Mercedes-Benz SL-Class (R230: 2001-2011)

Sensotronic Brake Control malfunction

The R230 SL-Class cars were fitted with a brake-by-wire system called Sensotronic Brake Control (SBC). This system caused one of the biggest Mercedes-Benz recalls. Once the affected cars have been sorted out by the recall, the system is reasonably reliable. However, the SBC pumps in these cars have a finite lifespan, which is monitored by the car’s computer.

Once the pump reaches a fixed number of braking cycles (number of times the brake pedal is pressed) a warning light will come on. At that point, the pump needs to be replaced because it has reached the end of its design life. If not replaced, the pump will eventually fail. A new pump will cost you around £900. Reconditioned SBC pumps are also available for a smaller price.

If the SBC pump fails, a warning message is displayed on the dashboard and braking power is massively reduced – this could happen at motorway speeds. In case of pump failure, be aware that you have toapply a lot of force to the brake pedal to get the car to stop. Also, the SBC pumps occasionally fail before reaching their design lifespan.

The SBC was dropped permanently in the W211 E-Class and C219 CLS-Class vehicles in 2006 because the customers lost trust in this system after the recalls. However, it was never removed from the R230 SL-Class.

In my opinion, SBC was a failed experiment. When buying a second-hand car, it would be better to have standard hydraulic brakes that are far less complex and have been proven over decades of automotive use.

If you are planning to buy the R230, be aware that it has SBC. If you don’t want to spend money on a new pump, and you want to be sure that your brakes won’t fail anytime soon, look for a car which had the SBC pump replaced recently (with an invoice to prove). With a new SBC pump, you should not have any trouble for a few years.

Corrosion (water-based paint)

Rust was one of the top problems with Mercedes-Benz vehicles produced between 1993 and 2004.

This is because around 1993, Mercedes-Benz implemented a more environmentally friendly, water-based paint. Coupled with steel that isn’t very corrosion resistant and no galvanizing process, the early R230 models are susceptible to corrosion.

Mercedes-Benz eventually started galvanizing all their cars, which improved the corrosion resistance significantly. In the case of SL-Class cars, Mercedes-Benz started galvanizing the R230 sometime in 2004.

Therefore, if you are planning to buy an R230, look for a car produced in 2005 or later. It will have much better corrosion resistance, which is important if you are planning to keep the car for a couple years or longer.

Complex hardtop roof

The hardtop “Vario” roof is operated by eleven hydraulic actuators, powered by a hydraulic pump. That’s six more hydraulic cylinders than in the R171 SLK-class. Your two biggest concerns are hydraulic system issues (mainly leaks) and weather intrusion (fancy name for a leaking roof).

Things to check regarding the Vario roof:

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

• Open and close the roof a couple times to make sure that it works. The roof should take approximately 20 seconds (16 seconds after the 2006 facelift) to open or close. If it takes much longer than that, the pump may be on its way out.

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

• 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.

• 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.

• Accident-free history is very important for any hardtop convertible. If the car participated in a large crash, the roof may be out of alignment, which can be a nightmare to waterproof if the chassis of the car isn’t “straight” anymore.

The hydraulic rams last only so long before the seals deteriorate and start leaking (15 years reliably at most as I see it). The roof lock cylinder is usually the first one to fail. When it does, the headliner may get soaked in hydraulic oil.

The OEM rams are expensive but there are companies that sell reconditioned units. Also, you can buy just the seals and replace them yourself (advanced DIY).

Along with the hydraulic system, there is plenty of electronics to manage the roof on the R230. Some of these cars are getting quite old so the roof can be a bit fussy sometimes.

ABC suspension failure

This car may be equipped with hydro-pneumatic suspension called Active Body Control (ABC). The ABC is standard equipment in the SL 500, SL 550, SL 600 and AMG models. It was an optional extra in other models.

When it works, the ABC exceeds what standard coil spring suspension can do.

The ABC suspension is very comfortable, and at the same time, there is very little body roll when cornering or braking (hence the word “Active” in the name). The system can adjust its position up to 5 times per second (5 hertz). Also, it is self-levelling, which means that it will maintain the ride height despite the load in the car. The ride height can be adjusted manually too.

If you think the Airmatic suspension is complex, never buy a car with the ABC suspension. The ABC is a lot more advanced than the Airmatic, let alone a standard suspension. Thus, you can expect the maintenance costs to be accordingly higher.

Here’s how the ABC is different from coil spring suspension:

Traditional suspension

• four springs – they support the car and allow suspension movement (suspension travel)

• four shock absorbers – they dampen the spring oscillations and prevent excessive suspension movement (bouncing)

Active Body Control

• • hydraulic piston pump (generates pressure for the entire system)

  • oil reservoir (stores hydraulic oil)

  • two valve blocks – one per car axle

  • four struts (a strut is coil spring, shock absorber and hydraulic actuator integrated into one unit)

  • two hydraulic accumulators (nitrogen spheres) – one per car axle

  • pulsation damper and return accumulator (two more nitrogen spheres)

  • ECU (the brain of the system)

  • hydraulic hoses that connect all of the above

  • over a dozen electronic sensors

Let me try to briefly explain how this system works:

The suspension struts contain a regular coil spring and a shock absorber – just like in a typical car. Additionally, there is a hydraulic cylinder in each strut that can change the height of the suspension (each wheel individually).

The system is powered by a hydraulic pump, monitored by sensors and controlled using solenoid valves. This allows for real-time adjustments to ride height and vehicle body lean, for example during cornering or braking.

There are four nitrogen spheres (accumulators) that have a membrane inside and pressurized nitrogen gas behind it. The membrane is flexible. On one side of the membrane there is gas and on the other, there is hydraulic fluid. The membrane can deflect to store energy (pressure) because nitrogen gas is compressible.

There are two accumulators that store the pressure to feed the system as explained above. The other two are there to dampen pressure spikes in the system. The hydraulic pump is a piston pump so it generates pressure pulses with every stroke. The pulsation damper, which is just another nitrogen accumulator, evens out these pulses (membrane deflecting). The return accumulator does a similar job after the oil leaves the struts.

In essence, the ABC is really a coil spring suspension with added hydraulic levelling. It’s the coil springs inside the struts that do the job of absorbing the shocks from the road and the hydraulic system is there just to adjust car body lean and height.

If you compare the ABC to other hydro-pneumatic suspension setups:

• In the Mercedes-Benz R129 SL-Class, the ADS suspension has conventional coil springs but no conventional shock absorbers. The hydraulic accumulators serve the function of the shock absorbers. The gas behind the membrane inside the nitrogen sphere is compressible and can absorb the shocks from the road (membrane deflects).

• In older Citroen cars, which were famous for their hydro-pneumatic suspensions, there is a nitrogen sphere at each wheel that serves the function of both the shock absorber and the spring. You won’t find any coil springs in Citroen cars with hydro-pneumatic suspension, and when a nitrogen sphere fails, there is virtually no damping in a Citroen car.

Similarly, the damping is lost when an accumulator fails in the R129 SL-Class with the ADS suspension. Citroen just took it one step further by getting rid of the coil springs altogether.

I hope I made this clear enough…

Here’s a video of an accumulator taken apart. It’s not from an ABC car, but its construction is the same.

Anyway, the key points are:

• The ABC is a complex, high-performance system and it will be more expensive to maintain than standard suspension or even the Airmatic.

• The ABC was improved in 2007. If the high maintenance cost doesn’t scare you, and you still want a car with the ABC, look for 2008 or newer cars.

• Age of the vehicle is important – the rubber hoses and seals age, which increases the likelihood of failure. The hydraulic hoses deteriorate and have a finite lifespan (they can burst). If a hose blows, the suspension shuts down. If you don’t stop on time, the hydraulic pump may die from running dry. As you can see, it’s important to keep the hydraulic hoses in good condition.

• The nitrogen spheres last around 60k 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.

• The hydraulic pump will need replacing at some point (very expensive). The pump has two sections – one that powers the suspension, the other runs the power steering.

• There are some options available on the market today that can help you save money servicing the ABC suspension: repair kits, aftermarket parts (Arnott has a good reputation) and reconditioned parts. Please be aware that it’s still going to be expensive…

If you’re still convinced that you want an SL-Class car with the ABC suspension, here’s what to look out for:

• Car dropping to quickly (on one corner or more than one) – visually check the car after it has been standing still for a couple days – the chassis should not be on the ground. According to Mercedes-Benz, a system in good working order should be able to hold the oil pressure at least for one week. If the car drops earlier, something is wrong with the valve block or there is a leak somewhere (probably a strut).

• Any ABC warning messages, appearing for a couple seconds when cornering, braking or after hitting a pothole, are bad signs. Before buying a car with the ABC, I recommend taking it to a professional to check the diagnostics log for any stored errors.

• Check the hydraulic fluid – it should be green. If it’s dark brown, black or the fluid level is too low, it means the car has been neglected or maintained poorly. Do not buy such a vehicle. Running the hydraulic pump without oil will kill it very quickly. The dark colour of the fluid means that it is dirty and full of abrasive particles – the entire hydraulic system could be worn out.

• Also, do not buy a car without maintenance history (including filter and hydraulic fluid replacements).

• Check the struts and the underside of the car for any oil leaks. Watch out for cracked dust covers on the struts (any damage there will accelerate the wear by allowing dirt and moisture in).

• Check that the car responds correctly, with no warning messages, to manually adjusting the ride height.

• Humming or howling noise between idle and 2000 RPM means that the pulsation damper has failed.

• The ride should be smooth and at the same time, there should be very little body lean when cornering or braking. If the car feels bouncy or hard, something is wrong.

M272 V6 & M273 V8 engine – soft balance shaft gears

The M272 V6 engines are equipped with a balance shaft. A balance shaft is an eccentric weighted shaft, which is used to eliminate engine vibration. It is driven by the timing chain as it needs to in sync with the engine pistons. A balance shaft is needed to make a V6 engine smooth because they are inherently unbalanced.

The M272 engines manufactured between 2004 and 2008 were fitted with balance shaft gears and idler gears that can wear out prematurely. The M273 engines don’t have balance shafts but still have the defective idler gears.

As these gears wear, the engine timing is altered due to increased slack in the timing chain. The first indication of a problem is the “Check Engine” light and P0017 or P0016 error codes. As the sprockets continue to wear, the engine will develop a rattle from the loose timing chain. All the while, more and more metal is being ground away from the gears.

If not fixed, one of the gears may fail eventually and take the engine out (valves hitting the pistons).

It is a problem that you should not ignore. The fix is to replace the gears with updated ones, which don’t wear out. It is very labour intensive, which makes it a very expensive repair.

Not all M272 and M273 engines made before 2009 are affected. Engine serial numbers below have updated gears, which are free from the problem:

• M272 engines with serial numbers higher than 2729 . . 30 468993

• M273 engines with serial numbers higher than 2739 . . 30 088611

Some engines seem to last despite having the unlucky gears, but I would not take the risk. In my opinion, affected engines are fine only as long as there is proof that the faulty gears (along with a number of other parts) have been replaced, and the engine is running well after the repair.

M272 and M273 engine – variable intake manifold

The intake manifold in these engines is very complex. It’s called a Variable (Length) Intake Manifold (VIM or VLIM), and it consists of three vacuum actuators, multiple levers, tumble flaps and valves that adjust the length of the intake tract.

It’s not a very reliable design with lots of plastic parts that can fail. When there’s a problem with the intake manifold, the engine performance is reduced (increased fuel consumption, loss of power, poor idle), and the “Check Engine” light may come on.

The typical cause of failure is increased friction in the mechanism from the accumulation of oil and carbon deposits in the intake manifold. Additionally, over the years the plastic parts in the mechanism become weaker and more brittle.

A couple of years ago you had to buy an entire manifold assembly to fix this problem (£800 at the dealership). Luckily, there are repair kits available on the market these days, which makes this a much smaller issue.

If you are curious how carbon deposits and oil appear in the engine intake manifold – they come from the Exhaust Recirculation Valve (EGR) and Crankcase Ventilation System (CVS). These are standard systems used in virtually all road cars.

M156 engine – head bolt corrosion (SL 63 AMG)

There have been cases of head bolt failures due to corrosion in the M156 engines. The head bolts are the fasteners that hold the cylinder heads in place. They are in constant tension to create a seal between the cylinder head and the engine block (engine block – head gasket – cylinder head).

When the bolts start corroding, the clamping force may be reduced, which could allow coolant to leak into the combustion chamber or mix with the engine oil. A typical symptom is dropping coolant level while the oil level is rising. If left untreated, the oil/coolant mix will eventually turn to sludge.

Coolant entering the combustion chamber can also cause increased smoking and a rough idle.

If any of the bolts holding the cylinder heads break off, a severe head gasket leak may appear. If the leak is large enough, the coolant entering the combustion chamber has the potential to hydrolock the engine.

Hydrolock is a situation when the piston tries to compress the incompressible liquid. The liquid is not going to give in but something in the engine will – usually the connecting rods. To illustrate, here’s a photograph of a bent connecting rod after hydrolock (not from a Mercedes-Benz):

Mechanical engine damage is the worst case scenario, usually, the cars start losing coolant before failing like this.

The head bolts were eventually updated by AMG around 06.2010. All engines made before this date are affected – engines with serial numbers beyond 569xx 60 060658 have the updated bolts. All cars produced before 2010/2011 may develop this problem. I believe that some 2011 cars are affected too.

Therefore, I recommend avoiding the pre-2011 SL 63 AMG cars unless you can get a warranty that covers potential engine meltdown or replace the head bolts preemptively.

As I see it, the percentage of engines that fail is small, but it’s not worth the risk to me.

M156 engine – camshaft wear (SL 63 AMG)

As I see it, another weak point of the M156 engines are the camshafts, which are susceptible to premature wear. A typical symptom of a worn camshaft is ticking noise, initially only when the car is cold started. Over time, it becomes more persistent as the wear progresses.

I believe that this is caused by a combination of too soft camshaft lobes and too hard cam followers. The camshaft lobes are quite sharp (small nose radius), which may contribute to the problem. It’s not only the camshafts that can wear. Once a camshaft is scored and loses its profile, the cam followers can wear too – in severe cases to the point of breaking a hole in the centre.

If you decide to get one of these cars, listen for a ticking noise for the first couple seconds after a cold start. It’s the same procedure as checking for timing chain issues, so look out for any chain rattle too. If you hear a ticking noise, it’s likely the camshaft. If you can hear a rattle which disappears after a couple seconds, it’s most likely the timing chain. Both types of noises are a bad sign.

Also, regular oil changes are very important for camshaft life. You should avoid cars with incomplete service history.

Here’s what a worn-out camshaft in the M156 engine looks like:

M275 engine maintenance costs

The M275 is based on the M137 – a naturally aspirated V12 engine that had some issues. For example, replacing a leaking oil cooler in the M137 required the cylinder heads to be removed, which made it a very expensive job.

The M275 is better in this regard, but it’s still a huge engine that fills out the engine bay completely, so replacing parts can be a nightmare. Something as simple as changing the motor mounts may require the engine to be removed from the car. It is possible to do it with the engine in the car if you get creative, but my point is that even small repairs can be pretty expensive.

There are 24 spark plugs and two ignition coil packs in this engine – one coil pack per cylinder bank. Each coil pack is an integrated unit that provides ignition for 6 cylinders. Imagine that you’re trying to fix a rough idle or misfire. Replacing the spark plugs alone will cost £500 or more because it’s not as straightforward as in an average engine and because there is 24 of them.

The coil packs fail occasionally and sometimes when there’s a misfire, replacing the spark plugs alone might not be enough. So you’ve already spent a lot of money on the spark plugs, and now you’ll try replacing the coil packs…

… a single coil pack is around £800.

If you compare this to the V6 M272 engine – a Delphi (OEM) ignition coil for the V6 is £40. There is one coil pack per cylinder and you can replace them individually, so it’s unlikely that you’ll need all six at the same time. I hope this gives you an idea what you’re getting yourself into when you buy a second-hand V12 Mercedes-Benz R230.

Summary of problems & additional information

• The R230 SL-Class is fitted with Sensotronic Brake Control, which is a brake-by-wire system. The SBC pumps have a finite lifespan and they are expensive to replace. Also, there have been cases of SBC malfunction before the pump reached its design lifespan (it’s a complex system after all). When SBC fails, the ability to brake is greatly reduced.

• Mercedes-Benz started galvanizing the R230 roadster in 2004. Newer cars have improved corrosion resistance.

• 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 balance shaft and idler gear issues in the M272 and M273 engines. The balance shaft horror story and the overly complex variable intake manifold mechanisms are the only real issues with these engines. Once fixed, they are not bad units. Unfortunately, Mercedes-Benz improved the balance shaft gears only in 2008. If you are going to buy an R230 with the M272 or M273 engines, be very careful not to buy one with worn gears. Look for one that had them already replaced or one with updated gears – check the engine serial number.

• The M156 was the first engine designed fully by AMG and not based on a Mercedes-Benz engine. Despite having excellent performance and winning multiple awards, this engine has its problems. I recommend avoiding the SL 63 AMG unless you are planning to do something about the head bolts that may corrode.

• As for potential camshaft wear in M156 engines, the safest thing to do would be to remove the valve covers and inspect the camshaft for wear before buying a second-hand SL 63 AMG. Many of these engines can reach a relatively high mileage without problems. If you are certain that the camshafts are in good shape, you should not have any issues for quite a while, provided that you maintain the engine well.

• Please be aware that Mercedes-Benz is a manufacturer of high-performance luxury cars. High performance usually goes hand in hand with increased complexity. This is particularly true for the SL-Class, which is filled with state-of-the-art technology like the ABC suspension or SBC braking system. When things go wrong, you can expect the servicing costs to be way above average.

• All R230 engines are fitted with timing chains, which normally don’t have a specified replacement interval. The timing chains in the R230 are generally reliable, with the exception of M272 and M273 gear problems, but they will not last forever. Read more about timing chains.

• Depending on the model, the R230 could be one of the most expensive to maintain older Mercedes-Benz cars you could buy. There is the complex Vario roof, the even more complex ABC suspension and the SBC braking system. If you choose the V12 engine, the maintenance costs will be even higher. Pick your model wisely – there will be a huge difference in maintenance costs between an SL 350 and an SL 600.

• It’s also the best looking SL-Class car up to date, in my humble opinion.

• For lowest cost and highest long-term reliability, I recommend a 2005-2006, pre-facelift SL 350 with standard suspension. A post-2008 SL-Class with a V6 engine, which has the updated balance shaft gears, is also a good option as long as you like the updated looks and don’t mind a bit more complexity. The key boxes to tick for reducing maintenance costs are:

  • the reliable M112 engine or the updated M272 engine

  • no ABC suspension

  • galvanized body

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