How to Replace the Rear Lower Control Arm on a Mercedes W123 & W126
Replacing the lower rear A trailing arm is a beast of a job. Here is a loooong video with the whole process.
Video Chapters
[00:01] Introduction: Addressing rust-through for state inspection.
[00:54] Scope of Work: The extensive disassembly required.
[01:57] The Brake Cable Dilemma: Why the emergency brake must be loosened.
[03:15] Interior Disassembly: Removing the rear seat and backrest.
[06:58] Accessing the Upper Shock Mount: Locating the cover under the seat.
[08:10] Measurement Tip: Recording suspension height for proper bushing torque.
[10:01] Exterior Prep: Loosening exhaust, half-shafts, and brake calipers.
[12:21] The "Van Trick": Removing a stubborn rusted exhaust with a tow strap.
[13:50] Hardware Details: Identifying the 16-point spline axle bolts.
[19:49] Suspension Links: Disconnecting the stabilizer (anti-sway) bar.
[22:42] Removing the Rotor and Caliper.
[26:58] Spring Compression: Using a specialized internal spring compressor.
[31:33] Emergency Brake Service: Removing springs and cables from the hub.
[36:18] Lowering the Subframe: Gaining clearance for the trailing arm bolts.
[39:50] Reinstallation: Mounting the new arm and compressing the spring.
[44:46] Upper Shock Finishing: Installing the rod, washers, and lock nuts.
[50:24] Closing Advice: Managing rusted hardware and "faint of heart" tasks.
Video Summary
Ralf tackles a "Roy pain in the neck" job [01:57] to save his W126 from being condemned by inspectors. The replacement of a trailing arm is a high-effort task that requires dismantling nearly the entire rear suspension and even parts of the interior.
Key Technical Highlights:
Interior Access: To reach the upper shock mounting nuts, the rear seat and backrest must be completely removed [03:15]. Ralf shows the hidden cover that allows access to the threaded shock rod [07:06].
Bushing Torque Secret: Before lifting the car, Ralf measures the distance from the fender to the wheel [08:10]. This is critical because suspension bushings must be tightened only when the car is at its natural ride height; tightening them while the suspension is hanging can pre-twist and ruin the new rubber [08:59].
Dealing with Rust: Ralf shares an unconventional "van trick" for a rusted exhaust, using a tow strap and another vehicle to slowly pull the pipes apart [14:43]. He also emphasizes using plenty of penetrating oil (WD-40) on the 16-point spline bolts that secure the half-shafts to the differential [13:54].
The "Magic" Tool: Removing the rear springs on a Mercedes requires a specific internal spring compressor [27:11]. Ralf demonstrates how to thread the plates into the coils to safely collapse the spring within the control arm pocket [30:20].
Subframe Clearance: A significant hurdle is that the trailing arm mounting bolts often cannot be removed without lowering the rear subframe slightly [36:28]. Ralf explains how to support the differential and subframe with a jack to gain the necessary clearance [37:21].
The Result: Despite the difficulty and "dirty" nature of the job, Ralf successfully restores the structural integrity of the rear suspension, ensuring the W126 is safe for daily driving [51:26].
Video URL: • How to Replace the Rear Lower Control Arm ...