A u-joint, or universal joint, is a component that connects the driveshaft to the differential, and sometimes connects the driveshaft to the transmission or two driveshafts to each other.
It’s a cross shaped piece with a bearing at each end, that allows for lateral and side to side movement of the driveshaft.
A u-joint allows for slight positions changed between the driveshaft, transmission, and differential. In other words; the driveshaft connects the transmission to the differential, but it can’t be a static, solid connection.
Since the differential moves independently from the rest of the vehicle over terrain, the transmission and differential don’t stay perfectly aligned with one another at all times. The u-joint compensates for motion, keeping the driveshaft connected and spinning freely.
Here are some symptoms of a bad or failing universal joint you may notice, in approximate order of severity:
Difficulty turning – If you’re having trouble making turns, it could be a driveshaft issue. A failing driveshaft can prevent the wheels from properly turning, making it difficult to control the vehicle.
Loud clunking noise – If you hear a loud clunking noise when shifting your vehicle, this is another sign of driveshaft damage. The likely culprit is a worn u-joint.
Car shudders upon acceleration – If you experience shuddering when accelerating from a stop or low speed, a loose u-joint or bad center bearing could be to blame.
Squeaking noise – Not just annoying, a squeaking noise at low speeds can be caused by a u-joint that needs lubricating. A little bit of grease will get rid of the noise.
Clicking or knocking noise – Can indicate a worn or failing CV joint. See your mechanic for a repair.
First of all, it’s important to remember bad u-joints only really apply to rear wheel drive cars, trucks, and SUVs. To a technician that knows what they’re looking for, a bad u-joint is obvious, since it displays specific symptoms.
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.
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.
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.
Fortunately, a u-joint is a straightforward and inexpensive fix. If it breaks completely, the driveshaft will disconnect from the vehicle, which can damage your transmission, the underside of your car, and anything else around it - plus, the vehicle won’t be going anywhere until it’s fixed.
We would hate for that to happen to our drivers, so that's why we want to provide competent repairs and service beforehand.