For this month’s blog post, we shed some light on the notorious biodegradable wiring harness issue, and how it may affect your R129 SL, and other Mercedes Benz models of the era.

One of the most frequent questions we receive at The R129 Co is, ”Does the wiring harness need replacing on my R129?” This concern is not without merit, as the use of biodegradable wiring insulation has become a heavily documented issue among 90s Mercedes owners and enthusiasts.

You’ll see all sorts of conflicting information online about the famous misstep from Mercedes. It was an odd, performative ‘environmentally friendly’ choice from the marque, intended to reduce environmental impact by using materials that would break down over time. While the intention behind using biodegradable insulation materials was noble, the execution has left much to be desired.

As the insulation ages, it becomes brittle and begins to disintegrate, leading to exposed wires that can cause shorts, electrical malfunctions, and even fires. This problem doesn’t just affect the R129 SL; it also impacts other Mercedes models from the early to mid-1990s, including the W124, W140, and W202. The insulation’s degradation process can vary depending on factors such as climate, vehicle usage, and storage conditions, making it a somewhat unpredictable issue.

This isn’t something to fear – If your car is affected, taking preventative measures can ensure its longevity and reliability. Replacing the biodegradable wiring harness with modern, non-biodegradable alternatives is a common and effective solution.

How do I know if my car is affected?

People often cite model years to watch out for (approximately 1993-1995), and this is the correct approach to an extent, but the devil is in the details – it’s not always possible to be 100% sure on whether the car is affected – on crossover model years for example, or where registration year differs to model year – without a careful check of the VIN number, engine number and implicit part numbers the car left the factory with, on top of a visual inspection for degradation, or to see if your car has already had the biodegradable wiring harness replaced.

Biodegradable wiring insulation disintegration – typically looks like this

That said, here is an approximate guide based on chassis codes for the R129 SL:

What are the symptoms of a failing or failed biodegradable wiring harness?

The wiring harness on your R129s engine is its nervous system, connecting various sensors and the electrical aspects of your injection system to the engine control unit. Any electrical fault can cause perplexing symptoms, and this is no exception – there are myriad signs to be weary of – but, the most common are related to the elements which get the hottest – the wiring for the injectors, which can lead to the following:

• The car won’t start

• The car struggles to start

• The engine cuts out

• Rough running at idle

• Poor or uneven throttle response

• Engine hesitation

• Misfires

Now, you may have other undiagnosed faults on your car contributing to any of the above – but crucially, you cannot proceed with any meaningful diagnosis until the wiring harness is replaced. So before scratching your head, or better yet, before you even need to start scratching your head – check if your car needs the work done (with our help if you’re not sure), and get ahead of it.

What work needs doing?

Heat and age are the culprits, and so it’s the engine wiring harness, upper (the main harness to the injectors) and lower (the smaller starter/alternator harness) that must be replaced on affected models.

Many of the cars still in circulation today have had these issues addressed with updated harnesses, typically produced by Delphi.

An updated Delphi wiring harness

Delphi supplied most of the updated versions to Mercedes in the 90s and onwards. Or better yet, the car may already have a completely rebuilt harness, like the motorsport grade, bulletproof ones we offer, backed by a lifetime warranty.

One of our guaranteed for life rebuilt harnesses

What about other wiring harnesses on my car?

On affected V8 models with ASR, the throttle body cable must be replaced. Affected V12 models often require more attention. Both throttle body cables require replacement, and it is advisable to replace the first section of the harness which connects the MAF sensors to the module box. The larger body-mounted harness is generally unaffected on most cars, except for V12 models due to the immense heat generated by the M120 engine. If the body harness on your V12 model needs replacing, it is a significant task but certainly achievable. Feel free to get in touch via Whatsapp or email, or via our contact form for bespoke help regarding the surplus issues outlined above.

I don’t have an R129, but I do have a Mercedes from the early 90s – how do I know if my car is affected?

If you own a different Mercedes model from the early 90s and are concerned about the biodegradable wiring harness issue, you can Whatsapp or email with your VIN number, and we’ll check it for you. Some models, such as 4-cylinder M111 W202 and W124 variants, have a slightly different setup with a separate harness for the ignition system, which also requires replacement. Provided you can supply us with some semblance of the original cabling, we can rebuild any harness and offer you peace of mind with a lifetime warranty.

To conclude, while the biodegradable wiring harness issue has caused considerable concern among Mercedes owners, it is a manageable problem, and best handled with preventative maintenance. By understanding the symptoms, identifying if your car is affected, and taking the necessary steps to replace problematic wiring, you can ensure the longevity and reliability of your vehicle. If you have any questions or need assistance, don’t hesitate to reach out to us at The R129 Co. We’re here to help you keep your Mercedes running smoothly for years to come.