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What to do when your Mercedes-Benz battery drains!

One of the most severe issues you may encounter with your Mercedes-Benz motor is an issue with the battery, which of course, makes the vehicle practically unusable.

Don’t get us wrong; there’s a reason why Mercedes-Benz are one of the most recognisable car brands on the planet, and to their credit, they create sleek and easy on the eye vehicles year after year consequently.

Now, we understand that often your Mercedes-Benz is your pride and joy. Many of us treat our vehicles better than we treat ourselves! So when you encounter any issues with your Mercedes, especially issues as impactful as a battery drain, you should consider immediate remedy. 

Unfortunately, these severe problems with your vehicle can happen to any motor from any brand. Although the parts that go into your Mercedes-Benz are of the highest quality and any issues are less likely to occur; however, these serious problems can still be prevalent. 

When Mr Stephens sought our help with his Mercedes-Benz SL350 2003, the issue was clear, and the battery was completely drained. The battery drain was so bad the car would be completely flat after four hours. Fortunately for Mr Stephens, our expert Mercedes technician Jason Aldridge was on hand to diagnose and correct the issue.

Diagnosing the problem

Mr Stephens had actually taken the vehicle to another Mercedes garage, and despite replacing some parts, the fault was still persistent. Interestingly, no relevant fault codes were present; however, we diagnosed the problem by using the information and wiring diagrams from the online Mercedes WIS system.

We decided to fully charge the battery and test it, which of course, the battery failed the test. The next step was to replace the battery and carry out a battery drain test which resulted in a constant 2.2amp drain that rose to 4.5amps for about 5 minutes every 15 minutes or so.

Next, we started testing fuses to see which fuses had current flow when the vehicle was locked, as this would typically reveal one or two fuses which we could pinpoint down to one or two circuits that had current flow. In this case, it was soon self-evident we had a network issue as most if not all fuses on the driver’s rear and drivers front SAM module had current flow.

The decision was then taken to study the CAN network diagrams. We located a point to test the Engine CAN and interior CAN network. The oscilloscope identified that the interior CAN was active and awake even when the vehicle was locked. If the CAN network doesn’t shut down, then control modules on that network won’t shut down. In fact, they will keep communicating and operating as if the car was running.

Let’s fix the problem!

Our expert Mercedes-Benz technician Jason replaced the Pneumatic control module for the central locking system, causing the module not to shut down. We found that as soon as this module was isolated, the CAN network started to shut down. After 2 minutes, the network signal was dead, and the control modules could begin to shut down, and after 35 minutes, the battery drain was down to an acceptable 0.050 amps.

The next step was to then replace the control module with a completely brand new unit, which we programmed and SCN coded to the vehicle before re-performing the previous tests and finding that there was no drain and no live CAN network. We had to go into the interior CAN network location information and wiring diagram and discounted each of the 42 control modules until the network stopped communicating.

Did we stop there? No.

We value our customers

Here at Fergies, we care about our customers. As mentioned, the customer had visited another garage before contacting us. We used our fundamental understanding of electronics and network systems and specialised tools, and we fixed the problem entirely the first time. By completing the job at the first time of asking, our technician saved the customer money and time. 

Instead of performing an unnecessary total replacement, we completed the task at hand by fully diagnosing the problem and utilising our expertise.

That is just what we do!

Mercedes-Benz Battery Drain: In conclusion

If you are reading this and own a Mercedes-Benz, any year, any model, and you are experiencing problems with your motor vehicle, be like Mr Stephens and get in touch with one of our expert technicians here at Fergies.

When it comes to your Mercedes Benz, we always advise against allowing issues to build up, although, as in this case, issues can go undiagnosed, but there are warning signs to look out for. 

  1. Check engine warning light.
  2. Look out for signs of your Mercedes jerking and shaking.
  3. Listen out for unusual noises.
  4. Use your sense of smell for foul odours.
  5. Check for excessive smoke.
  6. Check for oil puddles.

These are just guidelines; if you have any of the issues above, get in touch! We have a team of certified Mercedes experts ready to get you back on the road in no time.

Call us now and we would be more than happy to offer advice or book you in for an assessment to save you money and find the cause of your issue.

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