w211 270CDI SBC Brake failure...battery related?

[Cipollini]

Hi all,

few days ago I changed the front rotors and after that, my SBC turned off.
I drove home carefully with little brake and engine power. After I stopped the car and start again, everything was ok. Next day everything seemed ok, but after about 20km ride the red brake failure warning came up again.

Car was not cranked for 2 days and I measured aux and main battery voltage:

Cold engine:

Aux battery 12.69V
Main battery 12.03V

After 5min idling:

Aux battery 13.97V
Main battery 13.93V

1minute after engine stopped:

Aux battery 12.97V
Main battery 12.49V


Am I right to suppose the Aux battery and alternator charging is ok, but main battery died?

Before my front brake change I sometimes saw the white warning ,,Battery Protection Convenience Functions Temporarily Unavailable´´ right after start.

Thanks all

 

[mattkh]

Check how low the voltage goes down to on the main battery when cranking the engine.

 

[Cipollini]

Using voltmeter or like this?

 

[mersum1es]

Isolation relay is welded

 

[Cipollini]

Thanks for input, are you sure just looking at the voltage numbers?

Where I can find it and how to diagnose (or repair) it?

I am not an electrician, but I managed to repair burned glow plug relay, so I hope I will manage it.

 

[mersum1es]

Aux battery is charged only shortly after start, if BCM finds low voltage. Well, WIS does not state how long charge takes place, but you should not see charging voltage long time if BCM is good. That is another potential culprit.

 

[Cipollini]

Good point, I let the car idle only for about 5 mins.
Tomorrow I can try more, or give it a short ride, then some idling and measure voltage.

Anyway I´ll buy new main battery just to be sure. Will the EXIDE PREMIUM 12V 85ah 800A EA852 do the job?

 

[Cipollini]

Check how low the voltage goes down to on the main battery when cranking the engine.

Today I measured the main battery before cold start again: 11.97V.
When cranking it went down to about 9.5V, checked with voltmeter.

Anyway I had already new battery ready in the trunk (EXIDE PREMIUM 12V 85ah 800A EA852), 100% condition, checked in the shop when buying.

So...everything as before even with new battery. Red brake (and ABS, ESP) warning and I wasn´t able to move the gear knob.

Tomorrow I´ll have a look at isolation relay. Is there any way how to diagnose it?

Anything else I should try (except Star of course)? For the diagnostics I´ll have to wait.