Experiencing the dreaded SRS light illuminating on your Mercedes Benz 500 Sl dashboard, followed by the car entering limp home mode, is a common frustration for owners of this classic vehicle. For me, this issue became increasingly frequent in my 1991 500SL, eventually escalating to a daily occurrence, sometimes multiple times a day. Like many others, I’ve researched this problem extensively in online forums, discovering that faulty Electronic Throttle Actuators (ETAs) are often cited as the primary culprit.
The cost associated with replacing these ETAs can be substantial. My long-term plan involves removing and inspecting them, with the intention of rewiring them myself, following guides like the one found at http://v12uberalles.com/throttle_actuator_rewire.htm. However, this is a time-consuming task that would leave my Mercedes Benz 500 SL out of commission for an extended period. Instead of fully cutting open the plug for a complete rewire, I considered using heat shrink over the existing wires, requiring only unsoldering one end, as the underlying problem is often attributed to insulation breakdown.
Seeking a quicker, interim solution to get my 500SL back on the road, I opted to disable the Automatic Slip Regulation (ASR) system. Drawing inspiration from a forum post detailing the installation of a RENNTech ASR defeat switch, I decided to cut three specific wires in the wiring loom leading to the ASR (and ABS) computer. The RENNTech switch essentially works by cutting these same wires and providing a toggle to connect and disconnect them as needed. In my Mercedes Benz 500 SL, these wires were identified as blue/white, brown/blue, and brown/yellow.
After carefully cutting these wires, my Mercedes Benz 500 SL now drives remarkably well, and the frustrating SRS dash light is gone. Since it was permanently illuminated anyway, I simply removed the bulb. The snow traction control switch’s red LED light remains on, but this is a minor inconvenience I can easily live with.
Historically, the traction control in my 500SL rarely engaged during my typical driving. Living in Australia, where ice and snow are non-existent, I realized I don’t miss the ASR system at all. Having driven high-performance cars predating traction control, I’m accustomed to managing wheel spin myself. Furthermore, I had previously installed a first-gear start device, which inherently increases the potential for rear-wheel spin.
My primary motivation for sharing this experience is to potentially assist other Mercedes Benz 500 SL owners facing similar SRS/ASR issues and limp mode problems. Additionally, I am curious if disabling the ASR system in a 500SL results in any performance gains, such as increased power. Are there any other advantages to installing a dedicated ASR defeat switch beyond simply resolving the fault condition?
