If you’re experiencing issues with your ’91 Mercedes 500sl where the ASR (Acceleration Skid Control) malfunction lamp is illuminated but the ABS (Anti-lock Braking System) light remains off, you might be encountering Limp Home (LH) Mode due to a problem with the Electronic Accelerator (EA). As a seasoned auto repair expert at xentryportal.store, I’ve encountered this issue frequently, and here’s a guide to help you understand and temporarily resolve it.
Understanding LH Mode and Electronic Accelerator (EA) Issues in Your 500SL
In this specific scenario, LH Mode can engage even with the engine off, simply with the key in the ignition. A key indicator of LH Mode is observable in the throttle linkage, which you can inspect by removing the air cleaner housing.
The EA system relies on feedback from a potentiometer within the throttle actuator to ascertain the throttle plate position. If the signal from this potentiometer fails to confirm that the throttle position corresponds to the controller’s command to the actuator, the electronic control system disengages. This safety measure forces the throttle into LH Mode, restricting engine power.
Temporary Fix for LH Mode: Throttle Linkage Adjustment on Your ’91 500SL
Should the EA system be the root cause, you can often restore normal engine power temporarily with a straightforward adjustment to the throttle linkage. Let’s examine the components involved.
Rod #97 in the linkage system (refer to the diagram below) is responsible for rotating the plate inside the throttle body.
A diagram illustrating the throttle linkage system of a Mercedes 500SL, highlighting components #97, #109, and #88.
Normally, rod #97 is actuated by the throttle actuator and rod #109. However, in LH Mode, the actuator becomes inactive. In this state, rod #97 is moved via an arm that engages it at its rear end, as circled in the accompanying picture.
A close-up view of the Mercedes 500SL throttle body, focusing on the linkage arm and its engagement point with rod #97.
With the air cleaner assembly removed and the throttle in the closed position while in LH mode, you’ll notice that pressing the gas pedal results in the arm traveling through a noticeable gap before it begins to rotate rod #97. This “slop” is the issue.
You can effectively eliminate this play by inserting a rubber hose or a similar wedge into this gap. Alternatively, and perhaps more effectively, adjusting the length of one of the linkage rods – I believe rod #88 is the relevant one – can reposition the arm and remove the gap altogether. This adjustment can provide a temporary return to normal throttle response.
Disabling ASR System in a ’91 Mercedes 500SL
For situations where you might want to disable the ASR system, it can be achieved by interrupting the engine-running signal to the ASR control module. This is a method employed by performance specialists like Renntech and commonly discussed in Mercedes-Benz forums.
On a ’91 500SL 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 in the instrument cluster.
To address the persistent malfunction lamps, you can also interrupt the wires to terminals #17 and #29. If you intend to permanently disable ASR without a switch, you might also want to disable the “!” indicator in the speedometer, which is done by interrupting wire #15.
Disclaimer: Please note that these are temporary solutions and for informational purposes only. Addressing the root cause of the ASR malfunction, especially issues with the Electronic Accelerator system, requires professional diagnosis and repair. Disabling safety systems like ASR should be considered carefully and may have implications for vehicle safety and road legality. Consult a qualified Mercedes-Benz technician for proper diagnosis and repair of your 1991 Mercedes 500SL.
