Mercedes SL350 Battery Drain
You know that sinking moment when you turn the key and nothing happens at all?
For Mr P, this became a daily occurrence.
His Mercedes SL350 appeared completely fine when he parked it up at night, but by the next morning, the battery would be entirely flat. Despite previous repair attempts elsewhere, the problem persisted.
Each recharge provided only a short window before the power vanished again. He couldn’t leave the car locked without it draining within a few hours, making daily use almost impossible.
Looking for clarity, he visited our expert Mercedes technicians at Shires Garage, Langport.
With the drain happening this fast, the underlying cause needed to be uncovered. Our experts were confident they could solve the Mercedes SL350 battery drain problem.
Diagnostic Starting Point: Understanding the Mercedes SL350 Electrical Drain
At Shires Garage, Langport, all accurate diagnostics begin with a clear understanding of the driver’s experience.
Our Mercedes experts achieve this by listening.
When Mr P brought in his Mercedes SL350, our technicians talked through the symptoms his car was experiencing, previous work attempted, and how quickly his car’s battery was losing charge.
These initial discussions often highlight where a Mercedes SL350 battery drain may be coming from and point our initial investigation in the right direction.
We followed this by carrying out a visual inspection of the Mercedes to look for any obvious concerns, such as:
- Damaged wiring sections.
- Corroded or oxidised connectors.
- Incorrectly fitted parts.
Any of these could cause the vehicle to draw power when it should be shut down. Nothing was found visually, so we continued with electronic diagnostics.
Using the official Mercedes diagnostic technology (the same one the main dealerships use), we conducted a comprehensive system scan to identify any fault codes.
No codes appeared, meaning none of the control modules were registering an issue. This suggested the fault was deeper within the Mercedes electrical network, likely involving the Mercedes Controller Area Network (also known as CAN).
To recreate the problem, we started the vehicle, let it warm to operational temperature, turned it off, locked it, and monitored how the electrical systems shut down.
During this process, we kept a close eye on the battery and performed a battery drain test to measure current draw. The readings showed a steady 2.2-amp drain spiking to 4.5 amps every couple of minutes.
This level of draw would quickly drain any battery, even a brand-new one. It was no wonder Mr P’s battery was draining overnight. The results confirmed the presence of a parasitic drain, and the next task was isolating the root cause.
In-Depth Testing: Locating the Mercedes SL350 CAN Network Fault
With no obvious external cause for the Mercedes SL350 battery drain, our next step was to dig deeper into the vehicle’s internal electrical systems.
Using the same official Mercedes resources, WIS wiring diagrams and component locations as the main dealers, we mapped out the interior CAN network, which coordinates communication between the SL350’s many control units.
Our team then connected an oscilloscope to the system to monitor network activity, which quickly confirmed what we’d suspected.
Although the engine CAN shut down normally, the interior CAN continued sending communication signals long after the ignition was off and the vehicle was locked.
This explained why Mr P’s Mercedes battery keeps draining, but it didn’t reveal which control module was causing the issue.
To find the source of the fault, our technicians disconnected each interior CAN module individually, checking the oscilloscope reading after each step. This slow, methodical approach ensures accuracy without guesswork.
After testing more than forty modules (yes… forty), the faulty component became obvious.
Once our team disconnected the Pneumatic Control Module, the network fell silent immediately. The CAN system entered sleep mode, and the battery draw stabilised.
The cause of the Mercedes SL350 battery drain had been identified.
This particular module, which controls the vehicle’s pneumatic and locking functions, had developed an internal issue that kept the network awake.
To solve the issue and restore the Mercedes to peak working condition, a Mercedes control module replacement was required. Doing so would stop the parasitic drain from leeching energy when it shouldn’t.
Carrying Out the Repair: Fitting and Coding the New Module
Once we identified the faulty component within the Mercedes CAN network, our team called Mr P to discuss our findings and outline the repair plan.
At Shires Garage, Langport, we always keep you in the know, ensuring you understand the work required before anything is carried out. Transparency is a fundamental component of our approach.
Mr P was glad to see we’d uncovered the root cause of the Mercedes SL350 battery drain and gave us the green light for the repair.
With the approval, our experts began the repair process.
As the Pneumatic Control Module had failed internally, a Mercedes control module replacement was required. However, simply installing the new module alone wasn’t going to be enough; it also had to be programmed and coded so that the SL350’s CAN network could recognise it.
Our technicians completed the repair following Mercedes’ approved procedures:
- Removed the faulty Pneumatic Control Module and checked every connector for corrosion or damage.
- Installed a genuine Mercedes replacement module to maintain full compatibility with the system.
- Used official Mercedes diagnostic software to code and program the new unit, ensuring the CAN network recognised it correctly.
With the module installed and coded successfully, the communication network was operating correctly. Each step had been performed with care to ensure long-term reliability, setting the vehicle up for the final stage of our repair process: validating the repair.
Verifying the Mercedes SL350 Electrical Fault Was Gone
After installing and coding the replacement module, our technicians repeated all earlier tests, including oscilloscope monitoring and current-draw measurements, to ensure the issue had been completely resolved.
Within two minutes of turning the vehicle off, the interior CAN network shut down as expected. By the thirty-five-minute mark, the battery’s resting draw had stabilised at 0.05 milliamps, which is normal for the Mercedes SL350.
These results confirmed that the Mercedes SL350 electrical issues were caused entirely by the faulty module. With the CAN network now shutting down correctly, the SL350 could finally rest without unnecessary power consumption.
Our team were confident the Mercedes SL350 battery drain had been fixed and that the car was ready for Mr P to collect.
Result: Electrical Fault Resolved and Mercedes Performance Restored
With the new Pneumatic Control Module installed and coded as required, Mr P’s Mercedes SL350 was finally functioning properly.
No more overnight battery failures, no more repeated charges, and no more confusion about the cause of the issue.
When Mr P arrived to collect his car, our experts reviewed the repair with him, showing the failed module and explaining how the CAN network was shutting down in the correct sequence.
After seeing the results, he felt confident that the Mercedes SL350 battery drain had been fixed correctly and that no Mercedes SL350 electrical issues remained. His SL350 was once again dependable for day-to-day use.
For our technicians, this repair demonstrated the importance of structured diagnostics. One faulty module had kept the entire network awake. But, through the use of Mercedes wiring data, oscilloscope checks, and a careful investigation, we delivered a first-time fix that prevented further inconvenience.
Why Local Drivers Choose Shires Garage for Mercedes Electrical Issues
Modern Mercedes models, like the SL350, depend on a complex communication network linking dozens of control units. When something within that network stops behaving as it should, accurate testing is essential.
At Shires Garage, Langport, our team uses years of experience, official Mercedes technical data, CAN network information, and dealer technology to locate electrical faults precisely.
Here is why drivers across Langport, Taunton and the surrounding areas choose our garage:
- 12-month parts and labour guarantee.
- Courtesy cars available.
- Skilled Mercedes experts.
- Dealer-level technology and technical information.
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If your Mercedes battery keeps draining, book a Mercedes electrical diagnosis at Shires Garage, Langport.
Our Mercedes experts will identify the root cause of your vehicle’s issue and restore its reliability.
