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Mercedes Support Case study

Autologic supports a technician with electrical fault malfunction on Mercedes-Benz W205 chassis

Using Autologic diagnostic software and his extensive product knowledge for Mercedes-Benz, Kevin identifies the cause and steps needed to fix the problem.

Case study for Mercedes (C- Class) 205 model series

Using AutologicLive support, the customer reported that a warning message on the instrument cluster was displayed. The warning message displayed was an auxiliary battery malfunction message.

Cause of the problem

A code read was performed and sent to Kevin via the customers Autologic device, which revealed the following fault:
  • b21dc01 buffer battery of the EIS malfunction there is a general electrical fault
The most likely cause of an EIS fault code is that one of the power supplies to the EIS switch (ignition switch) is not present.
Kevin also suggested that the most likely possible cause of a general electrical fault would be as a result of the internal c8 capacitor shorting out.
Kevin explained that the W205 does not have a separate auto start/stop battery, instead it has a current limiter device on the positive terminal of the battery under bonnet, and that this works in conjunction with the c8 park pawl capacitor. Therefore, the cause would most likely be because of no voltage to the ignition switch on the 30b circuit.
The voltage converter would therefore need to be inspected and tested along with the C8 Park Pawl capacitor.

Fixing the fault

The following steps were provided to the customer:
This C8 Park Pawl capacitor is in the N/S/F foot well area . Checks required for the following included:
  1. At  the c8 capacitor pin 2 = red 12v
  2. Pin 1 = earth brown
  3. Pin 3 = red/blue to EIS switch plug 1 pin 28, 30b voltage

To remove the C8 Park Pawl capacitor the following guidance was provided:

  1. remove the N/S/F kick panel trim 
  2. remove the glove box under dash panel
  3. remove the N/S/F section of carpet
  4. remove the plastic panel on the floor against the bulk head


The C8 Park Pawl capacitor is bolted to the left-hand side of the electrics panel, remove the 8mm nut and lift out the voltage converter. This feeds in to the EIS at plug 1, pin 28 =30b voltage.

The voltage supply to the C8 Park Pawl capacitor comes from the rear SAM control unit in the trunk/ boot from fuse 448.


Further remedies

Kevin explained that if the c8 capacitor has shorted internally then it is possible that the feed in to the EIS at plug, pin 28=30b voltage or additional battery voltage will still be missing after the repair, and still result in the same fault message on the dashboard.
If the voltage converter is replaced, and the fault remains, the following checks would be required.
  1. K40/5 f448 (Electrical fuse 448) rear SAM (signal acquisition module).
  2. Check electrical lines, connections and connectors for damage, correct connection, loose contact and corrosion, and repair or replace if necessary.
  3. Check regulator and alternator.
  4. Additional battery.
  5. Replace C8 (Park pawl capacitor).

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