How to fix an auxiliary battery malfunction in a Mercedes Benz C300?

This blog will explain auxiliary battery malfunction in C 300 and answer the following questions: where is the auxiliary battery located in the C 300? Can you drive a C 300 with an auxiliary battery malfunction? How much does it cost to replace the auxiliary battery of the C 300?

How to fix an auxiliary battery malfunction in a Mercedes Benz C300?

In order to fix an auxiliary battery malfunction in a Mercedes Benz C 300, the auxiliary battery’s internal resistance is checked with the help of a diagnostic scanner. If the internal resistance is too high then the battery must be replaced. In the latest C 300, a park pawl capacitor is replaced as an auxiliary battery is not installed in the vehicle.

The onboard electrical systems control unit is constantly monitoring the power supply and internal resistance of the auxiliary battery. When the power supply is not in range or the internal resistance is greater than 0.0 ohms. A fault code is stored in the fault memory and an ‘auxiliary battery has a malfunction’ message appears in the instrument cluster.

Depending on the manufacturing year, there can be a battery or a capacitor installed in the C 300. Regardless of the type of component installed, the warning message in the instrument cluster will be the same.

Mercedes Benz modified the electrical system in the C 300 after 2014, a auxiliary battery was no longer required for the automatic engine cut-off function. Mercedes Benz used a voltage sag limiting device in the main battery of the vehicle.

The voltage sag limiter prevented the voltage level of the main battery from dropping below 10 volts when the starter motor was cranking the engine. The capacitor that is installed in the place of the auxiliary battery is only used to engage the parking gear in the transmission if the main battery is disconnected or completely discharged.

Where is the auxiliary battery located in the Mercedes Benz C 300?

Mercedes Benz is using an auxiliary battery from the third generation of the C 300, the location of the auxiliary battery in the different generations of the C 300 is as follows

• C 300 third generation (2007)

In the third generation of the Mercedes Benz C 300, there were two types of auxiliary batteries. The pre-facelift model used a small auxiliary battery located in the dashboard. The battery capacity is 12 volts and 1.2 A, it is a small auxiliary battery as the power supply is only used for the ignition switch when the main battery is discharged.

The auxiliary battery in the facelift model is located in the luggage compartment. It is a bigger battery compared to the pre-facelift model. The battery capacity is 12 volts 200 A and it is used to support the onboard electrical components when the engine is turned off in the ECO mode.

There is an auxiliary battery relay, which is actuated by the front SAM. The auxiliary battery is charged from the main battery. A signal line from the positive terminal goes to the front SAM for the battery monitoring function.

• C 300 fourth generation (2014)

Mercedes Benz modified the onboard electrical network in the fourth generation of the C 300. The model series is W205 and there is no auxiliary battery in the W 205. The engine start-stop function can still work with the power supply from the main battery.

The Auxiliary battery malfunction is still present in the W 205 because a park pawl capacitor is installed in the place of the auxiliary battery. The front SAM monitors the voltage level in the capacitor, it should be between the 11-14.5 V range.

The capacitor is located in the front passenger footwell near the powertrain control unit. Replacing the capacitor can take up to an hour as the front passenger seat and the floor carpet is removed to gain access to the park pawl capacitor.

The location is the same in the facelift version of the C 300. The facelift model only has updated front and rear bumpers and few interior equipments. There are no changes in the 

• C 300 Fifth generation (2021)

Mercedes Benz recently launched the fifth generation of the C Class with a 48 V mild hybrid technology. There is a 48 V high voltage battery in the engine compartment and a 12 V battery is present in the luggage compartment.

The 12 V battery is used to power the onboard electrical system and a park pawl capacitor is present in the front passenger footwell. It is used for emergency parking brake operation when the 12 V electrical battery is completely discharged.

A DC to DC converter is used to charge the 12 V battery from the 48 V high voltage battery. The 12 V battery then maintains the charge in the capacitor. The capacitor discharges only when the parking brake emergency operation is needed so it does not need a state of the charge monitoring function.

Can you drive a Mercedes Benz C 300 with an auxiliary battery malfunction?

Yes, it is possible to drive a Mercedes Benz C 300 with an auxiliary battery malfunction. An auxiliary battery is only used to support the interior equipment like radio, blower motor, and onboard control units when the main battery is starting the engine.

Another function of the auxiliary battery is to engage the emergency parking brake. The  ECO start-stop and the emergency parking brake function become inoperative when the auxiliary battery malfunction is present in the vehicle.

As the auxiliary battery is charged by the main battery, the condition of the main battery must be checked before replacing the auxiliary battery. If the auxiliary battery is not replaced then the main battery will also get damaged.

How much does it cost to replace the auxiliary battery of the C 300?

The cost of the repair can depend on the generation and the capacity of the auxiliary battery in the C 300. If the auxiliary battery is small then it can cost 40 USD to replace it. The auxiliary battery in the facelift model can cost 250 USD and the park pawl capacitor costs 150 to 200 USD.

Mercedes Benz can replace the auxiliary battery in the C 300 for zero charges if the vehicle is covered by the standard warranty and there should be no accidental damage to the battery. If the customer is replacing the auxiliary battery then there is a 2-year parts warranty on the battery.

Conclusion

This blog explained the remedy for the auxiliary battery malfunction in the Mercedes C 300. The auxiliary battery plays an important role in the onboard electrical system of the vehicles and the fault must be rectified as soon as possible.