W221 S350L CDI red battery appears in speedometer

Stewart Gray

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S350L CDI W221 2011
I bought a pre-loved W221 S350L CDI 2011 . After 6 months or so the alternator went kaput. Warranty sorted out a replacement however, I had to pay for a replacement main battery. Got the car back from the garage and it drove fine for about 6 hours when the red battery symbol appeared in the speedometer. I returned the car to the garage where they looked again. Told me they had to replace the alternator again along with a new drive belt and tensioners. Got the car back from the garage and things worked fine for a week or so. I had stopped at a service station and when I got back into the car I plugged in my phone charger, the red battery symbol appeared in the speedometer. I called the AA and in due course the man arrived and when the car was started it worked fine however, it was taken back to the dealer for further investigation. They sent it to Mercedes who checked the car over and reported they could find NO fault with the motor. I reluctantly took back the car and started to check online for similar issues. I discovered that others had had a similar problem to do with the auxiliary battery. I learned that the car would drive and act fine as long as I did not run any ancillary systems, air con, CD player, DVD player etc. and the issue did not occur as long as the car was 'driving' but, if I ran any of these and the car was allowed to idle, the red battery symbol would appear. If I continued to drive this would remain ON and the main battery would run down but, if i switched the off engine and let it sit for a few moments then it would run normally as long as I kept all 'unnecessary' systems switched off. After much searching in various locations the aux battery was located and replaced. The car worked fine and I could run everything ... for about 2 months. The problem recurred. When running ancillary systems, and this includes sitting at a junction with a bit of 'lock' on the steering, at idle the battery symbol appears.

Has anyone else had this issue? I'm a layman but I'm of the opinion the alternator is not charging the aux battery. Why would this be if it has been fitted with a new alternator? I'd be grateful for any insight, please.
 

drmw

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That is nonsensical. I have no experience of this model, but what you say is points at the aux battery. Hang on - you will get people who can help. Whereabouts are you? There are several independent MB specialists - I think you need to go to one of them
 
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Stewart Gray

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S350L CDI W221 2011
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That is nonsensical. I have no experience of this model, but what you say is points at the aux battery. Hang on - you will get people who can help. Whereabouts are you? There are several independent MB specialists - I think you need to go to one of them

I'm in central Scotland. Bought the car through Evans Halshaw in Grangemouth. It's been inspected by Mercedes in Grangemouth.
 
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Stewart Gray

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That is nonsensical. I have no experience of this model, but what you say is points at the aux battery. Hang on - you will get people who can help. Whereabouts are you? There are several independent MB specialists - I think you need to go to one of them

Love the Basil photo BTW
 
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Stewart Gray

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Why haven't they replaced the AUX battery yet ?

I replaced the aux battery and all was well for 2 monbths but the problem has recurred. The aux battery is now unable to cope because it's so low. I can run the car as long as i don't use anything that will increase the 'hotel' load, so no CD, no air con etc.
 

hmang

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I replaced the aux battery and all was well for 2 monbths but the problem has recurred. The aux battery is now unable to cope because it's so low. I can run the car as long as i don't use anything that will increase the 'hotel' load, so no CD, no air con etc.
did you replace with genuine merc batteries ?

should be able to put a meter across the aux battery to see if it's charging.
is yours in the boot behind the seat ?
 
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Stewart Gray

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did you replace with genuine merc batteries ?

should be able to put a meter across the aux battery to see if it's charging.
is yours in the boot behind the seat ?

I did not buy replacement Mercedes batteries but equivalents. The aux battery is actually behind the glove box. Main battery is under the bonnet in front of the windscreen with the aux battery behind a fuse panel accessed through a removable semi circular panel on the edge of the glove box. Open the passenger door to get to it. My reasoning is that when the new aux battery was fitted the problem disappeared now it has been drained due to not being recharged the issue has come back.
 

hmang

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I did not buy replacement Mercedes batteries but equivalents. The aux battery is actually behind the glove box. Main battery is under the bonnet in front of the windscreen with the aux battery behind a fuse panel accessed through a removable semi circular panel on the edge of the glove box. Open the passenger door to get to it. My reasoning is that when the new aux battery was fitted the problem disappeared now it has been drained due to not being recharged the issue has come back.
Sure Stewart .
I saw the type . It looks bit like a burglar alarm back up battery.
I thought my car w221 L 2013 had the same ,but seems there's differences. Mines bigger . Looks like a motorbike battery (ytx12bs or similar ), and is in the boot like i mentioned . I think mine is rated at 12 ah whereas the smaller ones like yours were much less. I guess that my big 'un must run some stuff and yours is probably for keeping certain systems backed up .

Dunno much about batteries , but i don't think either type take particularly well to being deep discharged .
When i was in motorbikes and the new sealed batteries came in , we had to invest in a new style charger ( constant current vs constant voltage) which would push a high voltage ( up to 30 v) through a flat sealed battery to breakdown the sulphation ? on the plates .some times it worked , sometimes not .

I'm sorry you are having this problem . I guess it would be sensible to check that your new battery is up to voltage and is receiving charge from the car . The car will only charge at 13-14 volts though , which is great for keeping it topped up but may not recover it from deep discharge .

If all is well you may have the misery of finding the source of a drain on the battery
 

hmang

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Mercedes make cars .... not batteries ;);)
Yeah .
I get that Mr Dad .

My original battery was made in usa apparently and my new one is a Fiamm .
Both are OEM although , as i have mentioned , slight differences on the terminals
 

mioba

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W124/E200, W220/S320CDI, W205/C200, W251/R350CDI 4Matic
Is the alternator genuine/up to required specs. MB are v voltage sensitive.
are the terminals well seated and clean.
consider genuine MB batteries they last donkies and very well specced.
my w220 battery lasted 13 years and the w251 is still on original (10 years)
 

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