Driveshaft and U-Joints

On rear wheel drive and four wheel drive vehicles, a driveshaft connects the transmission or transfer case to the rear differential. The driveshaft is a hollow steel or aluminum tube that spins at engine speed and transmits all of the engine's torque to the rear axle. Universal joints — U-joints — at each end of the driveshaft allow it to transmit power through the angles created by the suspension and drivetrain geometry.

A U-joint is a cross-shaped component with four arms — called a trunnion or cross. Each arm has a bearing cap pressed into a yoke. The cross pivots inside the bearing caps and allows the driveshaft to change angle as the rear suspension moves. U-joints are a wear item — the needle bearings inside the caps wear over time, especially if the grease dries out or the seal fails.

A clunk when shifting from drive to reverse or when first accelerating from a stop — the worn U-joint has enough play to produce a single distinct clunk as the drivetrain loads and unloads. A vibration at highway speed that increases with speed — a U-joint with a seized bearing cap causes the driveshaft to run out of balance. In extreme cases you can see the driveshaft wobbling under the vehicle. A severely failed U-joint can separate completely — the driveshaft drops from under the vehicle, digs into the pavement, and vaults the rear of the vehicle into the air. This is why a clunking driveshaft is never a concern to ignore.

With the vehicle on a lift, grab the driveshaft near each U-joint and try to rotate it back and forth while holding the yoke still. Any play or clicking indicates a worn joint. Also push the driveshaft side to side near each joint. Movement means bearing cap wear. Replace U-joints in pairs — if one has failed, the other has the same mileage and is likely close behind.