Article > Tech

ASE P2 Test Prep: Driveline

By Larry Carley

Information to help parts professonals pass the ASE P2 test.

Joints used in the driveshafts of rear-wheel drive vehicles and many four-wheel drive trucks are called U-joints. Universal joints (also called "Cardan" joints) are located on each end of the driveshaft. Vehicles with a two-piece driveshaft typically have a center carrier bearing with one or two additional U-joints. The joint itself has a double yoke and four-point center cross with needle bearing cups. The swivel-action of the joint allows the angle of the driveshaft to change as the suspension moves up and down. Most original equipment U-joints are sealed, but some have grease fittings. A worn U-joint may cause a vibration at higher speed, or it may make chirping noises or produce a clunk when putting the transmission into gear. A U-joint failure may allow the driveshaft to fall out of the vehicle. Replacing a U-joint requires removing the driveshaft, pressing the old joints out of the driveshaft yokes and pressing in the new joints.

The driveshaft that connects the transmission to the differential on a rear-wheel drive vehicle, or it connects the differential or transfer case on a 4x4 truck to the wheels. Driveshafts usually have U-joints on both ends, but have a CV joint on one or both ends. An out-of-balance driveshaft can cause speed-related vibrations. A driveshaft must be replaced if it is bent (too much runout).

A type of joint used mostly in front-wheel drive cars that allows the operating angle of the joint to change without affecting the speed of the shaft or wheel is the constant velocity joint. The basic types include Rzeppa, crossgroove, double-offset and tripod.

Rzeppa, crossgroove and double-offset CV joints have six small balls housed between an inner and outer race. A cage with windows holds the balls in position as the joint flexes. Rzeppa CV joints are most often used as the outer joints on the halfshafts, and are "fixed" (they do not slide in or out).

Crossgroove and double-offset joints are designed to "plunge" in and out to accommodate length changes in the halfshafts as the suspension moves up and down, and they are often used as inner joints on halfshafts.

Tripod joints contain no balls but have three roller bearings mounted inside a "tulip" or "claw" shaped housing. Tripod joints may be fixed or the plunge variety. Tripod CV joints are mostly used as the inner plunge joints on domestic FWD applications, but they may also be used as outer joints on certain import applications.

CV joints are packed with special grease and are sealed by a protective rubber or plastic boot. Boot damage can allow the loss of grease and joint contamination with dirt or water, which could lead to joint failure. A typical symptom of a failing CV joint is a clicking or popping sound when turning. Failing inner joints may cause a clunk or vibration when accelerating. CV joints may be replaced separately or as part of the halfshaft assembly. If only the joint is being replaced, make sure the new joint matches the splines, shaft diameter and overall length of the original. If the vehicle is equipped with ABS, the sensor ring on the replacement CV joint must also be the same as the original.

Driveshafts in front-wheel drive cars, and some all-wheel drive cars are called halfshafts. The shafts connect the transaxle to the wheels, or the rear differential to the rear wheels. Halfshafts have CV joints on both ends and are often replaced as a complete assembly. Replacement halfshafts may have new or remanufactured CV joints on one or both ends. Right and left side shafts are usually different and are not interchangeable. The replacement shaft must be the same length as the original, with the same CV joint shaft and splines as the original. A new outer hub nut should be installed when replacing a halfshaft.

Rear axle bearings are used on the rear axle shafts of rear-wheel drive vehicles to reduce friction. Located near the outer end of the shaft, the bearing should be replaced if it makes noise, is loose or turns roughly. Bearings are a press fit on the axle.

Wheel bearings are used on all wheels to reduce friction. Either ball or roller bearings are used. Most are sealed construction and require no maintenance (greasing or adjustment). But many wheel bearings on older vehicles are serviceable and require periodic cleaning and repacking with wheel bearing grease. Worn wheel bearings can cause steering wander and noise. Sealed versions are replaced as a unit. The serviceable variety are replaced as matched sets (race and bearings). Inner and outer bearings are different. Old grease seals should not be reused because they can leak. On some vehicles equipped with ABS, the wheel speed sensor is part of the hub and bearing assembly. If the sensor fails, the entire assembly must be replaced.

Wheel bearing grease is a special high-temperature grease containing lithium and other additives to withstand high pressures and temperatures. Different types of grease should not be intermixed.

Gear oil is a heavy-weight oil for differentials and manual transmissions (85W-90, 75W-140, etc.).Differentials with "Posi-Traction" or limited-slip require a gear oil with special additives. Follow manufacturer's viscosity recommendations. Synthetic gear oils flow more easily at low temperature to reduce friction.

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