The R129 SL Killer

”It’s the last of the over-engineered SLs with an abundance of tech and ‘cool’, but there are several unknown gremlins that aspiring and current owners should be aware of.”

Enthusiasts and owners will be familiar with the commonly cited electrical issues found with the R129 SL. We’ve touched on these issues before on SLSHOP Journal, but as these cars get older, miles climb and components wear, there are more alarming things to be aware of. Whether you’re already an owner or looking to purchase an R129 SL in the UK, be mindful that one out of two cars that SLSHOP sees has a rusty rear end.  

THE STORY: 

Modern Classic craftsman Callum has been working with the R129 for over ten years; recently celebrating a decade of work with SLSHOP at the end of 2021. His model knowledge is unrivalled outside Mercedes-Benz, and it’s this quality that has allowed him to report on this issue and develop a solution for owners around the country.  

Due to the weather conditions in the UK and Europe, it has been common for the modern classics team to see R129 SLs with rusty rear sub-frame assemblies. The figure stands at one out of two vehicles being affected – these are just the vehicles located in the UK. The figure in Europe could be the same considering the similarity of weather across all countries during the winter.  

A LOW MILEAGE R129 SL: 

It would be wrong to assume that the older, more used vehicles are the only ones that are affected. The R129 SL in focus here, has only 60K on the clock and has been well-maintained and cared for by the owner via several well-known establishments in the UK. The vehicle arrived for a health check at SLSHOP, where it was highlighted that there was a significant amount of corrosion to the rear sub-frame and suspension components, as well as the hydraulic and brake lines.  

The construction of the R129 SL means that some sections of these lines are hidden by the rear sub-frame assembly, concealing further issues caused by rust. To replace hydraulic lines, actuators and brake lines, the rear sub-frame assembly needs to be removed which can take up to a whole day to carry out.  

REMOVING THE REAR SUB-FRAME ASSEMBLY: 

With the go-ahead from the customer, Callum and the team commenced a lengthy procedure to remove the rear sub-frame assembly from the vehicle. Once on the ground, it was clear that this drivetrain was in a bad state, with a rusty sub-frame, rusty control arms, suspension and differential. Similarly, the underside (concealed by the rear sub-frame assembly) was exhibiting patches of surface rust that would no doubt spread if left untreated. The customer recognised that while this section was removed from the vehicle to get to the brake lines, refurbishing these components and treating the surface rust would be essential to the future of their SL500. 

The amount of rust severely impeded the removal of the differential. As you can see, the sub-frame, bushes and control arms are severely corroded. Not something you would expect from a car that cost north of £100k in the 1990s. To do as much as we can to help this owner, SLSHOP will be refurbishing the sub-frame, differential and replacing all seals, bushes and control arms. 

The sub-frame, differential casing and wheel hubs will all be sandblasted, and powder coated for added protection, while minor welding to the spare wheel well will be carried out. Where the rust has begun spreading throughout the undercarriage, our fabrication team will grind them down to bare the metal, adding primer and then under seal. This will offer the SL500 more of a lifeline than originally provided by Mercedes-Benz.  

As you can see, significant progress has been made underneath the vehicle, with the underside now completely free of rust and with a protective coating. The new rear setup can now be fitted, bringing us closer to handing the keys back to the new owner.

AN EXTENSIVE ISSUE: 

Without causing too much alarm, we felt it was essential to alert owners of this extremely hazardous issue. The corrosion of this rear section could severely impact driveability, causing injury and financial detriment to owners. No doubt, there will be owners out there who need not worry about this issue, but a Vehicle Health Check could be the right route to put your worries to bed. 

Where this issue does become is extensive is when considering other models from the Mercedes-Benz back catalogue. Elements of the R129 SL’s rear end were interchangeable with those found on the W140s, W124, W201, CLK and SLKs. The design setup of these vehicles is also the same – five-link suspension on each side. We advise owners of these models to also keep an eye on the state of their rear sub-frame assembly and interdependent components – rust is clearly a common issue with this set-up.  

HOW CAN SLSHOP HELP? 

The first route to solving these issues is to have a specialist look over your vehicle. Our Modern Classic Vehicle Health Check has been designed to spot issues like this and advise on how best to rectify the problem. Our detailed reports will enable you to make an executive decision on how to invest in your vehicle; will also bolster the provenance of your vehicle, potentially adding value should you decide to sell it later down the line.