In this guide, you will find 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".
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. These instructions apply to models such as R350, R500, R320 and R63 AMG.
Turn off all of your 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 up the carpet. It will help to remove the carpet on the passenger side completely. At the minimum, lift up the carpet enough to get to the 10mm plastic nuts.
Remove the 10 mm nuts.
Lift up 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.
ML-Class W164 and GL-Class X164 Auxilary Battery Replacement
On the ML and GL models, you will need to remove the passenger seat bolts and tilt the seat forward. 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 all the way 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, keys removed from the ignition.
- Remove the two bolts at the rear of the passenger seat.
- Once all the 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. You do not need to disconnect the wires from the passenger seat.
- Lift the carpet up. On some models, there are lines where you need to 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 up the main battery and put it to the side.
- Access the auxiliary battery. Remove the positive and negative connections to the auxiliary battery.
- Remove auxiliary battery bracket.
- 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.
- 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.
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 that is connected to the negative post of the main battery.
- What does the auxiliary battery do?
- One of the primary purposes of this battery 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