If you’re experiencing problems with your 1991 Mercedes 500sl where the ASR malfunction lamp is illuminated but the ABS light remains off, you might be encountering Limp Home Mode (LH Mode) due to a fault in the Electronic Accelerator (EA). This situation, common in the 1991 Mercedes 500SL and similar models of that era, can even manifest with just the key turned on, engine off. A quick check under the hood with the air cleaner housing removed can confirm LH Mode by observing the throttle linkage.
The EA system in your 1991 Mercedes 500SL relies on feedback from a potentiometer within the throttle actuator to ascertain the throttle position. Should this feedback signal fail to validate the commanded throttle position from the EA controller, the electronic control system disengages, forcing the throttle into LH Mode. This is a safety feature designed to prevent uncontrolled acceleration in your 1991 Mercedes 500SL.
If a faulty EA is indeed the culprit in your 1991 Mercedes 500SL, a temporary restoration of normal engine power can be achieved through a straightforward adjustment to the throttle linkage. Refer to the diagram below for visual guidance on these components.
(Alt Text: Detailed diagram of the 1991 Mercedes 500SL throttle linkage system highlighting rods and actuator arm relevant to LH mode troubleshooting.)
Rod #97, as depicted in the diagram, is responsible for rotating and thereby moving the throttle plate inside the throttle body of your 1991 Mercedes 500SL (see accompanying picture below). Under normal operation, rod #97 is actuated by the throttle actuator and connecting rod #109. However, when LH Mode is engaged due to EA failure, the actuator becomes inactive. In this state, movement of rod #97 is governed by an arm that engages it at its rear end, indicated by the circled area in the diagram.
(Alt Text: Close-up view of the 1991 Mercedes 500SL throttle body linkage, showing the arm and rod interaction point critical for understanding LH mode mechanical bypass.)
With the air cleaner assembly removed and the throttle in the closed position, LH Mode in your 1991 Mercedes 500SL is characterized by a noticeable gap. As you depress the accelerator pedal, this arm must traverse this gap before it begins to rotate rod #97 and open the throttle. This lag is what causes the reduced power in LH Mode.
To temporarily bypass this issue in your 1991 Mercedes 500SL, you can effectively eliminate this slack by inserting a wedge, such as a piece of rubber hose, into the gap. This makeshift solution ensures immediate throttle response. Alternatively, a more refined approach, and one that many owners of a 1991 Mercedes 500SL prefer, involves adjusting the length of one of the linkage rods – often rod #88. Modifying this rod’s length repositions the arm, mechanically removing the gap and restoring a more direct throttle response.
For a more permanent solution, particularly if you are comfortable with disabling the ASR system in your 1991 Mercedes 500SL, it can be achieved by interrupting the engine-running signal to the ASR control module. This is a known method employed by performance tuners like Renntech, and widely discussed in Mercedes-Benz enthusiast forums as a common ASR disable procedure.
On a 1991 Mercedes 500SL equipped with CIS fuel injection, the engine-running signal wire is typically a blue/white wire connected to terminal #15 of the ASR module’s electrical connector. Disabling ASR in this manner will result in the constant illumination of both the ABS and ASR malfunction lamps on the instrument cluster of your 1991 Mercedes 500SL. To address these persistent warning lights, you would also need to interrupt the wires connected to terminals #17 and #29. Furthermore, if you intend to permanently disable ASR without using a switch, you might also consider disabling the “!” indicator in the speedometer by interrupting wire #15.
While these modifications can help resolve LH Mode and ASR issues in your 1991 Mercedes 500SL, it’s crucial to remember that these are workarounds, particularly the throttle linkage adjustment. Addressing the underlying EA system fault is recommended for long-term reliability and safety of your 1991 Mercedes 500SL. Consult with a Mercedes-Benz specialist for a thorough diagnosis and repair of the Electronic Accelerator system for your 1991 Mercedes 500SL to ensure optimal performance and safety.
