Replaced aux battery but stop/start still not working

[camcc]

Hi all,

Yesterday I replaced the auxiliary battery in my W204 C220 CDI estate as the stop/start function had not been working for some time. For reference I installed a Varta AUX14 12V 13Ah 200A battery in place of the original OEM Mercedes branded battery located next to the spare wheel in the boot.

Now when I turn on the ignition the ECO button on the dash centre console is lit green and ECO shows in yellow on the dial cluster, however when I start the car the ECO button light goes out and ECO disappears from the dial cluster. Pressing the ECO button does nothing (it should turn the function on/off). I read that you need to wait for several conditions to be met including oil up to temp, flat surface, foot on brake, air con off etc. before stop/start will engage, so I went on a ~20 mile drive but stop/start still wouldn't engage automatically or via the button.

I then reset the car to factory settings via the onboard computer but this didn't make a difference either... the owners manual mentions nothing about an auxiliary battery!

Any thoughts on what I should do next?

(I know many people dislike stop/start functionality, but I would like to get it working again as I plan to sell the car in the near future)

 

[OneForTheRoad]

Hi all,

Yesterday I replaced the auxiliary battery in my W204 C220 CDI estate as the stop/start function had not been working for some time. For reference I installed a Varta AUX14 12V 13Ah 200A battery in place of the original OEM Mercedes branded battery located next to the spare wheel in the boot.

Now when I turn on the ignition the ECO button on the dash centre console is lit green and ECO shows in yellow on the dial cluster, however when I start the car the ECO button light goes out and ECO disappears from the dial cluster. Pressing the ECO button does nothing (it should turn the function on/off). I read that you need to wait for several conditions to be met including oil up to temp, flat surface, foot on brake, air con off etc. before stop/start will engage, so I went on a ~20 mile drive but stop/start still wouldn't engage automatically or via the button.

I then reset the car to factory settings via the onboard computer but this didn't make a difference either... the owners manual mentions nothing about an auxiliary battery!

Any thoughts on what I should do next?

(I know many people dislike stop/start functionality, but I would like to get it working again as I plan to sell the car in the near future)

You've probably got a capacitor pack under the glove box . Or a small motor bike battery either under the bonnet or in the boot ???

 

[keith of kent]

The OP said that he's changed the auxiliary battery.
I wonder what the state of the main battery is?

 

[TC350]

The stop/start on my 2012 W204 became far more responsive when I renewed the main battery under the bonnet last year. Having a healthy main battery with sufficient charge is one of the conditions required for operation.

(I don't think I have an auxiliary battery in the boot.)

 

[camcc]

Thanks for your responses so far. I've just tested the new auxiliary (12.7V) and the main battery which I have not yet replaced (12.5V). Thought it's better test the old auxiliary battery and it read 12.8V!

Also checked fuses 16 and 91 listed against stop/start on the fuse diagram and they are both absolutely fine.

Car starts on the button every time and other features such as HOLD still work so I am reluctant to replace the main battery just yet.

Do I need to check for a small capacitor pack under the glove box, or is that only for models without an aux battery?

 

[Gazwould]

No capacitor on these .

Spending money to get a wrecking ball swinging again to shorten the life of components is beyond my logic .

It kills me that people waste money renewing healthy batteries , quite often two at a time .

 

[BTB 500]

No experience with W204s but as above it's generally the main battery that's the issue. Certainly the case with our W205 - it always started absolutely fine, but the battery voltage was below the threshold required for stop/start to operate. Replaced and all good ever since (nearly 4 years now).

As an aside the Mercedes Me app always reported the original battery as being 'partially charged' rather than 'fully charged'.

 

[BTB 500]

Do I need to check for a small capacitor pack under the glove box, or is that only for models without an aux battery?

The latter - the capacitor pack is instead of an aux. battery.

 

[Gazwould]

No experience with W204s but as above it's generally the main battery that's the issue. Certainly the case with our W205 - it always started absolutely fine, but the battery voltage was below the threshold required for stop/start to operate. Replaced and all good ever since (nearly 4 years now).

As an aside the Mercedes Me app always reported the original battery as being 'partially charged' rather than 'fully charged'.

My Stop Strain on my S204 was working at lower main battery voltages than the OP's .

Even a faulty brake booster sensor can be the culprit.

 

[OneForTheRoad]

The OP said that he's changed the auxiliary battery.
I wonder what the state of the main battery is?

So he did , sorry , made myself look like a Kent .

 

[BTB 500]

My Stop Strain on my S204 was working at lower main battery voltages than the OP's .

Even a faulty brake booster sensor can be the culprit.

If the W204 handbook is like the '205s then it should have a list of the criteria for start/stop to function ... worth checking those first, IMHO.

The 12.5V mentioned by the OP is only around 80% charge.

 

[rf065]

Thanks for your responses so far. I've just tested the new auxiliary (12.7V) and the main battery which I have not yet replaced (12.5V).

Mine stopped working when the main battery was 12.6v, they really need both batteries to hold a full charge.

 

[OneForTheRoad]

Mine stopped working when the main battery was 12.6v, they really need both batteries to hold a full charge.

This ^^^^^^^^. 100%. Since renewing my main battery AND the capacitor pack , my stop start works every time ,everyday ,every condition. No parameters whatsoever.

 

[geoffus]

I just replaced my auxiliary battery and now the stop start works fine, 4 years ago I replaced my main battery as although worked fine the Stop/Start didn’t work, fitting a new battery fixed it for a short time.
Now I have 2 newish batteries and all is well.
So you may find you need a new main battery to bring it back to life, you could try charging the main battery overnight and find it works for a few days, if it does then a new main battery is needed.
Personally I would leave at it is, Stop/Start is a pain, I only cured it because was fed up with the warning light on the dash after start up.

 

[OneForTheRoad]

Thanks for your responses so far. I've just tested the new auxiliary (12.7V) and the main battery which I have not yet replaced (12.5V). Thought it's better test the old auxiliary battery and it read 12.8V!

Also checked fuses 16 and 91 listed against stop/start on the fuse diagram and they are both absolutely fine.

Car starts on the button every time and other features such as HOLD still work so I am reluctant to replace the main battery just yet.

Do I need to check for a small capacitor pack under the glove box, or is that only for models without an aux battery?

Charge the main 24 hours with AGM mode. If stop start works after, it's that , if after a few days stop start stops again , I'd suspect the main to be tired.

 

[markjay]

STAR will tell you exactly which condition have not been met. It might be some other component, not the batteries. In my case it was the Brake Assist vacuum sensor.

 

[Justintyme]

My Stop Strain on my S204 was working at lower main battery voltages than the OP's .

Even a faulty brake booster sensor can be the culprit.

Would an OBD scan not show a fault?
*** OOPS< Just saw Markjay's post***
But the question remains?

 

[Gazwould]

Yes