In an automobile, a driveshaft is the rotating steel line that transfers power from the engine to the drive train. This shaft is not in a straight line, so a universal joint (u-joint) is needed to handle the flex and the torque. The u-joint is a weak spot in the drive line, and when it goes bad, you could be left stranded.
What is a U-joint?
In a typical passenger car with two-wheel drive, there will be one U-joint at either end of the driveshaft. The purpose of a u-joint is to transfer power from the transmission to the differential. It handles the twisting and torque of acceleration and rough roads. The u-joint looks like a cross or an X, and each tip of the cross contains needle bearings that spin at high speed and under extreme temperatures. Keeping this joint well greased is essential.
Unfortunately there is no way to tell if a u-joint is going bad just by looking at it. And tearing apart your driveshaft to physically inspect the u-joint is not a task most car owners care to do without reason. Therefore, most drivers rely on noise to let them know it's time to replace the joint. One way that may be telling is to turn your front tires all the way to one side or the other and drive around very slowly in a circle. Many times, if your u-joint is going bad, a distinct "clicking" sound can be detected by educated ears.
Another way to detect when a u-joint needs to be replaced is by highway vibration. The vibration could become worse when accelerating and lessen when you back off the throttle. Of course, vibration can come from many other sources such as bad wheel bearings or loose lug nuts. Therefore, any vibration must be investigated by a qualified mechanic.
Put You Car on a Rack
If you have noticed any of the signs that your u-joint may be going bad, putting it on a mechanic's rack will tell the tale. With the car in gear and the emergency brake engaged, firmly grasp the driveshaft and try to rotate it. There should be no play in the u-joint. Even as little as 1/32 of an inch indicates immediate replacement. A good mechanic will be familiar with checking the driveshaft in this manner and can offer expert advice.
When you have verified your u-joint is going bad, don't wait to replace it. Having one explode into pieces at highway speeds presents a dangerous, if not deadly, situation. Sharp metal parts will bounce down the asphalt at speeds rapid enough to injure those on the road following you, and you very well could loose control of your car when your heavy steel driveshaft hits the ground.