If you're lucky enough to have the rear set of wheels—or all the wheels—providing the motion to get your vehicle going, then you may already know about U-joints. The “U” in U-joint stands for “universal.” According to Webster's sixth definition of the word universal, this means being adaptable to many sizes or mechanical uses, or in this case: universal rotation.

U-joints transfer engine power and torque to the differential and drive wheels as the suspension travels.

Supporting Role

U-joints are usually found at either end of a driveshaft. Their purpose is to transfer engine power and torque from the transmission to the differential and drive wheels as the suspension travels. As the driveshaft spins, it has to do so as the suspension moves up and down. The universal joint allows the driveshaft to put a positive spin on engine power and handle the twist of torque, regardless of whether the road is smooth, or the going gets rough.

Spin Out

All that spinning under power eventually causes even the toughest U-joint to wear out. The heart of a U-joint is an “X-shaped” cross. The cross spins around in the driveshaft yoke, thanks to four caps that sit on top of the tips of the cross. Inside each of these caps are tiny needle bearings and high-temperature grease. Through the miracle of gravity and centrifugal force, the grease travels from inside the cross as the U-joint spins, and keeps these needle bearings happy and alive as they spin along.

Over time the grease can spin out permanently. Wear can set in, causing the needle bearings to eat into the hardened steel of the cross. Symptoms of worn U-joints include clunking when you put the vehicle in gear, or vibration that matches transmission output speeds. At best, a worn U-joint causes vibration and clunky operation. At worst, a suddenly broken U-joint can cause an accident. If a front U-joint suffers catastrophic failure, the vehicle can launch on the driveshaft much the same way a pole-vaulter uses a pole to launch into the air.

X-Ray Vision

While clunking, rattling and a rumbling vibration at speed are telltale signs of worn U-joints, there is no way to tell if a U-joint is worn by looking at it from the outside. Removing the driveline and moving the U-joints by hand can reveal crunchy movement or impending failure, but the only way to be sure that it's in tip-top shape is to replace the U-joints. Since most U-joints come from the factory full of grease, once they're done—they're done. Some replacement U-joints have grease fittings for service while others, like the OEM units, are lubed for life. Replacing U-joints is a relatively easy affair once the driveshaft is out of the vehicle. Whether the classic hammer, socket and vice method or a relatively inexpensive U-joint tool is used, the method is the same. Follow along the step-by-step guide and put the “universal” back into your U-joints.

Step 1

The first step is to secure the vehicle on jack stands and remove the driveshaft. Raise the rear to prevent transmission fluid from draining from the output shaft, or drain the fluid first.

Step 2

Index the assembly by marking or scribing before disassembly. This will preserve the driveshaft balance.

Step 3

Remove the retaining clips on all four caps. Most U-joints are supplied with new clips, so no need to be gentle.

Step 4

Line up the U-joint tool on the yoke. Turn the screw to push the cap and cross through the yoke just far enough to allow the opposite cap to be removed. Do not push the side cap all the way through the yoke.

Step 5

Remove the tool and the pushed-through cap from the yoke.

Step 6

A socket can be used to help center the cross as it pushes the other cap back out of the yoke. The socket must be slightly smaller than the cap to fit through the yoke.

Step 7

Reinstall the tool and turn the screw until the cross pushes the remaining cap back through the yoke.

Step 8

A little help is sometimes needed to remove stubborn caps. Note how the grease holds the needle bearings in the cap. Make sure the new caps have grease to hold the needle bearings in place.

Step 9

Installation is just the reverse of removal. Be certain the caps and cross are centered in the yoke before applying pressure to the U-joint tool. Also be sure all needle bearings are lined up. Set up the tool to push the caps into place.

Step 10

Turn the screw to push the cap in just far enough to clear the retaining clip groove. Repeat the procedure one cap at a time until the cross and caps are centered in the yoke. Install new retaining clips. Re-install the driveshaft.