Universal joints (“u-joints” for short) are components of the driveshaft assembly fitted to most rear-wheel drive trucks, four-wheel drive trucks and SUVs, or off-road vehicles. U-joints, which are found in pairs on the driveshaft, compensate for the height misalignment between the transmission and the rear axle, all the while transferring power to move the vehicle. This allows each end of the driveshaft, and its associated universal joint, to bend with each rotation of the driveshaft in order to handle the misalignment (as a side note, rear-wheel drive cars now mostly utilize constant-velocity joints for the same purpose, which permit far smoother flexibility of drive shaft rotation).
Here are some symptoms of a bad or failing universal joint you may notice, in approximate order of severity:
1. Squeaking noise when starting to move (forward or reverse)
The bearing components of each universal joint are greased at the factory, but may not have a grease fitting to allow further lubrication after the vehicle is put into use, limiting their service life. Since the bearing portion of each universal joint twists a small amount with each rotation of the driveshaft (but always in the same location) the grease can evaporate, or be thrown out of the bearing cup. The bearing becomes dry, metal-to-metal contact is made, and the u-joint bearings will squeak as the drive shaft rotates. The squeak usually cannot be heard once the vehicle moves faster than 5-10 mph because of other vehicle noise. The squeak is a warning that the universal joint should be serviced by a professional mechanic. You can certainly extend the life of your existing u-joints by doing so.
2. “Clunk” with a ringing sound when shifting from Drive to Reverse
This noise usually indicates that the universal joint bearings have enough excess clearance to allow the driveshaft to rotate slightly, then come to a hard stop, when power is reversed. This can be the next stage of deterioration after having insufficient grease in the u-joint bearings. Servicing, or greasing the u-joint bearings, won’t reverse damage to the u-joint, but may extend its life somewhat.
3. Vibration felt throughout the vehicle while moving forward at speed
This vibration means that the universal joint bearings have now worn enough to allow the driveshaft to move outside its normal rotational path, causing imbalance and vibration. This will be a higher frequency vibration than that of an out-of-balance wheel for example, since the driveshaft turns 3-4 times faster than the wheels. The worn universal joint is now causing damage to other components of the vehicle – including the transmission. Replacement of the universal joint by a professional mechanic is certainly in order at this point to prevent further damage. Your mechanic should, when possible, select quality replacement u-joints with a grease fitting, allowing for long-term preventative maintenance, and enabling extended life of the u-joint bearings.
4. Transmission fluid leaks from the rear of the transmission
Transmission fluid leaking from the rear of the transmission is often the result of a badly worn universal joint. The vibration noted above has now worn the transmission tailshaft bushing and damaged the transmission output shaft seal, which then leaks transmission fluid. If fluid is suspected of leaking from the transmission, the transmission should be inspected to determine the source of the leak, and the appropriate repair made.
5. Vehicle cannot move under its own power; driveshaft dislocated
You've probably seen it before: a truck on the side of the road, with the drive shaft lying under the vehicle, no longer attached to the transmission or rear axle. This is the ultimate failure mode for a universal joint – literally breaking, and allowing the drive shaft to drop to the pavement, no longer transferring power. Repair at this point will include much more than the universal joint, and may require full driveshaft replacement or more.