The most common symptom of a faulty auxiliary battery is a warning message on the dashboard displaying: Auxiliary Battery Malfunction.
The early 2006 and 2007 S-Class models have two batteries but no auxiliary battery under the dash. One battery is behind the rear seats; the second battery is in the engine bay.
This guide also applies to CL-Class W216 from 2007 to 2013. Applicable to models such as CL550 CL63 AMG.
What you will need
- Disconnect the negative battery from the main battery. The main battery will be on the engine bay, passenger side, or behind the rear seat (accessible through the trunk).
- Remove the plastic cover on the driver's side of the dashboard using a dash removal trim.
- Remove two Torx screws that hold the fuse box in place. Pull the fuse box out but do not unplug the electrical connector.
- Remove the 10mm bolts that hold the auxiliary battery bracket in place.
- Remove the auxiliary battery from the bracket. Next, unplug the negative and positive wires from the auxiliary battery.
- Install the auxiliary battery in reverse order.
There is very little space in the dash, which means you may find it difficult to remove the auxiliary battery. If that is the case, remove the panel from under the dashboard. This will allow you to access the auxiliary battery from under the dashboard.
Once you replace the auxiliary battery, the Auxilary Battery Malfunction on the dashboard should turn off immediately. Unless the auxiliary battery is not charged, in which case it may take a few driving cycles for the instrument cluster error message to turn off.