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Lower Control Arm Removal and Replacement
 
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Pelican Technical Article:

Lower Control Arm Removal and Replacement

Kerry Jonsson

Time:

4 hours4 hrs

Tab:

$500

Talent:

***

Tools:

13mm, 19mm, 22mm wrenches and sockets with ratchets

Applicable Models:

Mercedes-Benz R129 (1990-02)

Parts Required:

Lower control arm

Hot Tip:

Slowly lower the lower control arm until all the pressure is off of the coil spring

Performance Gain:

Improved handling, reduced vibration and even tire wear

Complementary Modification:

Replace the shock absorber at the same time if necessary

There are many different types of suspension systems. There is the older double wishbone suspension which uses two control arms, one upper and one lower to keep the spindle in the correct position while the suspension moves through its travel. This is the preferred suspension system for sports cars because the upper and lower control arms keep the wheel straighter throughout its suspension travel. This reduces the effect called bump steer. Bump steer is the change in the angle of the tire as the suspension compresses and expands. If the wheel changes its angle as the suspension is moved up and down the car is said to have a lot of bump steer. This changes the direction of a car as the outside suspension compresses under load in a turn. If bump steer is managed properly with suspension geometry it can actually improve the handling of a car. More often than not it leads to unnecessary steering adjustments while driving. The MacPherson strut is another type of suspension. In this suspension system there is no upper control arm. The upper spindle is mounted to the lower part of the strut and the strut controls the path of the spindle as the suspension moves. This usually increases camber (the angle of the tire relative to the road) but under hard cornering during spirited driving more camber is usually a good thing. The W129 chassis uses a MacPherson type shock but the spring is located between the lower control arm and the upper spring perch on the body of the car. Most MacPherson struts have the coil incorporated with the shock absorber. In either case each suspension system has a lower control arm. On the W129 the lower control arm has the outer ball-joint built into the lower control arm. If the ball socket is worn out you will need to replace the whole lower control arm. In this tech article I will go over the steps to remove your lower control arm.

Lift and support the front axle of the vehicle, after placing your trusty safety glasses on your face. You have to remove the tires to perform this job and it does make access much easier since you may not have access to an automotive lift. See our tech article on jacking and supporting your vehicle and removing the front tires.

The procedure to replace the front lower control arm is the same for both the left and right sides. These pictures are of the left side of the vehicle. It is recommended you replace both sides at the same time to keep your handling neutral.

Remember your car may have been serviced before and had parts replaced with different size fasteners used in the replacement. The sizes of the nuts and bolts I give may be different from what you have so be prepared with different size sockets and wrenches.

ThisPicture illustrates the left side wheel well looking back at the spindle.
Figure 1

ThisPicture illustrates the left side wheel well looking back at the spindle. Remove the two 13mm fasteners (green arrows) for the sway bar bushing.

Remove the bracket from the lower control arm.
Figure 2

Remove the bracket from the lower control arm.

ThisPicture illustrates the left side wheel well looking out at the spindle.
Figure 3

ThisPicture illustrates the left side wheel well looking out at the spindle. Support the lower control arm with a jack. Using a 19mm wrench and socket with ratchet, remove the lower control arm mounting fasteners (green arrows).

If your vehicle still has original parts, you will notice a sealant on the spindle to protect corrosion from getting into the lower ball joint mount.
Figure 4

If your vehicle still has original parts, you will notice a sealant on the spindle to protect corrosion from getting into the lower ball joint mount. You will have to remove this sealant with a flat head screwdriver.

With the sealant and fastener removed, separate the ball joint from the spindle.
Figure 5

With the sealant and fastener removed, separate the ball joint from the spindle. You may have to use a hammer and chisel to break corrosion free. If needed, drive a chisel into the opening (green arrow) just enough to free the ball joint. You do not have to remove the brake caliper and shock absorber to perform this step. I have done so to afford you a better view. Hammer on the spindle until the ball socket breaks loose from the spindle. Being well away from the jack and the underside of your car, slowly and ever so carefully lower the floor jack to take tension off of the coil spring. The coil spring may fall out after lowering the control arm. This is normal.

Here I am under the car looking at the lower control arm front mount.
Figure 6

Here I am under the car looking at the lower control arm front mount. Remove the 22mm fasteners (green and yellow arrows). You will probably have to hold one end still (yellow arrow) while loosening the fastener (green arrow).

ThisPicture illustrates under the car still on the left side at the rear lower control arm mount.
Figure 7

ThisPicture illustrates under the car still on the left side at the rear lower control arm mount. Remove the two 22mm fasteners (green arrows). You will probably have to hold one still while loosening the other.

ThisPicture illustrates the front of the front lower control arm mount.
Figure 8

ThisPicture illustrates the front of the front lower control arm mount. With the nut removed, mark the position of the camber / caster adjusting washer (green arrow), then pull the bolt out of the sub-frame.

ThisPicture illustrates the rear of the rear lower control arm mount.
Figure 9

ThisPicture illustrates the rear of the rear lower control arm mount. With the nut removed, mark the position of the camber / caster adjusting washer, then pull the bolt out of the sub-frame.

ThisPicture illustrates the rear of the left side wheel well.
Figure 10

ThisPicture illustrates the rear of the left side wheel well. Use a pry bar to lever out the rear lower control arm from the sub-frame. You do not need to remove the outer ball join from the spindle. I have done so to provide you with a better view.

ThisPicture illustrates the front of the left side wheel well.
Figure 11

ThisPicture illustrates the front of the left side wheel well. Use a pry bar to lever out the front lower control arm from the sub-frame. Once again you do not need to pull out the shock absorber, spindle and outer tie rod end but I have done so for photographic purposes. Installation is the reverse of the removal procedures. Install both lower control arm mounts in the sub-frame of the car. Install the fasteners, camber/caster adjusting washers and tighten down the fasteners. Position the front coil spring in its upper perch and lower perch and jack up on the lower control arm to compress the spring until the outer ball joint can be positioned inside the spindle. Install the mounting fastener and tighten them. Fit the wheel, lower the car and you are done.

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Comments and Suggestions:
Paymon Comments: That is not true. The ball joint is replaceable. it requires a compressor and a extractor. I once saw a dealer official Mercedes Benz instructional video of it being done. You can purchase new ball joints as well.
August 9, 2016
  Followup from the Pelican Staff: That tool being not readily available, it is best to replace the unit. - Nick at Pelican Parts  
Blackbeard Comments: So, you're saying I cannot just replace broke ball joint, I need to replace whole control arm? Ouch. My R129 is sitting in a parking lot. Not sure I can afford to wait for part delivery. Wow.
June 23, 2016
  Followup from the Pelican Staff: COrrect. The ball joint is not serviceable. Give The Pelican Parts parts specialists a call at 1-888-280-7799. They can figure out what part or repair kit you need.
- Nick at Pelican Parts
 
Brian Comments: Hello, I ordered and replaced both the lower control arms on my 2001 SL500, I can't find the Torque for the bolt that holds the lower ball joint in, I would like to use the correct specs if you have it available. Thanks!
January 7, 2016
  Followup from the Pelican Staff: I don’t have that info.

I would grab a repair manual. It will have the procedure, special tools and torque specs.

Give our parts specialists a call at 1-888-280-7799. - Nick at Pelican Parts
 
Nigel Comments: Nowhere in the article does it mention spring compressors. It doesn't appear that one is needed if you jack the car up below the springs. Comments? I didn't quite understand the added comment about the chain
June 19, 2015
  Followup from the Pelican Staff: Step 5 mentions removing the coil spring:

Being well away from the jack and the underside of your car, slowly and ever so carefully lower the floor jack to take tension off of the coil spring. The coil spring may fall out after lowering the control arm. This is normal. - Nick at Pelican Parts
 
Jim Comments: Very good detailed instructions. Very helpful and simplified. BTW, I did not us a spring compressor. I ran a chain through the coil spring and ran a bolt, washer and nut connecting the chain links, leaving enough slack to let the spring expand all the way. I've done 4 different automobile upper, lower control arms and always use a chain. I learned at an early age, these springs can do great bodily injury if not properly tied down. No problem here. Thanks for photos and article. Good Job. James
April 27, 2015
  Followup from the Pelican Staff: Thanks for the additional info and feedback. We appreciate it.
- Nick at Pelican Parts
 

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