If you’re experiencing an ASR malfunction light in your Mercedes 500sl without the ABS light illuminating, you might be encountering LH Mode triggered by an issue within the Electronic Accelerator (EA) system. This condition, LH Mode, can even occur when the engine is off but the key is in the ‘on’ position. A telltale sign of LH Mode is observable in the throttle linkage, specifically when the air cleaner housing is removed.
The EA controller relies on feedback from a potentiometer in the throttle actuator to ascertain throttle position. Should this feedback signal fail to confirm that the throttle position aligns with the controller’s command to the actuator, the electronic control system disengages, causing the throttle to default to LH Mode.
Should the EA be the root of your Mercedes 500SL’s problem, restoring normal engine power can be achieved through a straightforward adjustment to the throttle linkage. Refer to the diagram provided below for visual guidance.
Rod #97 in the diagram is responsible for rotating the plate within the throttle body (as shown in the image). Under normal operation, rod #97 is moved by the throttle actuator and rod #109. However, in LH Mode, the actuator becomes inactive, and rod #97 is instead moved via an arm that engages it at its rear end (highlighted in the circle). With the air cleaner assembly detached and the throttle in a closed position, you can observe in LH mode that pressing the accelerator pedal causes the arm to travel through a noticeable gap before it initiates the rotation of rod #97.
This slack can be effectively eliminated by inserting a piece of rubber hose or a similar material into this gap to act as a wedge. Alternatively, and perhaps more permanently, adjusting the length of one of the linkage rods, likely rod #88, can reposition the arm to remove the gap altogether. This adjustment ensures a more direct and immediate throttle response in your Mercedes 500SL even if the EA system is experiencing issues leading to LH Mode.
For a more permanent solution to ASR issues in your Mercedes 500SL, disabling the ASR system entirely is an option. ASR can be disabled by interrupting the engine-running signal input to the ASR control module. This method is utilized by Renntech and is a common practice discussed in various online forums dedicated to Mercedes-Benz vehicles.
In Mercedes 500SL models equipped with CIS fuel injection, the engine-running signal is carried by the blue/white wire connected to terminal #15 of the ASR module’s electrical connector. Disabling ASR in this manner will result in the continuous illumination of both the ABS and ASR malfunction lamps on the instrument cluster. To address these persistent warning lights, you would need to interrupt the wires connected to terminals #17 and #29. If a switch is not being implemented to selectively disable ASR and the intention is to keep it permanently off, it is also advisable to disable the “!” indicator in the speedometer. This can be achieved by interrupting wire #15.
Please Note: While these modifications can address LH Mode and ASR issues in your Mercedes 500SL, it is crucial to consult with a qualified Mercedes-Benz technician for a thorough diagnosis and repair, especially for complex electronic system malfunctions. These fixes may serve as temporary solutions or for off-road/track use and might not fully resolve the underlying electronic issues.
