Jwallaz

Hi! I have 2014 E250 Bluetech 4matic w/ 80,000 miles and no warranty. My issue started with an error message indicating aux battery issue. Of course I ignored this for a couple weeks. Yesterday, I replaced the auxiliary battery (it was def time to replace since it was the original one) and after driving the car for about 20 seconds the red battery light came on and said see operator's manual. Every time I turn off the car the light goes away, but it comes on as soon as I start driving for 20 seconds or so. Today, I replaced the main battery as it too was old and it was dipping below 12v when not running (car being off). Unfortunately, the red battery light is still present after driving for about 20 seconds. Everything works as intended when the light is displayed except the start stop. The light comes on even if I turn the start stop off immediately upon starting the car. Also, if the car sits idle the battery red light does not come on ever, it only comes on when driving.

I have checked the voltage information and my voltage reads in the 12v range when not started and after starting the voltage goes to 14.6v or so. So, I don't think it is an alternator issue, but maybe I'm wrong? I have read all the threads regarding this issue and I can't seem to find one like mine. I have seen others mention it could be a fuse, ecm, alternator (or voltage regulator), SAM, shorts and all sorts of other things.

I greatly appreciate any advice anyone may have.

Plutoe

yep its a FUYAA if you listen to all the mis directions by the weekend warriors----get the car scanned then you will know for sure

Jwallaz

Thanks a lot for the reply! Will the autozone free scan suffice or could I get away with buying something like this:
I'm trying not to go to the dealer to spend a hundred and fifty bucks or so just to use the scanner. If that's what I have to do, I'll do it, but I'd rather spend the same money on a scanner of my own to keep if possible.

Jwallaz

Not sure if this helps, but I let the car sit and idle for a half an hour or so. Upon start the voltage goes to 14.6/7 but when I went back at the end of the idle it was only reading 14v. With the car off and the static charge removed it reads 12.6 - 12.8v.

bmwpowere36m3

What's a FUYAA?

bmwpowere36m3

Sounds like both batteries were in poor shape... which might have taken a toll on the alternator which is more to maintain and not consistently charge very discharged batteries (essentially putting out higher amperage for long periods of time).

So both batteries are new and the voltage of the main battery seem to be right. What I'm not understanding is the 20 seconds and charging warning light. Does it come on 20 seconds or so after starting the car? Does is take on for only 20 seconds?

The red warning light indicates a charging issue. It would help to hook up a meter and monitor voltage and current before and after the light comes on.

Scanning for codes is a given with these cars.... its nuts how often it needs to be said.

Minh

Still alternator problem: there are 3 phases rectifier diodes, one of them fail can cause the alternator reduce power. In additional, voltage regulator also a suspected one...I'm sure that if you do alternator replacement then problem went away.

Jwallaz

Thanks for the reply! Yes, both batteries are now new and the light only comes on after about 20 seconds of actual driving. If I do not drive and let the car sit idle, the light never comes on. Also, the light turns off every time I turn the car off and restart it. I’ve tried to keep the computer going that displays the voltage but when the red light turns on it takes the voltage off the screen and I can no longer see it. However, I’ll leave the car on and put a meter on it once the light turns on later today. Not sure if this is important or not, but since covid hit the car does not get driven much at all, maybe only 20 minutes a week sometimes.

Anyways, I appreciate the advice! I didn’t think it was the alternator bc the car gets the battery up to the 14.6/7 v once it’s started and the battery is maintaining around 12.6v when off. Or, maybe I should say I was hoping it wasn’t the alternator bc of the adventure it will be to change it out myself lol.

Also, I’ll get it scanned today at an autoparts store.

Last, I’ve read that sometimes it takes a few days for the light to turn off when changing the battery or that I may need to “register” the battery via a diagnostic tool/obd scanner. Does either of these sound plausible?

bmwpowere36m3

The battery voltage being good is whats hanging me up on the alternator being flat out bad... maybe its as simple as the battery sensor on the negative terminal being defective and triggering the charging light. Pulling codes, you might need a MB-specific code reader to flesh out some of these as they don't all come thru on generic scanners.

Jwallaz

Ok, so I think I spotted the issue, not sure what the repair would be though. I believe, but am not positive, that the light comes on after a short time of driving bc the battery voltage dips below 13v. I hooked up a device to allow me to see the voltage while driving and while the battery initially gets up to 14.6/7 upon starting the car, once I start driving and accelerate a little bit the voltage drop below 13v. Is this the reason the light comes on? And does this mean it’s the alternator or voltage regulator?

Arrie

It is normal for these cars to show 12.6-12.8 V at battery. The smart charging system seldom has it over 13 V other than when you drive and slow the car down or pull heavy load from the battery.

bmwpowere36m3

Concur with @Arrie

Anywhere from 12.6 to 15 VDC is within the control range of the system... it can also let the system drop below 12.6 in in exceptional cases (e.g. stall prevention, cold start at high altitude, catalytic converter heating).

Pull codes using something better than generic scan tool... that's my advice.

Jwallaz

Thanks for the advice. Will pull the codes ASAP and go from there. I just think it’s odd the battery light comes on as the voltage drops, which seems to be when I accelerate. I’ll let you guys now what I find. Thanks again!

nikobimmer

I’ve got 210,000 miles on my 2014 e250 and have never had this issue.

Arrie

The warning light connection should be such that one lead is connected directly to battery positive terminal and the other lead to alternator (or regulator) positive output. When engine is not running and alternator not turning the lead thru alternator connects to ground and the light comes on. When alternator starts working this lead turns positive to about the same voltage potential as the battery and light turns off. For the light to come on one of the leads needs to drop potential enough and as the battery is new it must be the lead on alternator side. The voltage does not need to drop to zero for the light to illuminate, just low enough for enough voltage potential difference thru the light to turn it on.

Could it be that when you see the battery voltage drop below 13V it actually means your alternator stopped pushing enough voltage to keep the light out and you measure what the battery alone has?

If the above holds your alternator part itself is ok as it is capable of pushing 14+ volts but the regulator controls it too low when the smart charging system takes control when you drive. So, it could be the regulator that I don't know if you can replace without replacing the whole alternator as I think it is built in.

bmwpowere36m3

What your describing @Arrie is how the warning indicator on "older" charging systems worked... I suspect with these cars, there's "logic" that latches the warning and not simple circuit.

Arrie

Well,

It very well could be the case, I don't know but the light connection circuit is so simple why change it? And I'm not sure a battery can read 12.6V by itself. My S-Class showed 11.8V today when I checked before starting it.

bmwpowere36m3

If the ignition was on or any modules awake, depending on the load and health of the battery... it could be 11.8 VDC.

Ideally the ignition is off and all models asleep, so minimal draw, < 50 ma when you check SOC.

Arrie

Yes, ignition was on. I checked with the cluster menu that in S-class allows checking some things when engine is not running.