E Coupe (C207) Hold/StopStart problems

[colh107]

2012 C207 E350.
For the last couple of months I've been having problems with brake hold not working. Especially on the way to work (~7am); but tending to be less problematic on the way home at noon. Hard to be conclusive, but cold/damp mornings are perhaps worse too. Sometimes, but not always, accompanied by a "something deactivated/inoperative" message on the dash (flashes up too briefly to read properly, but symbols were brake/abs related I think. No warning lights stay on).
Once, I got back in the car at supermarket and tried it in the parking space - working fine. (It hadn't been on the way there). Got as far as the lights at the car park exit, and it had gone again!

Dealer charged me for diagnostics a few weeks ago and found no faults so they did a "re SCN coding of the system". (Sounded a bit like "try switching it off and back on again" to me...)
It certainly seemed to improve it at first, but I've now had the problem a few more times again.

Also, it's ages since my Stop/Start has gone to active state. Even at the tail end of the summer it seemed to be going green less frequently than I was used to. Have only seen it green a few times since, even on journeys - and even then it didn't stay green for long.
(I know there's a lot of factors like temperatures, electrical loads etc, but I'm comparing with the previous few years' behaviour, with similar journey styles).

I've done a bit of googling and a recurring area to do with these systems seems to sometimes be the aux battery. I did find mine in the boot (the large-ish ~12Ah size) and with ignition off it measures 13V.
There seems to be a lot of variation in size & location of the aux battery, even on E Class models alone - is there a second aux somewhere or just this one?

Reluctant to pay for more 'head scratching' if there's something I can diagnose/fix myself.
Thoughts/info/suggestions welcome! Thanks.

 

[markjay]

My car (W204) is mechanically similar to yours, and in my case the Stop/Start stopped working due to faulty brake vacuum sensor.

Said that, the most common cause is that one or both batteries (main or auxiliary) needs replacing.

Simple Voltage measuring won't detect a battery fault, it's the battery management module that decides if the batteries are good enough or not.

Unfortunately, if there are no fault codes present, the course of action I would take is

1. Replace main battery.
2. Replace aux battery.
3. Replace battery management module.

And yes, the HOLD function is related to the battery management system, so resolving the current issue will fix both your Stop/Start issue and the brake HOLD issue.

Good luck.

 

[markjay]

Also, some members on here simply disconnected the battery management module, which disables both the Stop/Start and the brake HOLD functions, and eliminates the error messages.

But from what I understand, you do want these functions to work, so this solution won't work for you.

 

[colh107]

Thanks for the thoughts. Yep, I do want them working (brake hold in particular).
Is that module something major/expensive? I googled the part no. of the small unit beside aux battery - seems to be just a relay?
I'd thought it odd/disappointing that they didn't find any fault codes. Has me wondering about not a hard fault, but more some set of conditions which deactivate the systems but is typically not so unusual so as to register a fault. e.g. battery not fully charged - "not charged enough for me to activate right now - its just one of those things that happen sometimes, not a fault. No doubt it'll be back to normal soon". But in my case, something never is.
I was hoping the aux battery would simply show a clear naff voltage, but yes I appreciate there's more to 'condition' than the static near-off-load voltage. FWIW, Eng Mode of COMAND shows [presumably] main battery voltage as ~12V, drops to ~11V for a few secs upon starting, then ~14.7V.
I could get main battery properly tested by an auto electrician I guess, but not sure if their kit would be too beefy to able to test the aux?

 

[markjay]

The battery management module is on top of the main battery terminal.

The reason that there are no fault codes is that from the ECU's perspective it is normal for the battery to have too low charge from time to time, so an insufficient charge condition does not trigger a fault code in itself, even if it's near-permanent condition - the ECU can't tell why the charge level is low because there could be many reasons for that.

The only way to know if the main battery is at fault is by replacing it with new.

There are load-testers for batteries, but to be honest I am not sure to what extent they can reliably test a modern Stop/Start AGM battery.

 

[Riva811]

Your battery should be 12V minimum when the car is switched off. The battery should be 13-14V when the car is on.
If your battery is less than 12V when switched off, e.g. climbs eventually to 12V after a few seconds, then you need to replace that battery. Also make sure your current battery is AGM otherwise it wont charge properly.
In all fairness you can see the voltages in engineering menu, if you dont have a multimeter
If your battery is ~12V when the car is on, then your alternator needs some love.
The other aspect to test on the batteries is the CCA and charging capacity, for that you will need additional equipment.

 

[Riva811]

I dont see a connection between brake hold and start/stop. My uncles W212 is happy to apply a brake hold but the start/stop never works.

 

[colh107]

Out of engineering curiosity, what's the purpose of the aux battery [for these systems] - to maintain '12V' to them whilst the main battery droops due to the starter motor load on restart? Why does brake hold need an aux?
Any possible clue from the fact that the Stop/Start probably started playing up a couple months before the Hold...?

 

[colh107]

The battery management module is on top of the main battery terminal.

The reason that there are no fault codes is that from the ECU's perspective it is normal for the battery to have too low charge from time to time, so an insufficient charge condition does not trigger a fault code in itself, even if it's near-permanent condition - the ECU can't tell why the charge level is low because there could be many reasons for that.

The only way to know if the main battery is at fault is by replacing it with new.

There are load-testers for batteries, but to be honest I am not sure to what extent they can reliably test a modern Stop/Start AGM battery.

Sounds like it could do with some smart hysteresis to detect a 'going-on-for-too-long' condition. Not that I'd fancy trying to devise that software algorithm myself...
At the terminal posts isn't it just a normal lead-acid battery...?

 

[Riva811]

Out of engineering curiosity, what's the purpose of the aux battery [for these systems] - to maintain '12V' to them whilst the main battery droops due to the starter motor load on restart? Why does brake hold need an aux?
Any possible clue from the fact that the Stop/Start probably started playing up a couple months before the Hold...?

Your battery is dying.
And low voltage will cause brake vacuum errors amongst others

 

[colh107]

Then I wish it would die quicker so that the car will detect where the fault actually is lol
As mentioned, I sometimes also get something inoperative/deactivated messages - I guess they could be brake vacuum related.
Main battery, do you think? (I've had the car for around 5 years, no new batteries in that time).

 

[markjay]

Sounds like it could do with some smart hysteresis to detect a 'going-on-for-too-long' condition. Not that I'd fancy trying to devise that software algorithm myself...
At the terminal posts isn't it just a normal lead-acid battery...?

Under some conditions it will be normal for the battery to be always borderline-charged and the Stop/Start never enabled (e.g. short journeys in cold weather). And an oversensitive algorithm will trigger false failed-battery alerts, not good news for the manufacturer while the car is still under warranty......

 

[Riva811]

Then I wish it would die quicker so that the car will detect where the fault actually is lol
As mentioned, I sometimes also get something inoperative/deactivated messages - I guess they could be brake vacuum related.
Main battery, do you think? (I've had the car for around 5 years, no new batteries in that time).

I took that approach as well, and this December, battery drained in no time. I had undervoltage errors in almost every module and some funny ones where brake vacuum disappeared, power doors didnt work, electornic brake malfunction etc. At no time did the car report that the battery couldnt hold its charge any longer. And funny enough the previous owner threw in a standard battery instead of AGM. I knew it was coming my way but I left it to the last minute. Just buy a multimeter and test it. If the battery is past its warranty then its CCAs are low anyway

 

[colh107]

Under some conditions it will be normal for the battery to be always borderline-charged and the Stop/Start never enabled (e.g. short journeys in cold weather). And an oversensitive algorithm will trigger false failed-battery alerts, not good news for the manufacturer while the car is still under warranty......

For sure. I've been asked to add extra checks/tests into software - gets to a point that you have so many layers of trying to make things better that the system doesn't know whether it's coming or going... some things are just hard to train a computer to do reliably (esp. when factoring in the chemistry of a battery too)

 

[colh107]

Yikes - £245 for main battery from dealer....!! Vs £134 from Tanya - still an AGM and also Varta, so by no means a dodgy/unknown brand [my only Go To when designing Lithium-powered kit].
And given that battery is >4 yrs old anyway, and another hour of random diags will cost £120.... ??

 

[ToeKnee]

My first thought is are you sure the battery in the boot is the aux battery, my main battery is in the boot and aux is behind the seats (different model though)? If you are, is it worthwhile charging the aux battery separately from the main battery to make sure you start your next journey with a full charge, if everything then works for a while you can be pretty sure it's the battery that's at fault.

 

[colh107]

My first thought is are you sure the battery in the boot is the aux battery, my main battery is in the boot and aux is behind the seats (different model though)? If you are, is it worthwhile charging the aux battery separately from the main battery to make sure you start your next journey with a full charge, if everything then works for a while you can be pretty sure it's the battery that's at fault.

Definitely just the/an aux in the boot - motorbike size, at best. I have a smaller smart charger I could use on the aux, and a proper big boy car charger I could use on main. Might indeed be worth a try to see if behaviour changes.

 

[markjay]

My first thought is are you sure the battery in the boot is the aux battery, my main battery is in the boot and aux is behind the seats (different model though)? If you are, is it worthwhile charging the aux battery separately from the main battery to make sure you start your next journey with a full charge, if everything then works for a while you can be pretty sure it's the battery that's at fault.

The C207 is essentially a W204, so unless they changed it, then the main battery is in the engine bay, and the auxiliary battery is in the spare wheel well in the boot (under black plastic cowling).

BTW my main battery is made by Varta and the auxiliary battery is made by FIAMM (both batteries are original).

 

[colh107]

The C207 is essentially a W204, so unless they changed it, then the main battery is in the engine bay, and the auxiliary battery is in the spare wheel well in the boot (under black plastic cowling).

BTW my main battery is made by Varta and the auxiliary battery is made by FIAMM (both batteries are original).

Yes, sounds right. Aux in the boot is FIAMM, haven't checked brand of main - will have to take off the bits of plastic. (Miss my 156 V6 where you opened the bonnet and saw gleaming chrome intakes, not a bunch of plastic covers!! Though electrics can be a real headache on Italian cars.... )

 

[Riva811]

Yikes - £245 for main battery from dealer....!! Vs £134 from Tanya - still an AGM and also Varta, so by no means a dodgy/unknown brand [my only Go To when designing Lithium-powered kit].
And given that battery is >4 yrs old anyway, and another hour of random diags will cost £120.... ??

I wouldn't sacrifice quality here unless you want to change battery every few years. Varta should do it, not sure why MB is so high as it's Varta as well. Make sure you henge the right Ah and CCAs

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