Universal joints—or U-joints—are located on both ends of a driveshaft on rear-wheel-drive (RWD) vehicles, or they could be used on both ends of the half-shafts. As the suspension travels up and down, they allow height misalignment between the differential and the transmission, and between the wheel hubs and the transaxle.
On some applications, they have been replaced with the more efficient constant velocity (CV) joints. A U-joint is an equal-legged cross with needle roller bearings on its four ends covered with a cap that retains grease for the bearings. A driveshaft or a half-shaft will have two U-joints—one on either side —or it may have a U-joint on one end and a CV joint on the other. The driveshafts are balanced for smooth, vibration-free rotation. The half-shafts, because they’re shorter, may or may not be balanced from the factory.
A symptom of an unbalanced driveshaft is a high-frequency vibration since this part rotates three to four times faster than the wheels. Even slight play on the needle bearings will allow extraneous movement of the shaft as it rotates, disturbing its precise balance. Well-worn bearings will make a characteristic squeak as the vehicle is starting to move forward or backward.
An audible clunk when shifting direction—from reverse to forward, for example—means there is excessive leeway on the motion of the driveshaft. The misalignment can be severe enough to damage the output shaft seal on the transmission, the differential shaft seal, or the transaxle output seal, in which case you may see oozing transmission or differential fluid.
Physically moving the joints with a pry bar will confirm that they are loose. If they are tight, look for other causes presenting the same problem signs. While you’re under, you may see grease around the joint that’s flung outside the needle bearings’ caps. If you notice it early while the joint is still tight, regreasing it is possible if the U-joints come with zerk fittings for a grease gun. Most original equipment U-joints have sealed bearing caps. When there’s no way to replace the grease, the bearings will wear out soon enough.
Replacing the entire driveshaft is the easiest but most expensive fix. If compatible U-joints are available, it will be worth the effort to replace all of them. A replacement U-joint with a zerk fitting will be a worthwhile upgrade, granted there’s room to work a grease gun around the joint.
If an applicable U-joint can be found, read on:
1) Lift the vehicle.
Park on level ground, leave the transmission in neutral, apply the handbrake, then put wheel chocks on both sides of the wheels. Jack up the car and settle all four corners on jack stands.
2) Remove the driveshafts and/or half-shafts.
Before unbolting, mark with paint the alignment and orientation of the shaft relative to the transmission and the differential flange. The driveshaft is balanced to spin correctly from the assembly line. It’s important to restore it exactly how it was assembled.
3) Disconnect the U-joint from the yokes.
The needle roller bearings are locked in with retaining clips on the outer side of the bearing caps. On other U-joints, there are C-clips on the inner bearing races, so you will need to unfasten all the clips first.
A bearing press will make the removal and installation of the U-joint effortless. Hammering it out gently will work, but it will require skillful finesse. Once one side juts out of the yoke’s hole, pull out the bearing cap. Press it out to the other direction and remove the opposite cap. Do that for all four bearings and the U-joint should come loose.
4) Press in the new joints.
Remove surface rust, especially where the U-joint mates with the yokes and the grooves for the retaining clips. Make sure all the needles on the roller bearings are complete and greased adequately. Do the exact opposite of removing the joint: Press the joint ends as smoothly as you pulled them out. Secure all retaining clips or C-clips.
5) Reconnect the driveshaft or half-shaft.
Align the markings you painted on the shaft or flanges. Tighten all bolts and nuts to specified torque settings.
6) Go on a test drive.
Verify that the clunking and vibration have disappeared. If you feel or hear none of it, then you’ve successfully completed the repair.