Mercedes SL350 Battery Drain Solved
You know that feeling when you turn the key and nothing happens?
For Mr B, this became a daily problem. His Mercedes SL350 battery would be fine at night, only to be completely flat by the next morning, even after another garage had attempted repairs.
Each recharge kept the car alive for only a few hours before the battery dropped again. He couldn’t leave the vehicle locked for long without it draining completely, making it difficult to rely on.
Frustrated and out of options, he brought the car to our Mercedes experts at Cole & Miller Automotives, Arlesey.
With the battery going flat so quickly, the real cause had to be found, and our Mercedes technicians were confident they had the tools, knowledge and experience to identify it.
Our First Checks: Confirming the Mercedes SL350 Battery Drain Issue
At Cole & Miller Automotives, Arlesey, every accurate diagnosis starts before we even touch a tool.
We do this by listening.
When Mr B arrived with his Mercedes SL350, our technicians spoke to him about the car’s history, previous repair attempts, and the symptoms he’d noticed. Early details like these often provide a few clues as to the cause of a Mercedes SL350 battery drain before any testing begins.
With this information in mind, we carried out a thorough visual inspection to look for common electrical concerns, such as:
- Damaged wires.
- Corroded connectors.
- Incorrectly fitted replacement parts.
Any one of these issues could be causing his SL350 to continue drawing power after switching the engine off and locking the vehicle. Nothing obvious appeared during the inspection, so the next step was to inspect for any fault codes.
Utilising Mercedes dealership-level diagnostic technology, we conducted a comprehensive system scan to identify any stored fault codes.
No faults were present, indicating the control systems weren’t detecting the issue.
This suggested the cause was deeper within the electrical network, possibly in the Mercedes Controller Area Network (CAN).
To recreate the problem, we started and warmed the vehicle, switched the ignition off, locked it, and observed how the systems shut down while keeping a close eye on the battery.
During this process, our Mercedes experts performed a battery drain test. The readings showed a constant 2.2-amp drain, with spikes of up to 4.5 amps occurring every few minutes.
Even a new battery would struggle under that load. It was clear the car was experiencing a parasitic draw, meaning something was still consuming energy after shutdown. The next step was to determine which part of the system was responsible for the issue.
Detailed Investigation: Tracing the Cause of the Mercedes SL350 Battery Drain
With no apparent reason for the Mercedes SL350 battery drain, our next step was to analyse the vehicle’s electrical system in greater depth.
To narrow down which part of the system was staying awake and causing the Mercedes SL350 battery drain, our team turned to official Mercedes technical resources – the same ones the main dealership uses.
Using WIS wiring diagrams and component location information, we mapped out the interior Controller Area Network (CAN), which links dozens of control units throughout the vehicle.
We then connected our oscilloscope to monitor network activity, and the issue became clear almost immediately.
The engine CAN shut down as expected once the car was locked. However, the interior CAN continued sending data, even with the keys removed and the vehicle fully secured.
This confirmed why the Mercedes battery kept draining, but not yet which specific module was causing the network to remain active.
To isolate the fault, our technicians disconnected interior CAN modules one at a time, checking the oscilloscope trace after each removal. Although this process takes patience, it’s the only reliable way to pinpoint the exact source of the problem without relying on guesswork.
After working through more than forty control modules, the culprit finally revealed itself.
As soon as the Pneumatic Control Module was unplugged, the network went quiet. The CAN system entered sleep mode, and the current draw returned to normal levels.
Our team had located the reason why the Mercedes battery keeps draining.
The control unit responsible for central locking and related pneumatic functions had developed an internal issue that kept the system communicating continuously.
A Mercedes control module replacement was required to restore correct network behaviour and stop the excessive battery drain.
Carrying Out the Repair: Fitting and Coding the New Control Module
Once our technicians confirmed the source of the parasitic drain, we contacted Mr B to explain the findings and outline the repair plan.
At Cole & Miller Automotives, Arlesey, we believe in honesty and clarity. No work goes ahead until you understand what is needed and approve the next steps. You’re always in total control of the situation.
Mr B was relieved to finally know what had been draining his Mercedes SL350 battery and asked us to continue with the repair.
With his approval, our experts got to work.
The Pneumatic Control Module had failed internally and required a Mercedes control module replacement. However, installing a new component alone wasn’t enough to fix the issue, as it also had to be programmed and coded correctly to communicate with the vehicle’s CAN network.
Our team followed Mercedes’ approved procedures to complete the repair:
- Removed the faulty Pneumatic Control Module and checked each connector for corrosion or damage.
- Installed a genuine Mercedes replacement unit for complete compatibility with the SL350’s electrical system.
- Used manufacturer-authorised software to code and program the module so it integrated correctly into the CAN network.
Once the programming was complete, the repair was carried out to manufacturer standards, and the vehicle was ready for final testing in the next stage.
Final Verification: Ensuring the Mercedes SL350 Electrical Issue Was Resolved
After completing the repair, our technicians repeated all earlier oscilloscope checks and current-draw tests to confirm the fault had been eliminated.
Within two minutes, the interior CAN network shut down properly, and all network activity ceased. Around thirty-five minutes later, the battery’s resting current had stabilised at 0.05mA, which is correct for this model.
These results confirmed that the Mercedes SL350 electrical issue had been solved.
With the network now shutting down as intended, the car could finally enter its sleep state without continuing to use power.
Our team was confident that the Mercedes SL350 battery drain issue had been fixed and that the vehicle was ready to be returned to Mr B.
Outcome: Mercedes Restored and Battery Drain Eliminated
With the replacement Pneumatic Control Module fitted, coded, and responding correctly, Mr B’s Mercedes SL350 was finally behaving as it should.
No more surprise flat batteries, no repeated jump starts, and no wasted money on parts that didn’t address the issue.
When Mr B returned to collect his vehicle, our technicians talked him through the findings, showing him the failed module and explaining how the Mercedes CAN network was now shutting down precisely as it should.
Seeing the evidence for himself gave him confidence that the Mercedes SL350 battery drain had been resolved and wasn’t hidden behind a temporary fix. His SL350 was once again dependable, free from the Mercedes SL350 electrical issues that had caused so much frustration.
He drove away, pleased with the repair and was now confident he could carry on with his day without worrying about his vehicle not starting.
For our team, this case reinforced the importance of a structured diagnostic approach. A single faulty control module had caused a dramatic electrical drain across the vehicle.
By combining Mercedes technical data, oscilloscope analysis, and systematic testing, we delivered a first-time fix that saved the customer significant time and expense.
Why Local Drivers Trust Cole & Miller Automotives for Mercedes Electrical Issues
Modern Mercedes vehicles like the SL350 rely on complex communication systems linking numerous control units. When something within that network stops behaving correctly, it takes genuine expertise to trace the cause.
At Cole & Miller Automotives, Arlesey, our technicians combine years of hands-on experience with Mercedes technical data, dealer-level technology, and a structured diagnostic approach. Every repair is guided by accurate testing rather than assumptions or guesswork.
Here is why drivers across Bedford, Hitchin, Letchworth, Stevenage and the surrounding areas choose our experts:
- 12-month parts and labour guarantee for complete confidence in every repair.
- Courtesy cars are available to keep you mobile while your Mercedes is with us.
- Mercedes-trained technicians using official wiring data and CAN network information.
- Advanced diagnostic equipment that allows us to pinpoint complex faults accurately.
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If your Mercedes battery keeps draining or you suspect an electrical issue, book a Mercedes inspection at Cole & Miller Automotives, Arlesey.
We will find the cause, repair it properly, and make sure your Mercedes performs as it should.
