Hello everyone, I’m reaching out for some assistance with my 1994 Mercedes Sl500 (R129). I’ve been using online resources to understand the control module layout in my vehicle, but I’m encountering some discrepancies, specifically when comparing my setup to diagrams intended for potentially different models, like the W140.
My 1994 SL500’s control module arrangement in the computer box doesn’t align with the layouts I’ve found online. To clarify what I’m seeing, I’ve included a picture of my module bay (94SL500 ECU Layout.jpg). Additionally, I’ve taken close-ups of the top three modules (94SL500 Top 3 Modules.jpg) and the bottom two (94SL500 Bottom 2 Modules.jpg) to clearly show their part numbers.
From my initial assessment, I’ve been able to identify the ‘Base Module’ and the ‘ASR (Acceleration Skid Control) module’. These seem to be in the first and second positions respectively. However, the order of the remaining modules appears to deviate from the expected layout.
Question 1: Could anyone with experience on the 1994 Mercedes SL500 R129 model help me accurately identify each of these five installed control modules based on their position and part numbers in the images provided?
Moving on to diagnostics, I was pleased to find that my vehicle is equipped with a Diagnostic Switch (94SL500 Diag Switch.jpg). It wasn’t mounted in a fixed position and is missing its cover, but it was present in the module bay.
However, when I attempted to retrieve diagnostic codes using the procedure outlined in online resources – specifically pressing the button for 2, 4, 5, and even 10 seconds – I encountered a problem: the diagnostic system is not responding. There were absolutely no blinking lights from the diagnostic switch, regardless of the duration I held the button.
Question 2: Does the complete absence of any blinking from the diagnostic switch on my 1994 Mercedes SL500 definitively indicate that there are no fault codes stored within the system? Or could there be another reason for this lack of response?
Finally, after investigating these issues, I decided to check the battery. I disconnected the negative battery cable to measure the resting voltage, which read 12.42vDC. This voltage seems reasonable for a car battery. Interestingly, after reconnecting the battery and starting the engine, the Check Engine Light (CEL) behaved as expected. It illuminated when the ignition was turned on and then switched off once the engine started. This normal CEL operation occurred after all the module and diagnostic switch probing.
Question 3: Is it possible that a battery that is slightly weak or marginal for a 1994 Mercedes SL500 could, in some way, cause the Diagnostic Module (or whichever module controls the diagnostic output) to incorrectly activate or ‘latch’ the output circuit for the CEL, even when there isn’t an actual fault present?
Any insights or information you can provide regarding module identification, the non-responsive diagnostic switch, or the potential influence of battery condition on the diagnostic system and CEL behavior in my 1994 SL500 R129 would be greatly appreciated. Thank you in advance for your help!
Cheers,
Jeff
