Learn how to replace the auxiliary battery (also known as the small computer or secondary battery) on Mercedes-Benz ML-Class W164, R-Class W251, and GL-Class X164 vehicles. A complete list of applicable models is shown below.
The most common symptom that the small battery needs to be replaced is the car starts and runs fine, but you get a battery warning light on the dashboard that says “Auxiliary Battery Malfunction.”
Use a Mercedes-Benz diagnostic scanner to read the codes. You should get a fault code pointing to a faulty G1/7 auxiliary battery.
What you will need
You will need a 12-volt battery. The original battery is lead-acid, but AGM batteries will also work. Here are a few options.
R-Class W251 Auxiliary Battery Replacement
Instructions on changing the auxiliary battery on the R-Class are slightly different than the ML and GL instructions. On the R Class, you can replace the battery without removing the main battery or passenger seat.
- Turn off all of the accessories and lights. Move the shifter to the Park position and remove the key from the ignition.
- Move the plastic door molding on the passenger side.
- Lift the carpet. It will help to remove the carpet on the passenger side completely. At the minimum, lift the carpet enough to get to the 10mm plastic nuts.
- Remove the 10 mm nuts.
- Lift the fiberglass panel.
- Locate the auxiliary battery and unplug the positive and negative connections.
- Remove the two 13mm bolts. Once you remove the two bolts, you can slide the old battery and the bracket out.
- Remove the auxiliary battery.
- Install the new auxiliary battery 000000004039 in reverse order.
These instructions apply to R350, R500, R320, and R63 AMG models.
ML-Class W164 and GL-Class X164 Auxilary Battery Replacement
You will need to remove the passenger seat bolts and tilt the seat forward on the ML and GL models. You will need a Female Torx Socket Set to remove the Torx bolts.
Another option is to move the passenger seat forward and up. Then remove the main battery under the passenger seat. This will allow you to access the auxiliary battery.
- Remove the key from the ignition.
- Move the passenger seat to the back. Move the headrest down.
- Remove the two E10 Torx bolts at the front of the passenger seat.
- Move the passenger seat all the way forward. Do not turn on the ignition. Seats operate with the door open and keys removed from the ignition.
- Remove the two bolts at the rear of the passenger seat.
- Once all four bolts are removed, tilt the passenger seat forward and rest it on the dashboard. It is important to lower the headrest to get the most room. Disconnecting the wires from the passenger seat is not needed.
- Lift the carpet. Some models have lines where you must cut the carpet to access the battery.
- Remove the air vent plastic. Remove the 10mm nuts and remove the battery cover. Once the cover is removed, you will see the main battery.
- Remove the main battery. Disconnect the negative and battery terminals. Remove the bracket on the side of the battery. Lift the main battery and put it to the side.
- Access the auxiliary battery. Remove the positive and negative connections to the auxiliary battery.
- Remove the auxiliary battery bracket.
- Replace the auxiliary battery and install a new battery ( 000000004039) in reverse order.
Auxiliary Warning Still ON
Sometimes, the battery warning light may stay on after replacing the battery. If that is the case, check the voltage provided by the alternator.
- Turn key to position II.
- Press the R button on the cluster three times.
- Start the engine.
- The voltage will display on the instrument cluster.
The alternator works okay if the voltage is between 13 and 14.8 volts. If the voltage is low, there may be a problem with the main battery, ground connection, alternator, or voltage regulator.
Also, check that the small negative (brown) wire is connected to the negative post of the main battery.
Frequently Asked Questions
What does the auxiliary battery do?
One of this battery’s primary purposes is to allow you to shift the car in and out of Park if the main battery fails.
On which models do these instructions apply?
G500 463.249 113.962
G55 AMG 463.246 113.982
G550 463.237 273.969
G55K AMG 463.271 113.993
GL320 CDI 4MATIC 164.822 642.940
GL320 CDI BlueTEC 164.825 642.820
GL350 CDI BlueTEC 164.824 642.820
GL450 4MATIC 164.871 273.923
GL550 4MATIC 164.886 273.963
ML320 CDI 4MATIC 164.122 642.940
ML320 CDI BlueTEC 164.125 642.820
ML350 164.156 272.967
ML350 4MATIC 164.186 272.967
ML350 CDI BlueTEC 164.124 642.820
ML450 4MATIC HYBRID 164.195 272.973
ML500 4MATIC 164.175 113.964
ML550 4MATIC 164.172 273.963
ML63 AMG 4MATIC 164.177 156.980
R320 CDI 4MATIC 251.122 642.950
R320 CDI BlueTEC 251.125 642.870
R350 251.156 272.967
R350 4MATIC 251.165 272.967
R350 CDI BlueTEC 251.124 642.870/950
R500 4MATIC 251.175 113.971
R63 AMG 4MATIC 251.177 156.980
How long does the Mercedes auxiliary battery last?
It can run up to 7 years, depending on the driving conditions and usage.
We hope you find the Mercedes-Benz ML R GL – Auxiliary Battery Replacement guide helpful. Check these troubleshooting and repair guides for more help on your Mercedes-Benz.
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