The vehicle's drive shaft is equipped with a u joint at each end so that the rear axle can move up and down without causing the shaft to bend or snap. In older vehicles, the universal joint can be removed from the shaft, disassembled, checked, and lubricated for maintenance. In modern cars, on the other hand, the joint comes permanently lubed.
Over time, the u-joints in your driveshaft will wear out. When this happens, you should have it replaced as soon as possible or you may run the risk of damaging the drive shaft, which calls for more expensive repair or replacement. But before you purchase a replacement unit, you'd better make sure that the u joint is really the cause of any problem you're experiencing in your vehicle.
Here are the ways to find out if your vehicle's universal joint has failed:
Drive your car to check for unusual u joint noise and vibration.
Take your vehicle out for a drive, preferably on a not-so-noisy and crowded street, so you can listen to any unusual sound your ride will make. If you notice squeaking sounds from under the vehicle, that means your u joint lacks lubrication or has already dried out.
Also pay attention to any vibration that may be caused by a worn-out universal joint in your drive shaft. Vibrations should not be taken for granted because once they occur, it won't be long until the joint will stop working.
However, squeaking noise and vibrations from underneath the vehicle can also be caused by other automotive issues, so you'd better do some more checks.
Test the u joint's performance when the vehicle is parked.
Pull over in a safe place. With your engine running, depress the brake pedal and shift the transmission from park to reverse. While shifting from one gear to another, listen for a clanking or banging noise. If you feel that the gear jumps as it bangs, that means you have a loose u joint in there.
Turn off the engine and set the gear in park. Chock the wheels to ensure safety and turn the drive shaft. While twisting the shaft, take note of any play in the u-joints. If the joints are loose, the shaft will move from about a quarter inch to half-inch, in either left or right, when turned.
Do a visual inspection.
Visually inspecting the u joint will be easier if you drive your vehicle on dual ramps or have it raised just so you'll have more room on the underside. Take a flashlight with you as you go underneath your ride and look into the differential yoke as well as the yoke connecting the drive shaft to the transmission. Inspect the yokes for rust, broken or chipped bearing caps, as well as missing or misplaced retaining clips and bearings. These are clear signs of severely worn-out u-joints.
Rid the u joint of mud and dirt build-up and re-weld any cracked weld if there's any. Get a large, flat screwdriver and insert it into the yoke that connects the drive shaft to the transmission or differential yoke. Push up or pull down on the screwdriver to turn the shaft. Any movement in the drive shaft signifies a loose joint.