Mercedes-Benz ML400 Control Arm Replacement Cost

The control arm is connected on one end to the steering knuckle (to which the wheel is attached by way of the hub and bearing assembly) by a ball and socket called a ball joint. It is attached at the other end to the vehicle frame or body. When a car goes over a bump, the control arm pivots up and down with the wheel and allows the vehicle body to remain stable. On the frame/body side, the control arm has a bushing (or pair of bushings) that allows it to pivot.

The actual control arm may last the lifetime of the vehicle. The control arm bushings, on the other hand, often wear out as a vehicle approaches 100k miles. A control arm is prone to damage if the vehicle is in a crash or encounters a large pothole. If the bushings are worn out, the suspension will make a squeaking sound when going over bumps. If they are severely worn, a knocking sound may be heard as the control arm makes metal-to-metal contact with the bushing bracket. If the control arm is damaged or bent, tire wear may be uneven and the vehicle may pull to one side or wander.

If the control arm on a vehicle is damaged in a crash or impact with a pothole or parking block, the suspension will be out of alignment and cause uneven tire wear and poor vehicle handling. The control arm should be replaced as soon as possible. If the control arm bushings are worn, the result will be an annoying squeak when the car goes over bumps. In this case, the car can be driven. But if the bushings wear out considerably, metal-on-metal contact may happen between the control arm and the bushing bracket. This can cause further suspension damage and the control arm or bushing should be replaced.

While there is no prescribed frequency for replacement of a control arm, the bushings may wear out and need to be replaced as a vehicle approaches 100k miles. Driving aggressively or over rough roads can decrease the lifespan of the control arm bushings.

A technician may diagnose a worn lower control arm during a test drive of the vehicle. The condition may also present itself during a bounce test, where the technician will press down firmly on one corner of the vehicle to bounce the suspension and pivot the control arms. A damaged or bent control arm may show up during a four wheel suspension alignment since the control arm determines the position and angle of the wheel.

A technician will replace a lower control arm by first lifting and supporting the vehicle in the air on a hoist and removing the wheel and tire. A lower control arm must be disconnected from the lower ball joint (attached to the steering knuckle) and from the vehicle frame or body. An upper control arm is disconnected from the strut tower on the body and from the top of the steering knuckle. Some control arms come with bushings permanently installed in them, while others require the bushings to be installed. It is common to replace related components - ball joints, tie rod ends, sway bar links - at the same time that a control arm is replaced.

Replacement of a control arm is generally an intermediate DIY repair. On some vehicles, the control arms come with the bushings already installed. On other vehicles, the bushings need to be pressed into their housings on the control arm before installation into the vehicle. In this case, the repair will be more difficult. Also, this repair requires that the vehicle be safely lifted and supported with a jack and jack stands. Failure to do so in a safe manner can result in serious injury or death.