Common Mercedes R129 SL Roof Issues

Introducing Callum Harkness – R129 SL guru and veteran SLSHOP employee:

Callum is our lead R129 SL technician. Callum has just reached his tenth year with the company, joining in 2011 as a young mechanic with passion and dedication to the Mercedes-Benz SL lineage. Having worked across the W113, R107 and R129 SL variants, Callum has a deep respect and bond with these cars, allowing him to quickly identify issues and launch a plan of action to solve them.

Callum now leads the Modern Classics team, working primarily with the R129 SL and other more modern variants in the saloon and coupe realms. As the R129’s appeal increases and more and more enthusiasts seek to enjoy their cars, SLSHOP continues to support new owners looking to buy an R129 or existing owners looking to solve a problem with their car.

In this first series of investigation, we discuss common electrical faults with the R129’s soft top roof.

Overview of the R129:

Launched at the end of the 1980s, the R129 was a distinct leap from its predecessor, capturing hearts and minds with an abundance of technology, not available from any other manufacturer at the time. The R129’s seats have over 20 patented electrical parts, with the seatbelts adjusting automatically to the height of the headrest. The R129 features brake assist, stability control, active damping, rain sensing windscreen wipers and occupant detection to disengage the airbags on the passenger side if there is no one occupying the seat. For a variant over thirty years old, this is quite an impressive set of driver amenities.

Of course, the quintessential Mercedes R129 SL feature is the electric retractable roof, which can be operated at the press of a button. No more standing in the rain following the step-by-step process required with the R107 SL. However, the complexity of the electrics throughout the car, combined with wear and tear over the years can present problems.

Common R129 SL Electrical Issues:

For aspiring, new, and existing owners, Callum has pointed out the following as the primary root causes for roof issues:

Roof Control Module: According to Callum, fifty percent of issues with the retractable roof originate from this module located beneath the rear seats.

Diagnosing and solving the problem:

1. A common cause for this module failing is water ingress. If your hood is ripped or your window and deck lid seals are old and failing, water will seep straight through and head down beneath the seats. Check for the presence of water after a wet day or moisture in the morning.
2. Do not jump start your R129 SL. Spikes in the current can cause the module to fail, as well as damage other electrical components in the car. This can lead to issues beyond the electric roof including the central locking. Use a trickle charger when storing your car to keep the battery topped up.

R129 SL Electric Micro-Switches: Mercedes-Benz engineers configured the roof operation with upwards of 20 micro-switches embedded in the roof frame, enabling it to move up and down.

1. With so many of these switches on the earlier R129 SL models, it is common for one or two to fail or get stuck, preventing the operation of the roof.

Diagnosing and solving the problem:

The earlier R129 SL models possess very basic ECU units, which report error codes to an equally basic code reader. This code reader flashes a specific number of times indicating a certain problem referenced in the vehicle handbook. Later cars (1996 onwards) have more comprehensive ECUs read by a STAR computer which identifies the correct switch that has failed or stopped working.

Once you have identified the switch that needs replacing, you can purchase an individual replacement or completely refit all the switches.

Hydraulic Ram Cylinders: Sitting at the top of the header-rail, the left and right hydraulic ram cylinder lockers allow the hood to lock into place before releasing at the driver’s discretion. Callum advises that of all the hydraulic ram cylinders across the R129 SL, these two are most likely to leak.

1. With the age of these cars only increasing and an abundance of vehicles still on the road, inexperienced owners and technicians alike can often neglect the required hydraulic fluid changing regime. Dirty fluid will cause problems for the functionality of the electric hood. Callum advises owners to look for drips emerging from the sun visors.
2. Ask a professional to change the fluid to insure its cleanliness. This will avoid leaks and a poorly operating roof.

In the next episode of common R129 issues, we discuss the hydraulic suspension found on the SL500 and SL600.

SLSHOP MODERN CLASSIC SERVICING