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      06-20-2019, 08:08 AM   #1
MrSweet1991
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E90 Alternator Field/Excite Wire?

Hi everyone,

So to cut it the chase I either get a good 14.1/14.3v at alternator and battery, or I'll get a low output of 12.3v tested with a multimeter. So, does the alternator use a field/excite wire to supply 12v at ignition to make the alternator charge? It seems strange to me that the alternator only functions one of two ways, perfectly fine or 12.1/12.3ish which can be solved by simply turning the car off and back on (rarely requires a couple of cycles) so I'm wondering if the alternator has an excited wire, there's two positive wires, one to glow plug controller, the other probably to starter motor and there's also a little wire at the bottom of the alternator, it's hard to see but I'm thinking about inspecting that first if it is that wire?
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      06-20-2019, 12:51 PM   #2
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The alternator will have two positive connections (glow plug module & starter), it will also have a one wire plug connected to it (BSD line).
If you think the alternator is on it's way out easiest test method is to connect a multimeter to the charge posts in engine bay, run it with no consumers on and you'd expect to see around 14-14.5V.
Load it up by turning all lights on, blower to max, heated rear windscreen, etc.. it should remain around 14-14.5V without too much of an issue - I was working on an E91 330D the other day, when loaded up it slowly counted back voltage wise from 14.4V to below 11V (at which point is started running rough because the voltage wasn't enough to power control units, etc.. properly). Rectified issue by replacing alternator (and consequentially battery).
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      06-20-2019, 01:13 PM   #3
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Quote:
Originally Posted by N5GKP View Post
The alternator will have two positive connections (glow plug module & starter), it will also have a one wire plug connected to it (BSD line).
If you think the alternator is on it's way out easiest test method is to connect a multimeter to the charge posts in engine bay, run it with no consumers on and you'd expect to see around 14-14.5V.
Load it up by turning all lights on, blower to max, heated rear windscreen, etc.. it should remain around 14-14.5V without too much of an issue - I was working on an E91 330D the other day, when loaded up it slowly counted back voltage wise from 14.4V to below 11V (at which point is started running rough because the voltage wasn't enough to power control units, etc.. properly). Rectified issue by replacing alternator (and consequentially battery).
Thanks mate! I use an app called Caro0 pro connected to a carly adapter which ironically shows me the battery voltage as I'm driving, because of this I can quickly identify if the car has started with 12.1/12.3v or running good at 14.1/14.3v and I have to say for me it does seem that the alternator only does one of the two and the only time it can change is at startups, I don't think I've ever driven and seen it drop randomly. That said, I will do exactly as you have said and load everything I can while using the engine bay power points and see how it goes! Assuming my alternator can maintain the 14v with everything running what would that indicate? It does seem at this moment on time that each and every time I start the car something will trigger the 12v and other starts will load 14v.. quite strange.
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      06-20-2019, 01:34 PM   #4
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Right, measured 14.2v with nothing running. Turned heaters on full, AC on, radio on, headlights on and its providing a good 14.3/14.4 at high the alternator, battery and secret menu as showing the same. I'm going to do a journey and leave everything running and check again when I'm back but so far it's outputting enough power
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      06-20-2019, 03:19 PM   #5
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Well, done about 40 miles and it's been perfect, AC and heaters, radio and lights on and yeah the alternator had no issues maintaining above 14v so what does that leave? Battery is brand new as well and after a drive maintains around 12.7/12.6v
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      06-21-2019, 02:33 AM   #6
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So, if you can access the instrument cluster secret menu set it to the voltage screen.
Go for another drive and watch the voltage, when you accelerate does the voltage sporadically drop to around 12.3-12.6V? And then brake and see if it returns to 14V+,
If thats whats happening id be happy to say I don't see it as an issue. 'Smart' alternators tend to switch on and off quite often when driving, for efficiency's sake BMW have tried to work it so when the engine is under load, unless necessary the alternator will briefly turn off and then turn back on when on overrun / deceleration.
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      06-21-2019, 05:00 AM   #7
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Quote:
Originally Posted by N5GKP View Post
So, if you can access the instrument cluster secret menu set it to the voltage screen.
Go for another drive and watch the voltage, when you accelerate does the voltage sporadically drop to around 12.3-12.6V? And then brake and see if it returns to 14V+,
If thats whats happening id be happy to say I don't see it as an issue. 'Smart' alternators tend to switch on and off quite often when driving, for efficiency's sake BMW have tried to work it so when the engine is under load, unless necessary the alternator will briefly turn off and then turn back on when on overrun / deceleration.
Will test that soon, but I just went for another drive and alternator was stuck at 12v again, didn't move at all just 12.1v measured both the alternator and battery when I parked up while engine running and still 12v. What does that mean? The car has two states, it either starts up and outputs 14v or starts up and outputs 12v there's nothing in between
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      06-21-2019, 05:37 AM   #8
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No, no issues while accelerating. To be fair having monitored my voltages constantly using the Caro0 app I would have noticed the battery dropping into red but It never does.



Oh and for crying out loud, no, I wasn't recording while driving it was my partner.. I've been called out on this before.. I don't mess with my phone while driving so please don't jump the gun anyone
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      06-21-2019, 08:40 AM   #9
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And this is when the car only gets 12v.. I drive around for a minute to show the voltage while it's in this state and then I restart the car twice and it randomly goes back up to 14v

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      07-03-2019, 06:39 PM   #10
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Quote:
Originally Posted by N5GKP View Post
So, if you can access the instrument cluster secret menu set it to the voltage screen.
Go for another drive and watch the voltage, when you accelerate does the voltage sporadically drop to around 12.3-12.6V? And then brake and see if it returns to 14V+,
If thats whats happening id be happy to say I don't see it as an issue. 'Smart' alternators tend to switch on and off quite often when driving, for efficiency's sake BMW have tried to work it so when the engine is under load, unless necessary the alternator will briefly turn off and then turn back on when on overrun / deceleration.
Right, so just to update on this I "think" I've fixed the issue! So, knowing I have a good battery and I know the alternator (when it does output 14v) runs very well and doesn't fluctuate with things like AC, Lights etc we have to start thinking in between so my theory was, if it was a ground strap then from what I'm aware the car would probably randomly stall, or a bad ground strap can result in slower cranks where mine is pretty strong. So, I thought the voltage regulator may be somewhere to start, having done some research the voltage regulator is a common part to fail, then I found out the reason they fail is usually due to the brushes wearing out. This in itself was good to know because, assuming these brushes where on the very peak of just about touching the slip rings then maybe that's why sometimes it works and sometimes it doesn't

So, I purchased a Valeo alternator replacement brushes set (more specifically sv72) I took my alternator off and i could see that the inner slip rings where quite black, an indication that the brushes aren't making good contact, the top brush also was shorter. So I replaced the brushes and started the car and I got 12v but realised I had forgot to put the black connector into the alternator which actually connects to the voltage regulator and it rose to 14v so that in itself was interesting, if that plug isnt connected it wont output 14v, and of course if that connector connects to the voltage regulator and that is powered by brushes then if those aren't working good that would explain why it wouldn't always output 14v. Ibe started to car around 5 times now but tomorrow will be good, but man for a few hours work it cost 2.70!
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      07-04-2019, 05:08 AM   #11
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No scrap that after about 9 starts it went back to 12/12.3v so new battery, strong alternator that can maintain 14v new brushes on voltage regulator so what's left now I dont know unless something in the voltage regulator is actually broken
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      07-11-2019, 07:47 AM   #12
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Going through the same process as you. Regulator ordered, new battery is in. Hating the chasing! At least my problem is constant not random. I really hate the random ones.
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      07-11-2019, 10:12 AM   #13
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Quote:
Originally Posted by Ricthewrench View Post
Going through the same process as you. Regulator ordered, new battery is in. Hating the chasing! At least my problem is constant not random. I really hate the random ones.
That's interesting, so yours is stuck at 12/12.3v on startup? Did you also check with a multimeter that the alternator is actually outputting 12v and not 14v?
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      07-17-2019, 07:35 AM   #14
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My deal is new battery, not charging whilst running. Voltage regulator awaits my install, just found your callout of valeo brushes. Nice! Solder job, I get a spare regulator for cheap. Hoping it's just the regulator, lots cheaper than the alternator that's for sure. R
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      07-18-2019, 05:38 AM   #15
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Swapped regulator, 20 minutes. Too late and too hot outside to test it out I'll be doing that tonight. Hopefully that does it otherwise it's an alternator and that's about $300. Yay for BMW. R
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      07-18-2019, 12:13 PM   #16
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Quote:
Originally Posted by Ricthewrench View Post
Swapped regulator, 20 minutes. Too late and too hot outside to test it out I'll be doing that tonight. Hopefully that does it otherwise it's an alternator and that's about $300. Yay for BMW. R
Did you find it difficult to remove and install the alternator? There's like a metal collar which keeps the bottom mount tight so it took some persuasion to remove and it was quite hard to align the holes back up as it's not easy to maneuver

Oh and before you buy an alternator don't forget to test or buy a ground strap first because that would stop the alternator from charging.
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      07-19-2019, 07:29 AM   #17
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4 aluminium e12 bolts, out she came. Cradle type mount. Simple job, liking that. Ran out of time again last night cuz I'm working 14 hours 7 Days right now. Tonight I'll run it and gather data and make sure everything is as good as I think it is. R
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      07-19-2019, 07:30 AM   #18
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09 328i my ride.
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