Hi All, My I've had a fault recently and after much research on this and other Merc threads I found the problem (and also fixed it). Fault finding and rectifying this was due to others posting their issues and fixes, so I thought that I would post my experience and hopefully help someone else with a similar issue.
I've had the Red battery visit workshop message, which was intermittent to begin with, appearing every few weeks or so, my MOT is due soon and the warning came back .. so I decided to try and do something about it !! I believe this is a pretty common problem with W211.
I plugged in my icarsoft tool and found the fault code 9092 G1/7 (Auxiliary Battery) is defective or not installed. Both my Main battery and Aux battery are less than 6 months old, voltage was 12.8v and 12.9v with 14.1 on the main when running, so I know the batteries are (likely) ok. The 14.1 indicates that the Alternator is probably ok too.
I did some research + found the wiring diagram for the Aux battery/ Dual battery circuit which had been posted on the Benaworld forum (Thx Mersum for that), which led me to check the F30 Polyswitch fuse (it has continuity) so ... probably ok too.
I then tested the Aux Batt relay - ok (as the Aux batt has stayed charged I figured that the relay is working ok).
Next was the BCM (N82) which on inspection had no corrosion on or near the pins ... no evidence of moisture ingress, I cleaned out the contacts anyway. But warning message stayed.
I plugged in my icarsoft tool and looked through the live battery data (engine running) and noticed that the Aux battery voltage was being shown as fluctuating between 8.0v and 9.8v ..... I then re-checked (using voltmeter direct on the Aux batt.) the Aux batt voltage to rule out being low under load, but it was showing 12.9 at the same time as the icarsoft tool is showing the fluctuating voltage (through OBD port).
I figured this means that the BCM might be knackered but instead of blindly ordering a new one in the hope that it was the problem, I decided to assume that It is ok and carry on fault finding.
Next I decided to test the continuity between the BCM and both relay and the Polyswitch. Pin 4 on the BCM goes via brown/violet wire to pin 2 on the Relay .. this had good continuity.
Next test was between Pin 7 on the BCM via red/violet wire to the Polyswitch then back out the Polyswitch via red/yellow wire from to Relay pin 1..... this had almost no continuity between BCM and Polyswitch pin.
I traced this wire from the BCM through the car to the bulkhead and finally to the polyswitch looking for damage/pinch marks, corrosion .. there was nothing visible.
As I stripped the cloth electrical tape off the loom between the Polyswitch and the rubber grommet where the loom passes through the bulkhead (under the Aux Batt tray) I felt a section of the Red/violet wire was slightly less flexible than the rest ... to cut a long story short, I stripped 2 small areas either side of the hardened section and tested continuity back to the BCM. One side is good, one side has almost no signal.
The section of loom (wire) which was bad is located under the Aux battery tray. I have read other people having found issues with the wire in the same area too.
Anyway, I replaced the bad section and found that the continuity on the cable is now as it should be, and the reading for the Aux battery on my Icarsoft is showing 12.9v.
So 1-2 days of fault finding, then searching for the problem followed by the satisfaction of fixing it!!! Time for a beer...
I stripped the bad wire and found an area of green corroded material ... almost powder like ... no wonder there was no continuity !!
When its warmer outside (was snowing today) ill revisit the wire make double sure there are no more hidden surprises!!
With the luxury of hindsight, if I was doing this job again, and was going to trace the wire with poor continuity between the BCM and the Polyswitch, I would start at the bonnet end and work back toward the boot. Unfortunately ..... I started at the boot and followed the wire forward (stripping out the back seats, side trims, access to the footwells until I found the corroded section) I guess If I had though about the likelyhood of the wire being corroded in a dry boot/ interior compared to under the bonnet where it is exposed to heat, chemicals etc .... I may have done it the easy way .... Lesson learnt. (as you can tell I am no Auto electrician)
I have attached a couple of Images of the section of wire that I removed.
Hope this helps someone
I've had the Red battery visit workshop message, which was intermittent to begin with, appearing every few weeks or so, my MOT is due soon and the warning came back .. so I decided to try and do something about it !! I believe this is a pretty common problem with W211.
I plugged in my icarsoft tool and found the fault code 9092 G1/7 (Auxiliary Battery) is defective or not installed. Both my Main battery and Aux battery are less than 6 months old, voltage was 12.8v and 12.9v with 14.1 on the main when running, so I know the batteries are (likely) ok. The 14.1 indicates that the Alternator is probably ok too.
I did some research + found the wiring diagram for the Aux battery/ Dual battery circuit which had been posted on the Benaworld forum (Thx Mersum for that), which led me to check the F30 Polyswitch fuse (it has continuity) so ... probably ok too.
I then tested the Aux Batt relay - ok (as the Aux batt has stayed charged I figured that the relay is working ok).
Next was the BCM (N82) which on inspection had no corrosion on or near the pins ... no evidence of moisture ingress, I cleaned out the contacts anyway. But warning message stayed.
I plugged in my icarsoft tool and looked through the live battery data (engine running) and noticed that the Aux battery voltage was being shown as fluctuating between 8.0v and 9.8v ..... I then re-checked (using voltmeter direct on the Aux batt.) the Aux batt voltage to rule out being low under load, but it was showing 12.9 at the same time as the icarsoft tool is showing the fluctuating voltage (through OBD port).
I figured this means that the BCM might be knackered but instead of blindly ordering a new one in the hope that it was the problem, I decided to assume that It is ok and carry on fault finding.
Next I decided to test the continuity between the BCM and both relay and the Polyswitch. Pin 4 on the BCM goes via brown/violet wire to pin 2 on the Relay .. this had good continuity.
Next test was between Pin 7 on the BCM via red/violet wire to the Polyswitch then back out the Polyswitch via red/yellow wire from to Relay pin 1..... this had almost no continuity between BCM and Polyswitch pin.
I traced this wire from the BCM through the car to the bulkhead and finally to the polyswitch looking for damage/pinch marks, corrosion .. there was nothing visible.
As I stripped the cloth electrical tape off the loom between the Polyswitch and the rubber grommet where the loom passes through the bulkhead (under the Aux Batt tray) I felt a section of the Red/violet wire was slightly less flexible than the rest ... to cut a long story short, I stripped 2 small areas either side of the hardened section and tested continuity back to the BCM. One side is good, one side has almost no signal.
The section of loom (wire) which was bad is located under the Aux battery tray. I have read other people having found issues with the wire in the same area too.
Anyway, I replaced the bad section and found that the continuity on the cable is now as it should be, and the reading for the Aux battery on my Icarsoft is showing 12.9v.
So 1-2 days of fault finding, then searching for the problem followed by the satisfaction of fixing it!!! Time for a beer...
I stripped the bad wire and found an area of green corroded material ... almost powder like ... no wonder there was no continuity !!
When its warmer outside (was snowing today) ill revisit the wire make double sure there are no more hidden surprises!!
With the luxury of hindsight, if I was doing this job again, and was going to trace the wire with poor continuity between the BCM and the Polyswitch, I would start at the bonnet end and work back toward the boot. Unfortunately ..... I started at the boot and followed the wire forward (stripping out the back seats, side trims, access to the footwells until I found the corroded section) I guess If I had though about the likelyhood of the wire being corroded in a dry boot/ interior compared to under the bonnet where it is exposed to heat, chemicals etc .... I may have done it the easy way .... Lesson learnt. (as you can tell I am no Auto electrician)
I have attached a couple of Images of the section of wire that I removed.
Hope this helps someone
