What does auxiliary battery malfunction mean on a Mercedes E 350?
This blog will explain the meaning of the auxiliary battery malfunction on a Mercedes E350 and answer the following questions: how to fix the auxiliary battery malfunction on a Mercedes E 350? Can you drive an E350 with an auxiliary battery malfunction? How to tell which auxiliary battery is in your E350?
What does auxiliary battery malfunction mean on a Mercedes E 350?
Auxiliary battery malfunction on a Mercedes E 350 means that the voltage of the secondary battery of the onboard electrical system is not within the specified range. The specified range of voltage for the auxiliary battery is 11-14.5 V. A fault code is also logged in the electronic ignition switch or in the battery management control unit.
Mercedes Benz used different variations of the onboard electrical system network in the E 350. Depending on the generation of the E 350, the auxiliary battery is used to either support the onboard electrical network when the main battery is used to start the engine in Eco mode or engage emergency parking gear when the main battery is fully discharged.
The battery capacity and size also vary according to its function. The auxiliary battery concept is not new in Mercedes Benz. A dedicated starter battery was used in the models manufactured before 2006 in addition to the main battery.
The third generation of the E 350 model series W211 also used an auxiliary battery for the Sensotronic brake system. This model was the only exception as the auxiliary battery used in the W211 E350 was only for the fly-by-wire brake system and had no function in the vehicle power supply.
The W211 E 350 will not show an auxiliary battery malfunction message in the instrument cluster. Instead, a battery symbol will appear with an increased stopping distance message flashing in the instrument cluster.
How to fix the auxiliary battery malfunction on a Mercedes E 350?
The procedure to fix the auxiliary battery malfunction on a Mercedes E 350 is explained below:
W212 E350 pre-facelift
The fourth generation of the E 350 was launched in 2009. It has a small auxiliary battery located on the driver-side inside the dashboard. The capacity of the auxiliary battery used in the W212 E350 is 12 V 1.2 Ah and the part number is N000000004039.
This small battery engages the parking gear when the main battery is fully discharged or disconnected. This is a safety feature to secure the vehicle, the power supply from the auxiliary battery directly goes to the electronic ignition lock.
To fix the auxiliary battery malfunction in the W212 E350 pre-facelift model, the old battery is replaced. A 10 A fuse is also renewed if the condition of the old fuse is not okay. The fuse is present in the electrical line between the positive terminal and the BMS control unit.
If the fault occurs after replacing the auxiliary battery then it means the 10 A fuse is blown and the BMS cannot measure the voltage level of the auxiliary battery.
W212 E350 facelift
Mercedes Benz launched the facelift version of the W212 E350 with the Eco start-stop function. The auxiliary battery is located in the trunk of the vehicle next to the spare wheel. It is used to power the onboard electrical system and the auxiliary transmission pump when the vehicle turns off automatically in the ECO or engine cut-off mode.
The size of the auxiliary battery is more than the pre-facelift model. The W212 E 350 facelift uses a 12 V 200Ah auxiliary battery, part number is A0009829208. There is no need to disconnect the main battery while replacing the auxiliary battery.
When the vehicle is switched off, the auxiliary battery relay is open and the battery can be replaced. The BMS control unit only connects the auxiliary battery to the vehicle power supply when the engine is automatically started in ECO mode.
W213 E350/ E350 facelift
Mercedes Benz made several modifications to the onboard electrical network of the fifth generation of the E 350. The W213 E 350 does not have an auxiliary battery like its predecessors.
The voltage drop at the main battery is limited through a resistor built into a device called a voltage dip-limiter. When the engine is starting in ECO mode, the voltage drops at the main battery due to the starter motor being limited to only 2-3 Volts.
Hence there is no need for the auxiliary battery for the Eco start-stop function. Instead of a conventional lead-acid auxiliary battery, there is a voltage converter. The voltage converter is a capacitor and it only discharges when the emergency parking gear is engaged.
The message shown in the instrument cluster is the same as the voltage converter is connected in the same circuit as the auxiliary battery. Therefore, if the fault occurs the voltage converter is replaced to fix the warning shown in the instrument cluster.
Can you drive an E350 with an auxiliary battery malfunction?
Yes, it’s possible to drive an E 350 with an auxiliary battery malfunction. The auxiliary battery does not affect the ability of the vehicle to start. It is only used for emergency and ECO functions.
If the voltage of the auxiliary battery is below the specified range then the ECO start-stop will not work. In the pre-facelift model, the vehicle will not be able to engage the P gear when the main battery discharges.
The cost to fix the auxiliary battery malfunction depends on the type of battery and the generation of the E 350. On average, it can cost between 200-300 USD but if the auxiliary battery is not replaced soon then it can damage the main battery of the vehicle.
The battery management system control unit is always monitoring the voltage of the main battery and auxiliary battery. It connects the auxiliary battery to the main battery in order to charge it.
If the new auxiliary battery malfunctions again then the auxiliary battery relay may be the cause since the relay is used to connect the main battery to the auxiliary battery.
How to tell which auxiliary battery is in your E350?
One of the easiest ways to check which type of auxiliary battery is in your E 350 is to check the vehicle identification number on the driver’s side B- pillar. If the starting series is W213, it is the voltage converter.
The W212 has a conventional lead acid battery, if there is no ECO start stop button in the W 212 then it has the smaller 1.2 Ah battery in the dashboard also known as a buffer battery.
This blog explained the meaning of the auxiliary battery malfunction and how to fix it in different generations of the E 350. Although the auxiliary battery is used for the support function, it is recommended to fix the fault as soon as possible. The vehicle can easily be stolen if the main battery is discharged and the emergency parking gear is not engaged.