W211 Battery Control Relay

[dursleyman]

I am getting the dreaded red battery alarm with Visit Workshop message on our W211 2005 estate and a 9071 diagnostic code.
The alternator and battery control relay were both replaced by my local specialist in 2018 and I fitted new main and auxiliary batteries last year.

The 9071 code says "Component K57/2 (auxiliary battery relay) or the signal wire to the component is faulty"
Is it possible to test the battery control relay or do I have to just try fitting a new one to see if that fixes the fault?
If it's the signal wiring that sounds like a messy job.

With the engine running I see 14.1v on the live (red) terminal on the relay but only 12.4v on the other so it's obviously not switching on to charge the auxiliary battery.
I need to find if that actual relay faulty or the signal wire.

 

[mersum1es]

Car will charge aux battery only short time after engine start, if aux battery voltage is low enough. SDS you can force the relay on, perhaps other diagnostics can do the same. Did you check all wires around relay, they are prone to corrode. Also there is a fuse nearby, did you check that?

 

[dursleyman]

Car will charge aux battery only short time after engine start, if aux battery voltage is low enough. SDS you can force the relay on, perhaps other diagnostics can do the same. Did you check all wires around relay, they are prone to corrode. Also there is a fuse nearby, did you check that?

I was wondering if I could activate the relay manually, its got that two wire plug so the wires must be live and earth. I have taken the relay off and cleaned the terminals but they looked pretty good anyway. I do have some of those plug in connectors so may try feeding it that way to check it operates.
Got a used relay coming in the post so will also try experimenting with that off the car.
There was no sign of any kind of fuse nearby under that plastic cladding that covers the wiper mechanism?

 

[Kudly]

My 211 came up with that warning on Friday. I hadn't been using it so batteries went flat. Revived main in situ with a Ctek but red battery warning appeared. Took it for a good spin but warning remained, no other issues. Popped in to my Indy, batteries & fuses ok, BCM disconnected & reconnected, problem solved. I will try to get a used BCM on fleabay just in case as a backup.
Hope your issue is as easy to fix.

 

[mersum1es]

I was wondering if I could activate the relay manually, its got that two wire plug so the wires must be live and earth. I have taken the relay off and cleaned the terminals but they looked pretty good anyway. I do have some of those plug in connectors so may try feeding it that way to check it operates.
Got a used relay coming in the post so will also try experimenting with that off the car.
There was no sign of any kind of fuse nearby under that plastic cladding that covers the wiper mechanism?

https://cimg4.ibsrv.net/gimg/www.mbworld.org-vbulletin/805x560/capture_1689e7c7c0b950f050ba132adc7a3bb3afb024de.jpg

 

[dursleyman]

Thanks mersum1es, I saw that but never recognised it as a fuse so I will check it.
I have tested the relay by providing 12v onto the plug terminals and it does operate and switch power on to the aux battery.
Seems it is not getting the signal to switch on. I read that signal comes from the Battery Control Module N82 so need to find that next.

 

[mersum1es]

BCM is in trunk, cigarette box size thing. Check box wires too, also prone to corrode...

 

[dursleyman]

BCM is in trunk, cigarette box size thing. Check box wires too, also prone to corrode...

OK found it. All looks dry and clean, but is there any way to test it apart from swapping with a known good one?

 

[mersum1es]

looking live data, what BCM reads... by SDS you can do also a relay test, dunno if other devices can do that