R230 SL55 Rear Consumer battery not charging (front battery charging fine)

[Goliath]

Any ideas?

If I put the multimeter on the batteries the reading are as follows....
With the engine OFF - Front battery showing 13.2v, Rear (consumer) battery showing 12.2v
With the engine RUNNING - Front battery - 14.5v, Rear (consumer) battery 11.8v

The alternator is obviously working as it is charging the front battery.

I changed the alternator 11 months ago for a factory refurbished unit and it has been fine until yesterday.

 

[flowrider]

Have you tried charging the consumer battery seperately? Could be a bad battery.

 

[John Laidlaw]

Rear battery is goosed

 

[television]

A battery does not have to be flat

 

[Goliath]

I will take it out and charge it up, but I'm fairly sure it is ok as it is not very old. Also even if the battery was goosed, surely I would still see 14+v going into the battery with the engine running?

 

[John Laidlaw]

In my SL I had almost identical readings even when trickle charging the consumer every night or at least every other. Consumers would occasionally go offline too. Changed the battery and all solved.
Not sure on the answer to your query on the engine running TBH...

 

[Goliath]

I've just been on a short drive in it (with no error messages) and when I tested it as I got home the rear battery is now reading 14.2v with the engine running.

Very odd, I will keep checking every now and then over the next few days and see what's happening.

 

[flowrider]

My old consumer battery would only last 2 days before it gave me the offline warning, even after a full charge. I changed it for a new MB battery and now i no longer get the offline message even after nearly 2 weeks of not using the SL.

 

[Goliath]

I use it every day and it has been fine, until yesterday where I got the consumers offline message very quickly followed by a constant beeping and a 'stop car now' brake warning message - it also wouldn't change gear in manual mode. I parked it up and when I started it this morning it was fine (but the battery was showing the low reading in the first post). It has been fine all day though with no warnings and now the battery is giving good readings. Very odd, but I have come to expect that of this car!

 

[Goliath]

I used it again this evening and it came up with the consumer offline message, very quickly followed by the red warning to stop the car.
Is it possible for the regulator thing on the alternator to only give out a charge for a short time? That seems to be what is happening, battery charging ok when first started but then after anything more then a very short drive it stops charging. (I think, although the front battery still seems to be charging fine all the time)

 

[John Laidlaw]

Or the rear battery not holding the charge?

 

[drmw]

+1 ^

 

[alexanderallen1]

I used it again this evening and it came up with the consumer offline message, very quickly followed by the red warning to stop the car.
Is it possible for the regulator thing on the alternator to only give out a charge for a short time? That seems to be what is happening, battery charging ok when first started but then after anything more then a very short drive it stops charging. (I think, although the front battery still seems to be charging fine all the time)

Hi bud, did you ever get to the bottom of this ? My s320 w221 is having the exact same issue and have changed the voltage regulator but has done nothing.

 

[ulyssesking4]

Hello, I’m having the same problem with my2008 sl 55. Front battery is charging and rear is not. I have checked the fuses under the glove compartment on the passenger side and both are good. With car running I’m getting 14.2 v at the front battery and 12 v at the back. Any ideas?

 

[LostKiwi]

Bad rear battery, bad earth or failed BCM (battery control module).

 

[alexanderfoti]

As above. BCM actuates a relay to switch rear battery in/out of charging circuit, potentially this relay is stuck but this often gives red battery warning on dash. Have you read the fault codes?