Mercedes SL350 Battery Drain
You know that horrible moment when you turn the key and your car doesn’t respond at all?
For Mr M, that became a regular frustration.
His Mercedes SL350 appeared fine at the end of the day, yet by the next morning, the battery would be completely flat, even after another garage had replaced parts.
This issue persisted for weeks.
Each recharge only kept the SL350 running for a few hours before the battery went flat again. He couldn’t lock the vehicle for long without it draining entirely, making daily use difficult.
Running out of options, he turned to our Mercedes experts at RS Autotechnik, Dursley.
With the Mercedes SL350 battery drain making his vehicle unreliable, the real cause needed to be identified, and our technicians were confident they had the expertise to get to the bottom of it.
Our Initial Evaluation: Confirming the Mercedes SL350 Battery Drain Fault
At RS Autotechnik, Dursley, every accurate diagnosis begins with listening to the driver.
When Mr M brought his Mercedes SL350 to us, our technicians discussed the vehicle’s recent symptoms, previous repair attempts, and how quickly the battery was going flat. These early conversations often reveal clues about a Mercedes SL350 battery drain before testing begins.
With this information, we conducted a visual inspection to determine if there was anything obvious causing the Mercedes SL350 battery drain issue.
Our team looked for:
- Damaged or deteriorated wiring.
- Corroded electrical connectors.
- Components that may have been fitted incorrectly elsewhere.
Any of these could keep the vehicle drawing power when it should be resting. However, nothing concerning was found, so our team proceeded with the next step of our investigation.
Using the same Mercedes-approved diagnostic technology as the main dealers, we performed a comprehensive system scan to identify stored fault codes.
No codes were present, indicating none of the control units were detecting a fault. This suggested the problem was deeper within the electrical system, likely involving the Mercedes Controller Area Network (also known as CAN for short).
To reproduce the battery drain issue Mr M was experiencing, our team warmed the vehicle, turned it off, locked it, and observed how the systems shut down, keeping our eyes on the battery activity.
During this process, we performed a battery drain test to measure the current draw. The readings showed a steady 2.2-amp drain with spikes reaching 4.5 amps every few minutes.
Even a new battery would struggle under that load, so it was no wonder his vehicle’s battery was draining every couple of hours. The test confirmed the presence of a parasitic draw (something leeching power when it shouldn’t), and our next job was to identify which part of the system was responsible.
Detailed Investigation: Identifying the Cause of the Mercedes SL350 Drain
With no obvious explanation for the Mercedes SL350 battery drain, our next step was to examine the electrical system in greater depth.
Using official Mercedes technical information, including WIS wiring diagrams and component location data, our technicians mapped out the interior CAN network, the communication system linking the vehicle’s control modules.
When we connected the oscilloscope, the issue started to reveal itself.
The engine CAN shut down correctly once the vehicle was locked. However, the interior CAN continued to transmit signals even with the ignition off.
This behaviour confirmed why the Mercedes battery keeps draining, although it didn’t yet provide us with the specific module causing the issue.
To isolate the fault, our team disconnected the interior CAN modules one at a time, checking the oscilloscope trace after each step. This controlled and systematic approach is the only reliable method for pinpointing the source of why the Mercedes battery keeps draining.
After working through more than forty modules, the culprit finally appeared.
The moment the Pneumatic Control Module was disconnected, the network fell silent. The CAN system entered its sleep mode, and the current draw returned to expected levels.
Our team had found the source of the battery drain.
The module responsible for the SL350’s pneumatic and locking functions had developed an internal fault that kept the network awake.
A Mercedes control module replacement was required to restore proper CAN network behaviour and eliminate the drain issue.
Repairing the Fault: Installing and Coding the Replacement Module
Once we identified the control module responsible for keeping the network awake and causing the Mercedes SL350 battery drain problem, we called Mr M to explain what we had found and recommend a solution.
At RS Autotechnik, Dursley, we ensure you fully understand what is needed before any work begins. Transparency sits at the centre of our process, and you remain in complete control throughout. We only start the repair when you say so.
Mr M was relieved to finally understand the cause of the ongoing Mercedes SL350 battery drain and gave our technicians the go-ahead to proceed with the repair.
It was clear to our Mercedes experts what was needed to fix the battery drain issue.
The Pneumatic Control Module had developed an internal issue and required replacement. Fitting the new unit alone wouldn’t solve the problem; it also had to be coded and programmed so it could communicate correctly with the SL350’s CAN network.
Our technicians completed the repair following Mercedes-approved procedures:
- Removed the faulty Pneumatic Control Module and checked each connector for corrosion or damage.
- Installed a genuine Mercedes replacement module to maintain compatibility with the vehicle’s systems.
- Used authorised diagnostic software to code and program the new module so it communicated correctly within the CAN network.
With the control module installed and coded correctly, the communication network was functioning as intended. Every stage of the repair was completed with care to ensure long-term reliability, preparing the vehicle for final verification.
Validation Stage: Confirming the Mercedes SL350 Electrical Drain Was Fixed
After the repair was completed, our technicians repeated all previous oscilloscope and current-draw checks to ensure the fault had been eliminated.
Within two minutes, the interior CAN network shut down exactly as it was supposed to. After around thirty-five minutes, the battery’s resting current stabilised at 0.05 milliamps, which is correct for this Mercedes model.
These results confirmed that the issue was fully resolved. The Mercedes SL350 electrical issues caused by the faulty control module were no longer present.
Now that the CAN network was shutting down as it should, the vehicle could finally enter its “sleep state” without continuing to draw power.
Our team were confident the Mercedes SL350 battery drain had been fixed and that the car was ready to be returned to Mr M.
Result: Mercedes SL350 Restored and Battery Drain Eliminated
With the new Pneumatic Control Module installed and coded correctly, Mr M’s Mercedes SL350 was finally operating as it should.
There was no more waking up to a dead battery, no repeated jumpstarts, and no wasted money on parts that didn’t address the actual issue.
When Mr M returned to collect his car, our Mercedes experts walked him through our findings, showing him the faulty control module and explaining how the CAN network was now operating exactly as it should.
After seeing this for himself, he felt confident that the SL350 battery drain had been resolved permanently, not just temporarily masked. His vehicle was once again reliable and free from any Mercedes SL350 electrical issues.
For our technicians, this repair reinforced the importance of precise diagnostics. A single failing control module had caused a significant drain across the entire network.
By using official Mercedes data, oscilloscope testing, and a structured diagnostic approach, we delivered a first-time fix that saved the customer time, stress, and unnecessary costs.
Why Local Mercedes Drivers Choose RS Autotechnik for Electrical Diagnostics
Modern Mercedes vehicles rely on a network of control modules that must communicate perfectly. When one of those modules develops a fault, accurate diagnosis becomes essential.
At RS Autotechnik, Dursley, our technicians use Mercedes wiring data, CAN network information, and high-level diagnostic equipment to identify electrical issues correctly. Every conclusion is based on testing, not guesswork, saving you time, money and stress.
Here is why drivers in Dursley, Bristol, Tetbury, and Gloucester choose our garage:
- 12-month parts and labour guarantee.
- Courtesy cars are available.
- Dealership-level technology and technical data.
- Experienced Mercedes experts.
Join the {{review-count}} customers who’ve rated us {{average-rating}} stars on Google for excellent value, expertise and service.
If your Mercedes battery keeps draining, book a Mercedes electrical inspection at RS Autotechnik, Dursley.
We’ll identify and resolve the root cause of your Mercedes’ issue.
