One of the reason why the power electronic modules fails frequently on the Mercedes-Benz S400 is due to frequent and unnecessary start/stop cycles. 

Disabling the start/stop on the Mercedes S400 W221 is a fairly straightforward process which can have many benefits: 

  • extend the life of the car's hybrid components
  • give you the power you need all the time rather than having a pause every time you stop
  • reduce the cycles on the power electronics module, starter, and hybrid battery
  • overall make the car a lot more reliable

Procedure

The Process is straightforward given you have the right tools. Though I was able to do it using Vediamo and a chinese Star C4 unit, any other unit that can do variant coding is capable of doing this as well.

  1. The first step is to disconnect the power cables leading to the driver side seat belt buckle. You will find these under the seat of the car as labeled in the photo below. Raise the driver seat to the maximum level and access these from behind. The connecter on the left is a bit difficult but use a slim flat head and take out only the one with the arrow on top. power buckle
  2. Now you will get three warnings: perpetual seat belt unbuckled, airbag light, and pre safe warning. These will all need to be coded so that they don't show up anymore. Don't worry, the airbag and seat belt still work and you can verify by pressing the brakes hard after this procedure, the seat belt still locks up as it should. 
  3. Connect Vediamo to the car, and open the ARCADE221 ECU. Init contact to the ECU and then expand DiagJob and dobule click the DJ_Zugriffsberechtigung_1 and DJ_Zugriffsberechtigung_2. This unlocks the ECU allowing for us to code in what we want. vediamo
  4. Now click the button shown below, which shows us the error messages currently on the ECU. You should see two error codes 9176 and 9013 as pictured below, for both the airbag and power to the seat belt buckle. (This step is just for informational purposes, later we will need to come back to this menu and hit clear to clear the codes). err
  5. Go to the Variant Coding menu of the ARCADE 221 ECU and select the HW-Eingang Gurtschlob Fahrer verne borhanden and select nein. Then press ECU coding. Don't exit after that. This is for disabling the seat belt warning light. coding
  6. While you are there in the same variant coding menu select ZK Gurstraffer Fahrer Vorhanden then hit nein and ECU coding as well. This is for the airbag belt tensioner. coding2
  7. Then do ECU reset by going to Function -> FN_ECU_reset and double click. resetecu
  8. Then you need to go to STAR DAS and open the Instrument Cluster module developer menu actuations and click variant coding. You need to unlock the ECU so go to Control Unit AB block and look for the same. I forgot which SAM module it was that gets here but there you will find DJ_Zugriffsberechtigung_1 and DJ_Zugriffsberechtigung_2 variables. You need to click F3 and reset them to unlock ECU. 
  9. Then go to the Instrument Cluster module and go to variant coding and scroll all the way down to the presafe setting (the one right under Fahrdynamischer Seitenhalt) and change it to nicht vorharden. 
  10. Then go to the control module actuations (under the same Instrument Cluster module) and do a control module reset, and you should be all good to go! The ready symbol will stay yellow, which is normal. It will stay yellow the entire time you are driving the car which indicates that the start-stop will never engage. control module reset

Note that once you disable the auto start/stop feature the READY symbol on the dashboard will always stay on as yellow.

Disable mercedes auto start stop feature

What this means is that the start/stop is disabled. It does not mean the hybrid system is disabled. The electric motor still functions properly even with the auto start/stop feature disables. 

One thing to keep in mind is that the seat belt tensioner that deploys in case of an accident to tighten the seat belt, may not ignite since the vehicle think you are not wearing a seat belt. Although i tested it by pressing brakes hard, the seat belt still tightens, I can give no guarantees. Youcanic is not responsible for any damages caused by doing this and you do this at your own risk!

This post is for information purposes only.