S-Class W222 - Problem With Start Stop Function

[Tahushi]

The main battery for my S-Class S350 (2014, Diesel, LWB) was showing low voltage and cranking Amps. The start stop function was also not working (the light wasn't turning green) So I put a new genuine Mercedes battery. The start stop function still does not work (not turning green). I read somewhere there could be an issue with Auxiliary Battery AKA Voltage Converter. There are no warnings or fault sings. Any thoughts

 

[markjay]

The next thing to try is replacing the auxiliary battery.

On newer cars, it's actually a capacitor, but on your 2014 car it should still be a smallish lead-acid battery.

There could be other reasons why the Start/Stop doesn't work, but a weak main or auxiliary battery is the most common issue.

Obviously, you should make sure first that both batteries are fully charged, either by taking the car on a motorway drive, or by using a trickle-charger (e.g. CTEK).

 

[Atavus]

It’s probably the aux battery. Lack of stop/start is a common sign of it.

 

[Tahushi]

As I mentioned, the main battery is new so no issue with that. I have also tried but can't find any info where the others small battery is , if there is one.

 

[Atavus]

Scan the code under the fuel flap. It’s probably located in the boot.

 

[markjay]

Scan the code under the fuel flap. It’s probably located in the boot.

The QR code is a link to the rescue guide which shows the location of the batteries in the car.

 

[Diesel Benz]

The start/stop battery is at the trunk, on the right side behind the panelling. But I'd except a warning message on the cluster if the small battery fails. Better to run diagnosis on the car first although the small battery is not that expensive and relatively easy to change (have changed mine).

 

[Sweetooth]

My stop start hasn't worked for 5 months,took it for an mot today,tester had it idling for about 45 mins,drove it locally today on country rds for an hour restricted to 40 mph the green light came on in slow stop start traffic . I should add I do a40ml trip on the m4 every week.Any one have a plausible answer.

 

[Mercstarrr]

If anyone here has RAC breakdown cover, you can call them out to do a battery health check free of charge. They will then do a full test which takes about 15 minutes and will tell you the exact problem. When my stop start died, the RAC patrol did the check and said only the main battery needs replacement. Once he fitted the new Bosch battery, stop start started working absolutely fine and never had problems since.