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Autologic Live: Electrical fault malfunction on Mercedes C-Class

By autotech-nath on February 4, 2019

Autologic’s OEM trained Master Technician, Kevin Hardy, supports a customer with an electrical fault malfunction on a 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.

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


A code read was performed and sent to Kevin via the customer’s 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 (ignition) switch is not present. Kevin also suggested the cause of a general electrical fault is likely to be as a result of the internal c8 capacitor shorting out.

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, working 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 need to be inspected and tested along with the C8 Park Pawl capacitor.


The following steps were provided to the customer:

This C8 Park Pawl capacitor is in the N/S/F foot well area. Checks required include:

  • At the c8 capacitor pin 2 = red 12v y Pin 1 = earth brown
  • Pin 3 = red/blue to EIS switch plug 1 pin 28, 30b voltage


To remove the C8 Park Pawl capacitor the following guidancewas provided:

  • Remove the N/S/F kick panel trim
  • Remove the glove box under dash panel
  • Remove the N/S/F section of carpet
  • 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 boot from fuse 448.


Kevin explained that if the c8 capacitor has shorted internally then it is possible that the feed into 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).








About Autotechnician
Autotechnician is a magazine published nine times a year, delivering essential information to independent garage owners and technicians in the UK. Delivered both digitally and in print, autotechnician provides readers with technical, training, business advice, product and news, allowing our readers to keep up to date with information they need to run and work within a modern workshop.
Aftermarket Media Solutions Ltd, The Joiners Shop, Historic Dockyard Chatham, Kent ME4 4TZ
T: 01634 816 165
Company no. 09625886
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