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This guide provides instructions on how to change the auxiliary battery (also referred to 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."

Table of Contents


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

mercedes ml gl r class auxiliary battery

Auxiliary Battery

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. These instructions apply to models such as R350, R500, R320, and R63 AMG.

  1. Turn off all of the accessories and lights. Move the shifter to the Park position and remove the key from the ignition.

  2. Move the plastic door molding on the passenger side.

    mercedes r350 r500 r320 small auxiliary battery replacement
  3. 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.

    lift carpet to access aux battery
  4. Remove the 10 mm nuts.

    remove 10mm bolts to access aux battery on r class
  5. Lift the fiberglass panel.

    access aux battery on r class
  6. Locate the auxiliary battery and unplug the positive and negative connections.

    r350 r500 auxiliary battery location
  7. Remove the two 13mm bolts. Once you remove the two bolts, you can slide the old battery and the bracket out.

    aux battery on ml r gl
  8. Remove the auxiliary battery.

    aux battery on mercedes x164 w251 w164
  9. Install the new auxiliary battery 000000004039 in reverse order.

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.

  1. Remove the key from the ignition.
  2. Move the passenger seat all the way to the back. Move the headrest down.
  3. Remove the two E10 Torx bolts at the front of the passenger seat.move two bolts at the front of the seat
  4. Move the passenger seat all the way forward. Do not turn on the ignition. Seats operate with the door open, keys removed from the ignition.move seat forward
  5. Remove the two bolts at the rear of the passenger seat.remove seat bolts e10
  6. 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.
  7. Lift the carpet. On some models, there are lines where you need to cut the carpet to access the battery.tilt passenger seat forward
  8. 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.main battery location r ml gl class
  9. 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.main battery removed ml gl r class
  10. Access the auxiliary battery. Remove the positive and negative connections to the auxiliary battery.battery connection
  11. Remove auxiliary battery gl battery location
  12. Replace the auxiliary battery and install a new battery ( 000000004039) in reverse order.

Auxiliary Warning Still ON

In some cases, the battery warning light may stay on after you replace the battery.

If that is the case, check the voltage provided by the alternator.

  1. Turn key to position II.
  2. Press the R button on the cluster three times.
  3. Start the engine.
  4. The voltage will display on the instrument cluster.

If the voltage is between 13 and 14.8 volts, the alternator is working ok. 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.

battery warning light on after changing the battery


  • 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