Ball joints are a part of your vehicle's suspension that connects the steering knuckles to the control arms. A ball joint is essentially a flexible ball and socket that allows the suspension to move and at the same time the wheels to steer. Cars and trucks without strut suspensions typically have four of them (one upper and one lower on each side). Cars and minivans with strut suspensions have only two (one lower ball joint on each side). Some front-wheel drive cars also have ball joints on the rear suspension.
Like any other suspension component, ball joints eventually wear and become loose. Excessive play in the joint can affect wheel alignment and tire wear. Loose joints can also cause suspension noise (typically a "clunking" sound when hitting a bump).
WARNING: If a ball joint fails, the suspension can collapse causing a loss of control. So don't put off having a bad set of joints replaced.
Joints should be inspected before they're greased (since grease takes up some of the slack in the joint). Ball joints are pretty easy to check, but each type requires a different inspection procedure. Use the wrong procedure and you'll get misleading results. The procedure that needs to be used depends on the location and loading of the joint:
* LOWER LOAD CARRYING ball joints are found on front- and rear-wheel drive vehicles where the coil spring or torsion bar is on the lower control arm. You'll also find them on the rear suspension of 1985 & up FWD Buick, Cadillac, Pontiac & Oldsmobiles, too.
Joints with built-in wear indicators (most GM and Ford RWD cars, rear joints on the FWD GM cars, and GM RWD vans, S10 & S15 Blazer) must be checked with the full weight of the vehicle on the tires on the shop floor or on a drive-on style ramp -- not with the wheels up or the suspension supported by jack stands.
No measurements are required if a joint has a wear indicator because internal play is indicated by the position of the grease fitting boss. The boss protrudes about .050 inches on a new joint. As the joint wears, the boss recedes into the housing. The joint is considered "good" as long as you can see or feel the edge of the boss protruding from the housing. But if the top of the boss is flush or below the housing, it's time to replace the joint.
On lower load carrying ball joints without a wear indicator, the joint is checked in the unloaded condition with the wheel raised off the ground and the lower control arm supported by a jack stand. A dial indicator is then used to measure play in one of two directions: sideways (horizontal or radial play) or vertically (axial or up-and-down play). The direction to measure depends on the application (refer to a manual for the exact specs).
Sideways play is measured with the indicator positioned against the inside of the wheel rim near the joint. The wheel should be pushed in and out by hand to check sideways play, and lifted with no more than 25 lbs. of force to check vertical play. Many joints allow up to .250 in. of sideways (radial) play, but some allow no play or only .015 in. of play. Always refer to the vehicle manufacturer's specs.
Vertical play is measured with the dial indicator positioned against the knuckle stud nut (Ford & GM) or the joint housing (Chrysler). A joint that has more than .050 in. of vertical play doesn't necessary require replacement because the specs range from zero play to as much as .125 inch of play.
The most common mistake that's made here is to use too much pressure on a pry bar or to insert a pry bar between the control arm and knuckle rather than under the wheel. Pry hard enough and any joint may appear to be bad.
* LOWER FOLLOWER NONLOADED ball joints are found on two kinds of applications: RWD cars where the spring is over the upper control arm, and vehicles with MacPherson strut suspensions. On both applications the lower joint is checked with the wheel raised off the ground hanging free (no stand under the lower control arm). Rock the wheel in and out by hand. A good joint should show no movement.
One exception here is 1978-80 Omni & Horizon which allows up to .050 inch of sideways play. Another exception is Chrysler FWD minivans and FWD cars ('81 & up). On these applications, the lower joint has a wear indicator grease fitting. Joint play is checked with the wheels on the ground rather than raised. If the grease fitting can be twisted with your fingers, the joint needs to be replaced.
* UPPER LOAD CARRYING ball joints are found on vehicles where the spring or torsion bar is on the upper control arm. Like the lower follower nonloaded ball joints, the upper joints are checked in the unloaded condition with the wheels off the ground -- but with a wedge or block between the frame and upper control arm to support the upper arm. On most applications, any movement calls for replacement. But on some Fords, up to .250 in. of radial play is allowed.
* UPPER FOLLOWER NONLOADED ball joints are also checked with the wheels off the ground but with the lower control arm supported. Any movement usually calls for replacement.
Any joint that exceeds the vehicle manufacturer's maximum allowable wear needs to be replaced. The greater the amount of wear, the greater the urgency to replace it.
Ball joints are often replaced in complete sets, or at least in matched pairs on both sides (both lowers or both uppers). This is because the joints on both sides of a vehicle usually have the same amount of wear. If one is bad, the other usually is too. Load carrying ball joints usually wear out before ones that don't carry a load, so it may only be necessary to replace the loaded joints instead of the complete set.
Replacing a set of ball joints requires separating the control arms from the steering knuckles, a job which can be difficult depending on the design and age of the vehicle. At the very least, it usually requires a special "ball joint fork" tool to loosen the ball joint stud from the knuckle. If this sounds like more of a a job than you want to tackle, let a professional do it the work.