ASE P2 TEST PRIMER: Drivetrain Components

By Larry Carley

Driveshafts may be one piece or two pieces with a center carrier support bearing, and have either U-joints or CV-joints on the ends.

Drivetrain components include driveshafts (that connect the transmission and differential), half shafts (that connect a transaxle to the wheels), U-joints, CV-joints and four wheel drive systems.

Driveshafts may be one piece or two pieces with a center carrier support bearing, and have either U-joints or CV-joints on the ends. The joints allow the driveshaft to follow the vertical motions of the suspension. Each U-joint consists of a four-point center cross with needle bearing cups mounted in a pair of yokes. Most late-model U-joints are sealed and require no maintenance, but some have great fittings that allow periodic lubrication with chassis grease.

A worn U-joint may cause a vibration at speed or make chirping noises, or produce a clunk when putting the transmission into gear. Replacing a U-joint usually requires using a press to remove and install the joint on the yokes.

CV-joints are used on Front-Wheel Drive (FWD) halfshafts because the outer joints have to accommodate high operating angles that occur when the front wheels are turned to either side. The inboard and outboard CV-joints on a halfshaft may be a different design. Ball-style CV-joints that are typically used for outboard joints include Rzeppa, crossgroove and double-offset. Tripod joints with three roller trunnions are most often used for inboard joints, but may also be used for outboard joints.

CV-joints are sealed inside a rubber or plastic boot and filled with a special CV-joint grease. No maintenance is required, but if the boot is damaged or leaks, the CV-joint may have to be cleaned and rebuilt or replaced. Typical symptoms of a failing CV-joint are a clicking or popping sound when turning. Failing inner joints may cause a clunk or vibration when accelerating. CV-joints may be replaced separately, but a complete halfshaft assembly is faster and easier to install.

Replacement CV-joints and halfshafts must have the same number of splines as the original, and be the same length as the original. On ABS applications, the wheel speed sensor rings must also be the same. A new outer hub nut is also recommended when replacing a shaft.

Other driveline parts that may need to be replaced include the wheel and axle bearings. On older vehicles and some 4WD trucks, the wheel bearings may be serviceable and require periodic inspection, cleaning and repacking with special wheel bearing grease (never ordinary chassis grease). But on most vehicles, the wheel bearings are sealed and have to be replaced as a cartridge or as part of the hub assembly if bad. Worn or loose wheel bearings can cause steering wander and noise.

Required to pass this section of the P2 test:
1. Identify major drivetrain components.
2. Identify component function and common reasons for replacement.
3. Identify related items, including lubricants.
4. Provide basic use, maintenance, installation and warranty information.

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