Experiencing the dreaded SRS light illuminating on your Mercedes Benz 500 Sl dashboard, followed by the car entering limp home mode, can be incredibly frustrating. For owners of the classic 1991 500SL, this issue is becoming increasingly common. Like many, I recently encountered this problem with growing frequency over the last year, eventually escalating to a daily occurrence, sometimes even multiple times a day.
The online Mercedes Benz forums are filled with discussions about this exact problem, pointing towards the Electronic Throttle Actuators as the most probable culprit. However, replacing these actuators in a Mercedes Benz 500 SL can be a very expensive undertaking. My intention is to eventually remove and inspect them to determine if rewiring, as detailed in guides like the one found at http://v12uberalles.com/throttle_actuator_rewire.htm, can resolve the issue. This is a time-consuming project that would render my 500SL unusable for a period. Instead of completely cutting open the plug for rewiring, which seems complicated, I considered using heat shrink over the existing wires, requiring only unsoldering one end. The consensus is that the breakdown of the wire insulation is the root of the problem.
In the interim, seeking a quicker, temporary solution, I opted to disable the Automatic Slip Regulation (ASR) system. This was achieved by cutting three specific wires in the wiring loom leading to the ASR (and ABS) computer, after carefully exposing the wires by pulling back the insulation. This approach was inspired by posts from other Mercedes Benz owners who had installed the RENNTech ASR defeat switch. This aftermarket switch functions by cutting the same wires, offering the ability to connect and disconnect them as needed with a simple flick. In my 500SL, the wires in question were identified as blue/white, brown/blue, and brown/yellow.
Since performing this modification, my Mercedes Benz 500 SL now drives exceptionally well. To address the permanently illuminated ASR dash light, I simply removed the bulb. While the snow traction control switch’s red LED light remains on, this is a minor visual inconvenience I can easily accept.
Prior to this modification, the traction control system in my 500SL rarely engaged during my typical driving. Living in Australia, where ice and snow are non-existent, I find that I don’t miss the ASR functionality at all. It’s worth noting that I have a history of driving high-performance vehicles predating the widespread adoption of traction control systems. Furthermore, I had previously installed a first gear start device in my 500SL, which arguably made it even easier to induce rear wheel spin.
My primary reason for sharing this experience is to potentially assist other Mercedes Benz 500 SL owners facing similar SRS/limp mode issues. Beyond this, I am also curious: does disabling the ASR in a 500SL result in any performance increase? Are there any other advantages to installing a dedicated ASR defeat switch, such as the RENNTech unit, beyond simply bypassing the system? Any insights from fellow Mercedes Benz enthusiasts would be greatly appreciated.
