The Weakest Link - What to Check When Buying an R129 to Avoid Expensive Repair Bills

Our R129 has just spent 3 weeks at an independent Mercedes specialist being serviced and checked for any issues that may need looking at. We didn't expect to come through this unscathed, and indeed one of the things that was discovered was that one of the catalytic converters was failing. Not a quick fix, as the replacement had to be ordered from Germany, hence the wait to get the car back and came with a lovely price tag of around £900 plus VAT attached (cat converter + O2 sensor which couldn't be separated from the old cat converter +labour). Now, you can probably buy some aftermarket ones faster and cheaper, but our approach is not to be stingy when it comes to replacement parts. 

I can imagine that for potential buyers, spotting something broken that is so expensive to repair can be a deal breaker. There are two catalytic converters in the car as well, so if they both fail, you may have to tighten your belt for a while to get that fixed. Unfortunately, this fault is not easy to discover, unless you can raise the car up and knock on the catalytic converter. If it's broken, you will hear rattling. It doesn't seem to be a part that goes bad frequently though - our 1998 Mercedes still had its original converter, so even if you have to have one fixed, you can rest assured the new one will last you a good few years!

Apart from this, there are other things that are expensive to fix if they go wrong, and they are easier to check when buying. Of course, with it being a convertible with an automatic soft top retraction, that's an obvious things to check. The R129 soft top is operated by an electrically driven hydraulic pump, a bank of 15 hydraulic actuators, 11 solenoids controlled by microprocessors and 17 end-position switches. Complicated and not cheap to repair if things go wrong. Therefore, make sure you test the roof mechanism before buying the car.

Adaptive Damping System (ADS), a computer-controlled suspension system controlling the car's level was an optional extra (standard on SL600), which again can give you an impressive repair bill if its valve body fails.

The automatic transmission on R129 was marketed as sealed for life. Unfortunately, if you treat it as such and never change the fluid, its life is going to end up being rather shorter that you would expect. You may get told that there is no need to change the fluid, but don't believe that. Ideally you would want to change it every 40,000 miles, so check if it has been done when buying a car with a high mileage. Servicing your transmission regularly will spare you a massive bill to have it replaced if it fails. We saw no bills to prove the transmission fluid on our SL500 had been changed, but the car has only done 70,000 miles so far, so we weren't too concerned and simply had it done now.

Finally, make sure the A/C is working properly, as issues may end up being quite labour-intensive to sort out.