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Discussion Starter #1
So, I had the dreaded high discharge idrive message for 2-3 yrs now, changed battery, made no difference.
A couple of months ago, car basically stopped driving due to low battery (although I never got a warning about it).
My radar detector displays voltage so I observed that upon startup the voltage was at 11.8 or so and then it would climb to 13.9 within 30 seconds or so.
More recently, I noticed that my IDrive screen would go blank.
Also, the AC Fan would cut in and out.
Within the last month I started getting lots of system failures.
I noticed that when those things happened the voltage would be moving up to the high 14s low 15s.
I gave in and got a brand new alternator. I know the culprit was the regulator but If I was gonna spend the money on labor to get the thing replaced, I wanted to put a new one in there.
Right after the replacement, I noticed that the voltage was 13.9-14.0 right upon startup. And, the voltage has stayed rock solid between 13.8-14.1 most of the time.
Except today. I started the car up for its 2nd trip of the day and it started at 11.8 again, took a few seconds before it shot up to 14.1.
This is making me very nervous, is there something else messing with the alternator output? Can you guys chime in as to what you alternator does on startup?

Thx
 

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What is code ,,did you read any code,
Did you get 279D codes

Thanks
 

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The dme controls when the alternator turns on after engine starts. Target voltage is controlled as well based on batt temperature etc. Normal operation imo.
 

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Discussion Starter #4
What is code ,,did you read any code,
Did you get 279D codes

Thanks
Have not checked for codes as I did not get any errors on the IDrive. I just noticed the voltage on my radar detector was below 12 for a few seconds after startup.
 

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Discussion Starter #5 (Edited)
The dme controls when the alternator turns on after engine starts. Target voltage is controlled as well based on batt temperature etc. Normal operation imo.
Can you please verify if you car does that upon startup? I would not expect the dme to allow voltage to be below 12.

Thx
 

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The dme doesn't 'allow' that voltage. That is just your battery voltage with all loads on and alternator not running yet. If memory is right yeah 11.5-12 is not unusual.
 

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I had similar issue. Had to code the cas module for the battery I installed. For some reason ista would correctly identify battery. Weird problem, simple fix. Now I have no alternator codes and no more voltage fluctuation problems.
 

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Discussion Starter #8
More info: On 1st startup of the day, the alternator immediately kicks in and provides at least 13.8V.
On 2nd startup after the vehicle has sat for about an hour or so, the alternator does not kick in until after 15 seconds have passed.
 

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Twice my alternators fried the computer modules. Be sure to keep the battery fully charged ( I use BMW trickle charger), otherwise the alternator will be forced to work harder to keep charging the weak battery. So the never sleep systems killed 2 batteries, 2 alternators, and 2 computer modules. When alternator about to die, it most likely start to overcharging electronic modules.

Last week we tested 2020 Acura MDX with dead battery. It killed the AC, and the car was underpowered. Most, it not all, today cars never sleep!
 

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Discussion Starter #10
ok, i have more evidence that something is wacky with BMW's charging set up.
Like I mentioned earlier, on 1st startup of the day, Voltage goes to 13.8/13.9 immediately upon startup with no input from driver.
On a subsequent startup though, it's where the weirdness lives.
Today I restarted the car after having sat for about 1.5 hours.
I started it and the voltage was at 11.8.
I did not touch the throttle. I let the car run. The voltage slowly dropped as the minutes went by. After a few minutes, my experiment was over. I blipped the throttle and immediately the voltage climbed up to its usual 13.8. So, it seems that this could be a 'bug' in the BMW software. Can anyone else confirm ?
 

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Discussion Starter #13
Been testing this since yesterday and long story short my alternator starts 10 seconds after engine starts, worst case being the overnight case.

Here's the few videos I recorded, measured at the jump start terminals.

And it starts regardless of whether you touch the throttle? For me, after it sits for about 1.5-3 hrs I have to blip the throttle to get it going. At idle it just sits at < 12V basically draining the battery. On cold start, the Alternator is at full force immediately. So, different behaviour than yours it seems. What year/country is your car?
 

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Correct never touched the throttle. Looks different indeed. I believe there's a screen in inpa where you can monitor commanded vs measured voltage. I suspect your new alternator is at fault based on your description.
 

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ok, i have more evidence that something is wacky with BMW's charging set up.
Like I mentioned earlier, on 1st startup of the day, Voltage goes to 13.8/13.9 immediately upon startup with no input from driver.
On a subsequent startup though, it's where the weirdness lives.
Today I restarted the car after having sat for about 1.5 hours.
I started it and the voltage was at 11.8.
I did not touch the throttle. I let the car run. The voltage slowly dropped as the minutes went by. After a few minutes, my experiment was over. I blipped the throttle and immediately the voltage climbed up to its usual 13.8. So, it seems that this could be a 'bug' in the BMW software. Can anyone else confirm ?
Mine was behaving the same before it damaged the computer module. Then next year when it was waiting several days in the dealer for the ignition coils, the battery died (most likely they left the fob inside the car. Then the dealer jumped started the car. Couple weeks later the alternator and computer module died.

Try replacing the Voltage Regulator first. It is an easy DIY.
 

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Discussion Starter #16
Mine was behaving the same before it damaged the computer module. Then next year when it was waiting several days in the dealer for the ignition coils, the battery died (most likely they left the fob inside the car. Then the dealer jumped started the car. Couple weeks later the alternator and computer module died.

Try replacing the Voltage Regulator first. It is an easy DIY.
The whole unit is brand new.
 

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Discussion Starter #17 (Edited)
Correct never touched the throttle. Looks different indeed. I believe there's a screen in inpa where you can monitor commanded vs measured voltage. I suspect your new alternator is at fault based on your description.
That's a good idea. I think ISTA has that as well, I will take a look

BTW, it seems the DME does control the Alternator, I found this:
 

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mine sits at 12.2 even with a new alt. If i drive its almost at 14. However, once i replaced the alt, i no longer get the high discharge code and its been a month now.
 

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Discussion Starter #19
mine sits at 12.2 even with a new alt. If i drive its almost at 14. However, once i replaced the alt, i no longer get the high discharge code and its been a month now.
12.2 with the engine running/idling?
I am going to check it out with INPA today.
 
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