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E46 M3 (2001-2006) Engine: S54 - Max Hp: 333 hp at 7,900 rpm / 262 lb/ft at 4,900 rpm
Total Produced: 45,000+ - Years Produced: 2001 to 2006.


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Old Fri, Dec-25-2009, 03:10:07 AM   #1
mpegripper
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Default Possible bad alternator?

So over the past couple of months, I've noticed that my battery would drain quite quickly if the car wasn't driven for a couple of days. I used my optima warranty and swapped it out (D34/78 yellow top), but the issue still persists. I don't have a battery light or any errors in my ecu/dme etc.

So I grabbed a multimeter and started to see what was draining it. While doing so, I decided to see how much the battery was being charged while the car is on. To my amazement its only charging at 12.4 volts. I tested my friend's M3 and his was charging around 13.9 volts.

Does this mean that the alternator is on its way out? My car is an 01 with 76k miles on it.

Also, while testing the current through the car while it was off, i found that my car was pulling about .4 amps, but my friends car was about .8 amps. His car never dies though. Does anyone know what the correct amount is about? I know different options and mods can alter this, but .4 amps seems high to me. To test the current, i took the ground cable off of the battery and put the multimeter to the ground cable and the ground pole of the battery. I believe this is the correct method iirc since the potential would be infinite if the cable were connected, but please let me know if I'm wrong.

Please also note that my sound system, angel eyes, aftermarket fog light harness, ipod, and flashlight in the glove box were all disconnected completely from the car.

Last edited by mpegripper; Fri, Dec-25-2009 at 03:15:01 AM.
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Old Fri, Dec-25-2009, 03:18:07 AM   #2
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I also, plan on getting my new battery tested at Autozone as I believe they are the only place with the proper tester that can tell me if any of the 6 cells are bad.

and FYI - Bentley Manual says the following for voltage:

12.6VDC for battery ful charged..engine at rest

13.2-14.5VDC for engine running and regulated voltage

Now the question is, at what regulated voltage with the engine running causes the battery light.

Last edited by mpegripper; Fri, Dec-25-2009 at 03:31:13 AM.
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Old Fri, Dec-25-2009, 03:26:50 AM   #3
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Your numbers don't sound good - but low numbers can be caused by a heavily discharged battery - which becomes a load across the alternator, drawing the voltage down. Normally if this were the case I'd expect to see high charging current - and in this case we're not.

What does the alternator/battery voltage read at 1,500 RPM?

The alternator really isn't overcoming "house loads" (the power needed to keep the engine running) at idle. It doesn't start to charge until somewhere around 1,500 RPM or so.
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Old Fri, Dec-25-2009, 03:29:06 AM   #4
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i haven't measure the current while the car is charging (dunno how i would do that), but i'll definitely check the voltage when the car is at 1500rpm.

does the current while the car is off sound ok? also, is my method for measuring the current correct?

Just found out that the Bentley Manual says that anything over 50mA should be investigated

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Old Fri, Dec-25-2009, 04:38:47 AM   #5
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after 16 minutes of the car being off the load is 60mA and at 2000rpm the voltage was at 12.6 volts

does a rebuilt bosch alternator come with a new voltage regulator?

Last edited by mpegripper; Fri, Dec-25-2009 at 04:43:25 AM.
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Old Fri, Dec-25-2009, 07:07:19 AM   #6
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problem is the yellow top. mine did the same thing too. once i replaced it with the bosch battery, the problem went away. yes, the regulator mounted on the side of the altnerator is included if you buy a new alternator.
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Old Fri, Dec-25-2009, 06:19:44 PM   #7
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Quote:
Originally Posted by mpegripper View Post
after 16 minutes of the car being off the load is 60mA and at 2000rpm the voltage was at 12.6 volts

does a rebuilt bosch alternator come with a new voltage regulator?
Current sounds OK. You do know all the modules in the car keep talking to each other (on) until 15 minutes after the last electrical action in the car (like closing the door..) right? To measure standing current draw - best to bring a pair of leads out from the battery connection (in series with the negative terminal and the battery) through the closed trunk lid - then walk away for 30 minutes and come back to see what it's reading. If it's above 50-60mA, you have the fun project of repeating this many times - each time after removing one fuse. When you find the fuse where the current draw drops below 50mA, you're getting close to finding what's drawing the excess current.

That all said - the battery voltage at 2k RPM should be above 13V. Your alternator isn't charging. Unfortunately the failure is not one that triggered the "battery" (actually alternator) warning light, so I'd guess you have a failed diode on the diode bridge inside the alternator. Bad brushes or a bad regulator will normally light the light. A failed diode reduces the capacity of the alternator, but won't trigger the light.

Fix is a new alternator. I'd suggest a BOSCH rebuilt one - NAPA is a good source for these. I'd avoid AutoZone, Advanced Auto, etc.. they mostly spray paint and cross their fingers for the rebuilds. The Bosch rebuilds are as good as new.

Good luck!

BTW - DO have the battery tested. It sounds improbable to me that the battery is the problem given the voltage you read - but I suppose it's worth having it tested (especially since it's free..)
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Old Fri, Dec-25-2009, 06:27:24 PM   #8
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the 60mA was after 25 minutes of the car being off and doing the following process:

make the car think the trunk is closed, open the door, close the door, turn key to on position for 5 seconds, turn car off, open the door, close the door, see measurement after 25 minutes.

i have a rebuilt bosch alternator coming tomorrow from winchester auto parts (very reliable local auto parts store), unfortunately napa in the area only carries valeo alternators.

i will also get the battery tested at autozone tomorrow.
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Old Wed, Dec-30-2009, 06:55:32 AM   #9
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installed the new alternator today. after driving around town some its charging my battery at about 12.9-13v...leave it on the charger overnight to see if it charges it at a higher voltage tomorrow. then i'm swapping batteries with a buddy to see if anything changes as well

if none of this solves it, we're gonna hook up an oscilloscope and see what the alternator is doing. from what we're reading it should have a small ripple.
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Old Wed, Jan-06-2010, 06:08:29 AM   #10
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Any updates?
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Discussing Possible bad alternator? in the E46 M3 (2001-2006) Forum - Engine: S54 - Max Hp: 333 hp at 7,900 rpm / 262 lb/ft at 4,900 rpm
Total Produced: 45,000+ - Years Produced: 2001 to 2006. at BMW M3 Forum.com (E30 M3 | E36 M3 | E46 M3 | E92 M3 | F80/X)