Guide
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