Universal joints (u-joints) connect rotating shafts aligned at different angles. In many vehicles, they transfer power by connecting the driveshaft from the transmission to the rear drive axle.
U-joints respond to motion in the suspension, compensating for the difference in height between the rear axle and the transmission. These critical components are common in vehicles with rear-wheel and all-wheel drive, like trucks and SUVs. Some smoother rides replace u-joints with "constant velocity" joints, which offer even greater control.
U-joints tend to be highly durable, but if they get out of whack from hauling heavy loads or driving on tough terrain, they require prompt repair. Loose u-joints pose a serious hazard, and if they fully break, the vehicle they're in won't move at all. This can cause a bad accident if it happens suddenly.
To stay on the safe side and avoid an expensive towing bill and major repairs, pay attention to these potential symptoms of a bad u-joint.
1. Clunking and Clanging
If you hear a clunk with a ring or clang when you shift between drive and reverse, or when you speed up or slow down quickly, get your u-joints checked. This is a bad sound for a u-joint to make, and it's potentially a very dangerous sign.
It's also a convenient way to test whether you have an issue. From a stopped position, hold the brake and shift between drive and reverse. A clunking, knocking, or popping sound could indicate that the u-joint is worn down. Worn joints have too much room to rotate, which can cause them to fail.
It's possible that these sounds could mean a joint just doesn't have enough grease, but if that's the case, it's getting worn down by working while too dry. Adding more grease at this point is a Band-Aid solution, so you're better off going with a full replacement.
As much as you'd probably like to, you shouldn't ignore these clunking sounds. Take your vehicle in for a checkup as soon as possible.
If you notice a vibration or humming sound, especially when accelerating, decelerating, or driving at high speeds, that can also mean you have a u-joint that's getting worn. Wear and tear can cause an imbalance in the driveshaft, manifesting as a sensation of vibration that increases in intensity as your vehicle goes faster.
It can be easy to confuse this vibration with a wheel balance problem, but there are some key differences. Vibration caused by wheel imbalance will make a steady sound when the vehicle's in motion, while a bad u-joint will cause vibration will increase both during acceleration and deceleration. The sound of a failing u-joint will also be higher pitched, since the driveshaft rotates much faster than the wheels.
Vibration is less serious than clunking, but again, you shouldn't ignore this. U-joints can damage your transmission if they're getting outside their normal rotation.
3. Grinding or Chirping
Over time, the grease inside a universal joint can dry out and cause the roller bearings to wear small indentations into the section called the cross. This sound comes from the bearings then being forced over these indentations.
Squeaking when you're starting to move, either forward or backward, can be a sign of this wear. This issue is much more noticeable at slow speeds, since above about 10mph, you won't be able to hear this over other vehicle sounds.
Luckily, this one is a little less of a dire indicator than the heavier noises. Your mechanic might just add some lubricant and call it a day.
You should still service your vehicle, though, since u-joints will wear down much faster if they're not greased enough, taking you into the dangerous territory suggested by the first two noises.
4. Loud Banging
On the flip side, a loud bang is a serious sign that you need a repair right away.
When a u-joint breaks, it can separate the driveshaft from the transmission, causing a loud noise, like metal striking metal.
Should the U-joint sever completely, your vehicle will fail to respond to any power it receives from the transmission and will be unable to move forward or backward.
If you're moving when this happens, the break can cause a very dangerous situation. The safest thing to do is pull over immediately and either contact assistance or check out the situation yourself.
5. Joint Movement
You can only see this sign if you know what to look for, but if you prefer to diagnose the problem definitively yourself you can try it.
Turn off your vehicle and engage the emergency brake, then find the driveshaft and try to rotate it, or to lever the yoke connected to the driveshaft with a tool like a screwdriver. If you see any movement in the u-joint, replace it as soon as possible.
While you're down there, look for chips, cracks, or rust. These can all be indications of a dangerous situation.
Take your vehicle in for an inspection if you notice any of these symptoms. Removing and replacing a u-joint isn't exactly cheap (it should run you $200 to $300 at a garage), but it's much less expensive (and dangerous) than the damage it can cause if you ignore it.