Mercedes-Benz SL-Class (R129: 1989-2001)

Corrosion (water-based paint)

Rust is by far the number one problem with Mercedes-Benz vehicles produced between 1993 and 2004. This is because in 1993, Mercedes-Benz implemented a more environmentally friendly, water-based paint.

Unfortunately, the new paint offered less protection from rust. With no galvanizing process, and coupled with steel that wasn’t very corrosion resistant, the 1990s Mercedes-Benz cars are very susceptible to corrosion. When buying a Mercedes-Benz from that era, inspecting it for rust should be one of your top priorities.

On a positive note, the R129 is one of the least affected Mercedes-Benz cars. The SL-Class roadsters are not galvanized. Mercedes-Benz started galvanizing their cars only in 2003/2004, while the production of the R129 model stopped in 2001.

Still, the R129 SL-Class cars seem to hold up quite well when compared to other Mercedes-Benz cars from that era, for example, the W202 C-Class or the W210 E-Class.

Biodegradable wiring harness

In the early-nineties, Mercedes-Benz decided to use biodegradable wiring harnesses in their vehicles. Unfortunately, it turned out that the biodegradable wiring insulation made from soybeans degrades far too quickly.

It becomes brittle and falls apart over time. The wires become exposed, short-circuit and cause the electrical systems to malfunction. The cracking insulation also makes the wires susceptible to corrosion.

It’s the heat of the engine that accelerates the process of degradation. Therefore, the cables closest to the engine are the ones affected the most.

The wiring harness problem affects Mercedes-Benz cars from 1992 until early 1996. For most models, the problem started in 1993, however, it was earlier in the case of M119 & M120 engines.

The following Mercedes-Benz R129 models are affected:

• SL 280 (193 PS M104 engine) – introduced in 1993

• SL 320 (231 PS M104 engine)  – introduced in 1993

• SL 500 (320 PS M119 engine) – introduced in 1992

• SL 600 (394 PS M120 engine) – introduced in 1992

Only cars with electronic fuel injection are affected. Cars with the KE-Jetronic are fine.

The components that could potentially malfunction because of a damaged wiring harness are fuel injection, starter, alternator, throttle actuator and the ECU. A faulty wiring harness can disable the car, and it is a potential fire risk.

The typical symptoms of a failing harness are electrical errors, problems with starting the engine, poor running and rough idle.

Before buying a 1992-1996 Mercedes-Benz R129, check if the harness has already been replaced. Otherwise, you will have to replace it yourself.

The wiring harness is complex, and that makes it expensive if you buy a new one from the dealership. However, you may be able to find used ones in good condition on eBay. Just make sure that you get one with the updated insulation that doesn’t disintegrate when you touch it.

Soft top roof – complex hydraulic system

The soft top roof is operated by twelve hydraulic rams, powered by a hydraulic pump. This is the highest number of hydraulic cylinders out of all Mercedes-Benz convertibles. One of the hydraulic rams was removed in 1997, bringing the number down to eleven, which is the same as in the R230 SL-Class roadster.

Your two biggest concerns are hydraulic system issues (mainly leaks) and weather intrusion (fancy name for a leaking roof).

Here are the things to check regarding the soft top roof:

• Check for any dampness in the footwells, carpets and in the boot. It’s best to do it after rainfall.

• Open and close the roof a couple times to make sure that it works.

• Check the roll bar operation.

• Since you are already looking at the boot, check if the level of hydraulic fluid is correct. If it isn’t, then the car may have been neglected or there is a leak somewhere. The pump and reservoir are under the spare wheel (three nuts need to be removed). Also, look for any hydraulic fluid leaks.

• Check the condition of the roof fabric and the plastic windows.

• Check the condition of the roof seals. Ideally, they should be lubricated regularly – ask the previous owner about it.

• Look for hydraulic fluid leaks – in the boot, on the headliner and around the hydraulic rams.

• Get the car up to 60 mph with the roof closed and listen for excessive wind noise.

• Listen for rattling noises while driving with the roof closed.

• Take the car to a high pressure (touchless) car wash and see if any water gets inside the cabin or the boot.

The hydraulic rams last only so long before the seals deteriorate and start leaking (15 years reliably at most, as I see it). Replacement OEM rams are expensive but there are companies that sell reconditioned units. Also, you can buy the seals alone and replace them yourself (advanced DIY).

The R129 SL-Class cars came with a removable hardtop roof. Ask the seller if the roof is included. Ideally, put it on the car and see if it fits and latches on correctly.

SL 600 – ADS suspension failure

The SL 600 and 600 SL models are equipped with hydro-pneumatic suspension called Adaptive Damping System (ADS). As the name implies, the ADS can adjust the suspension damping stiffness based on readings from acceleration sensors installed in the car. Also, the damping can be set manually.

Here’s how it works:

In the V12 Mercedes-Benz R129 cars, there are no conventional shock absorbers. Instead, there is a hydraulic cylinder and a hydraulic accumulator at each wheel. The suspension height can be adjusted by changing the amount of oil fed into the hydraulic cylinders.

Oil is incompressible and this is where the hydraulic accumulator comes in. The hydraulic accumulator is filled with pressurized nitrogen, and it serves the function of a shock absorber.

The nitrogen accumulator is a sphere with a rubber diaphragm inside. On one side of the membrane, there is compressed nitrogen gas and on the other, there is hydraulic fluid. The gas behind the membrane is compressible and can absorb the shocks from the road (membrane deflects).

The system is powered by a tandem hydraulic pump that also supplies fluid to the power steering system. There are two ride height sensors (one per axle) that adjust the suspension level. Also, there are valves, acceleration sensors and the ECU that controls this system.

Here’s what’s inside an accumulator:

The ADS is fairly reliable but given the age of these cars, you will probably have to deal with hydraulic fluid leaks and failed accumulators. You should know that the suspension damping is lost when an accumulator fails in the R129 SL-Class with the ADS suspension. Driving like that can damage other components so it’s important to keep the nitrogen spheres in good shape.

The nitrogen spheres last around 60,000 miles or 10 years, whichever comes first. They are service items and will need to be replaced regularly. Even if you don’t drive the car, the nitrogen gas will eventually escape, just like air escapes from a seemingly airtight balloon.

In comparison, the ABC suspension in the R230 roadster also uses nitrogen accumulators but their purpose is different.

The ADS in the R129 is actually very similar to the hydro-pneumatic suspension Citroen cars were famous for. Citroen just took it one step further by getting rid of the coil springs altogether.

Without a doubt, a conventional spring suspension is cheaper to maintain. This is a V12 Mercedes-Benz though. It was a car meant for people who don’t care about such things. On a positive note, the ADS is still less complex than the newer ABC suspension used in the Mercedes-Benz R230.

If you are planning to buy an SL 600, here are the symptoms of ADS suspension problems or malfunction:

• illuminated ADS warning light

• hard, bouncy suspension usually indicates a failed accumulator

• the suspension can also be hard when some other component fails – the ECU then sets the suspension damping to “hard”

• incorrect ride height or the car leaning forward / backwards

• hydraulic fluid leaks – check from under the car

• low hydraulic fluid level – it’s a sign of neglect or a problem with the suspension

• lack of noticeable difference in ride stiffness when sports mode is activated

You can also check if the suspension is working correctly by pressing on each corner of the car. The suspension should not be hard or bouncy. Then, put the car in sports mode and check again. You should be able to notice a difference in suspension stiffness. The difference isn’t massive, but it should be noticeable.

It is possible to convert the SL 600 to conventional coil springs and dampers using SL 500 parts (heresy!). The ride won’t be as nice though, and it’s not cheap because you would need four new dampers, new springs and some other suspension parts that are different.

M103 & M104 engines – head gasket failure

The head gasket is the weak point of these engines. Considering head gasket replacement is inevitable at some point, often between 100,000 and 150,000 miles.

Before the gasket fails completely, these engines often develop an external oil leak from the head gasket at the back of the engine. Make sure to inspect that area when buying one of these cars.

Once the head gasket fails and is replaced, the new one should be good for another 100k – 150k miles. This issue mainly affects the inline-6 engines, like the M103 & M104 engines, because of their long cylinder heads. The 4-cylinder engines from that era are also affected but to a smaller degree.

If you’re curious why the head gaskets fail more often in the inline-6 engines – it’s because of the difference in thermal expansion between the cast iron engine block (11 m/m.K x 10^-6) and aluminium alloy cylinder head (22 m/m.K x 10^-6).

As you can see, the expansion coefficient is two times higher for aluminium. This means that when the parts get hot, the aluminium cylinder head expands twice as much as the iron block. The longer the engine, the more pronounced this effect is. The head gasket, which sits between the engine block and the cylinder head, has to deal with these expanding parts.

This was a relatively new engineering problem to solve at the time as former Mercedes-Benz engines had cast iron cylinder heads. Aluminium alloy started being commonly used in the 1980s to make lightweight cylinder heads. These days, the engine blocks are also made out of aluminium alloys.

KE-Jetronic fuel injection (CIS)

The M103, early M104, and early M119 engines were equipped with a mechanical, Continuous Injection System (CIS) called KE-Jetronic. It was designed by Bosch and it is well-engineered and reliable.

The problem is that most repair shops these days are clueless as to how this system works. Unless you can find a specialist or you understand it yourself, trying to get a KE-Jetronic problem solved might not be easy.

If you find a car that runs well, it will most likely keep going as the injection system doesn’t require any special maintenance. If it does break down though, it will be trickier to fix than a more modern electronic fuel injection system.

If you already have a car with KE-Jetronic and you are experiencing problems, often they can be fixed by replacing the vacuum pipes as the system is sensitive to vacuum leaks.

Make sure you check the following points before purchasing a car with KE-Jetronic:

• Idle quality – The idle should be even when cold and warm. The engine should not die or hesitate when coming to a stop (out of gear in a manual transmission car).

• Power delivery – it should be smooth throughout the rev range. There should be no hesitation when pressing on the accelerator pedal.

• Cold starts – see how well the car starts when the engine is cold.

• Hot starts – when warmed up – shut the engine down for 10 minutes and see how well it starts then. This step is important, repeat it twice.

It everything works well, you should be OK.

Engines with KE-Jetronic should not be run on LPG. It is possible to convert it to LPG, using a second generation system, but it is not a good idea. In fact, I recommend that you never buy a car with KE-Jetronic that has ever had an LPG installation fitted for two reasons:

1. The fuel injection system has a mechanical fuel distributor, which is lubricated by petrol. When you switch to LPG, the distributor will be running dry and start wearing out.

2. With a simple second generation conversion, there is a risk of gas explosion in the intake manifold, which has a large volume. If this happens, you would be left stranded on the roadside with half the vacuum pipes blown off the engine. Even when you fix the car, it will likely never run the same due to the damage that was done (bent air flow sensor plate, damaged vacuum pipes and likely air leaks).

M119 engine – plastic chain guides & oil tubes

The M119 engines are reliable apart from a couple issues related to the plastic parts inside the engine.

The first problem is that over time, the plastic upper chain guides in these engines may develop cracks and pieces of the guides can break off. If they do, the timing chain loses tension and is at risk of skipping. It is rare, but it can cause some serious engine damage if it happens (valves hitting the pistons).

In my opinion, the guides should be replaced as part of preventative maintenance at around 100,000 miles.

Another issue, which could potentially damage the engine if left untreated, is the plastic oil tubes – 16 of them in total. These little pipes become brittle and may crack from the heat of the engine and the exposure to oil. When they break or lose their end caps, the hydraulic lifters do not get enough oil, and the engine develops a ticking sound.

The oil tubes used to be metal, and they were changed to plastic at the end of 1992. The cars with the metal tubes are problem-free.

Cars with electronic fuel injection have the plastic oil tubes, while cars with KE-Jetronic have the more robust, metal ones. Fortunately, replacing the plastic oil tubes is a relatively small job.

M120 engine – high maintenance costs

The M120 engine was created in response to the BMW’s V12 engine in their 7-series cars. The Mercedes-Benz V12 M120 is based on two inline-6 M104 engines joined together. One of the key changes was making the engine block out of aluminium alloy instead of cast iron.

Mercedes-Benz did a good job with the M120 engine, as it doesn’t have any major flaws (unlike its successor, the M137 used in the W220 S-Class).

There is one thing that you should keep in mind when buying a V12 Mercedes-Benz R129. The M120 is a huge engine that fills out the engine bay completely, so replacing parts can be difficult.

Something as simple as changing the motor mounts becomes a tedious task. You can still do it without removing the engine from the car, but my point is that even small repairs can be pretty expensive.

The M120 is not that much more complex than the smaller Mercedes-Benz engines, but many components are doubled when compared to the six-cylinder engines. So, higher maintenance costs are something to consider when buying an SL 600 or one of the V12 AMG models.

On a positive note, the maintenance costs for the M120 engine should still be lower than for the newer M137, which is a lot more complex (it has six catalytic converters and eight oxygen sensors).

Summary of problems & additional information

• Corrosion is a big problem with Mercedes-Benz cars made between 1993 and 2004 because of the introduction of water-based paint and non-galvanized car bodies. While the R129 paint is also water-based, the R129 seems to be the least affected model. Its corrosion resistance isn’t great but these cars don’t turn into rust buckets as quickly as some other Mercedes-Benz cars from those years.

• The Mercedes-Benz R129 SL-Class shares many components with the W124 model, which was famous for its reliability. The inline-6 models should come close to the W124 in terms of maintenance costs, as the key components are the same or very similar (engines, steering system and suspension). This is as long as there are no problems with the soft top roof.

• The hydraulic soft top roof mechanism is complex (despite being just a soft top). Depending on the year of production, there are 11 or 12 hydraulic cylinders that operate the roof. The seals in these hydraulic rams degrade over time and develop leaks. If you’re planning to buy a Mercedes-Benz R129, inspect the roof mechanism thoroughly for hydraulic fluid leaks.

• The V12 models are expensive to maintain. The SL 600 and the AMG model are equipped with hydro-pneumatic suspension called Adaptive Damping System (ADS), which offers a better ride and handling at the cost of a higher maintenance burden. Also, access in the engine bay is restricted so you can expect to pay more for labour when replacing parts.

• Before buying a Mercedes-Benz R129 from 1992-1996, check if the biodegradable wiring harness has already been replaced. Replacing some wires may sound like a trivial job, but a new wiring harness isn’t cheap. For most models, the problem started in 1993, however, it was earlier in the case of M119 & M120 engines.

• The V6 M112 and V8 M113 engines – Mercedes-Benz got both of them right. They are closely related to each other and both are reliable.

• Watch out for head gasket leaks in the M103 and M104 engines. The head gasket is the weakest link in these engines. Other than that, they are robust units.

• All R129 engines are fitted with timing chains, which normally don’t have a specified replacement interval. The timing chains in the R129 are generally reliable, except for the M119 plastic chain guides, but they will not last forever. Follow this link to learn more about timing chains.

• The KE-Jetronic is an interesting piece of engineering and a part of automotive history, but it may be difficult to service. If you plan to use the car as a daily driver, it’s better to look for the newer engines with electronic fuel injection, which all car mechanics are familiar with.