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E39 (1997 - 2003)
The BMW 5-Series (E39 chassis) was introduced in the United States as a 1997 model year car and lasted until the 2004 when the E60 chassis was released. The United States saw several variations including the 525i, 528i, 530i and 540i. -- View the E39 Wiki

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Old 02-22-2015, 10:06 AM
TG.Goose TG.Goose is offline
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Location: Vancouver BC
Join Date: Nov 2014
Posts: 1
Mein Auto: e39 & e30
Alternator / voltage / idle issues

Car in question is a 10/98 1999 540i m62tu with water cooled alternator.

Been chasing a strange issue when ever coming to a stop. Not always, but sometimes when coming to a stop, the rpms dip 100-200 below idle and then jump instantly up to correct idle speed. I figured vacuum leak, but fixed everything that appears with smoke machine. Then I started checking voltages, running I get 13.8, when I come to a stop it drops to 12.2 and will usually jump to 12.8 and sometimes 13. When starting it drops to 9v and then comes up to 13-ish (does this with 2 brand new, fully charged batteries). I'm ruling out the battery itself, leaning towards the alternator, but the battery light has never once come on during all of this. Could I be missing a ground strap? I can't see the one that's supposed to be from near the A/C to frame. Voltage drop when starting seems really high, taken from OBC via instrument cluster.
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Old 02-22-2015, 11:15 AM
Willy7166 Willy7166 is offline
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Location: Limerick,PA
Join Date: Apr 2014
Posts: 44
Mein Auto: 2002 540i Sport
Same issue here with my 2002 540i sport . Thought it may be a bad connection somewhere but have been unable to find anything. Just happened a couple times last week when the temp dropped to less than 10 F. And it stalled twice on me. Started right back up with no issue, but idle bounce was pretty pronounced at stops.
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Old 02-23-2015, 08:42 AM
Sroor9001 Sroor9001 is offline
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Location: Bahrain
Join Date: May 2014
Posts: 137
Mein Auto: Bmw 520i 2002
Check red positive wire that connected to the alternator and the nut the screwd to the wire
Also may be your voltage regulators problem
Check for it's brushes if not replace voltage regulator
And for sure you'll be fine faced this before
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Old 02-23-2015, 11:06 AM
Spencercat Spencercat is offline
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Location: CT
Join Date: Dec 2010
Posts: 56
Mein Auto: '99 540i/6 '03 540i/6
Also check connections at the battery, could also be bad battery itself.
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Old 02-23-2015, 01:37 PM
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bluebee bluebee is offline
Seek to understand,^Value
Location: San Jose, California
Join Date: Mar 2008
Posts: 22,682
Mein Auto: 02 BMW 525i M54 auto 130K
These may help...
- Complete DIY for how to test a BMW battery & alternator (1) & a diagnostic procedure to test for overnight parasitic battery drain (1) & how to find all the fuses (1) & how to choose a good aftermarket battery (1) and a simple battery replacement DIY (1) & where to find the battery explosive charge emergency quick disconnect (1) (2)
Please read the suggested threads, where the best always add value to those threads, either by pictures or by descriptions, so the next person with the same problem stands on your shoulders.
See also: E39 Bestlinks & How to easily find what you need
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