New Alternator, Still Low Voltage at Idle

Hey guys, I am in need of some advice. I think I understand the charging system pretty well (and my mechanic of a father agrees with me) so we're both kind of stumped.

By the way, I'm in a 2003 325i Spt/Prem with 138,000miles on the clock.

About 10-12 days ago, my battery light started flashing at me. Replaced the alternator with an off-brand from some import shop here in Dallas. Everything seemed fine, as I drove it back and forth from Oklahoma with no issues. Then 3 days ago, I went for a drive at night, and was having a little fun. After about 30-45 minutes, I had flashing battery light (and subsequent random systems lights) again. Used the OBC voltage redout to watch my voltage and found that while driving, above about 800 rpm or so, voltage fluxuated between 13.1 and 13.4. Sitting at a light, at idle, in drive, with headlights and radio on, (no ac) voltage would fluxuate around 12.3-12.8. (Which seems low to me.) Then randomly, would dive down to 9.9 or so, and trigger the warning lights on the dash, and immediately recover. If I kept the revs above about 800, it wouldn't happen. Kill the engine, and restart, worked fine. The battery tested fine at Autozone with a load tester. (Which I would agree with since it starts the car just fine every time.) When I started it for the tech, he said that the alternator was bad, as I was only getting 12.7 volts at idle.

So, I'm thinking I got a shitty alternator and I've already ordered a rebuilt bosch from NAPA to put in it. Will check grounding straps before I install it, as I've read that can be an issue, but since the voltage is constantly too low, (never have I seen it at or above the 14V that everyone shoots for) I'm thinking it's the alternator.

My symptoms seem to be slightly different than all the others that I have read about on here so I figured I would start a new thread. And when I get this all worked out, I will definitely update and hopefully save someone else some frustration.

Any input would be much appreciated. Thanks in advance guys. I still LOVE LOVE LOVE my e46!!

go to a junkyard and buy an OEM Bosch alternator for your car. Then go to the dealer and buy a new voltage regulator for it. total will cost about $150 and it will last 100K miles and actually work.

I've fixed about 10 OEM bosch alternators on E46's and E39's by simply replacing the voltage regulator. One of them now has 70K miles on it since I replaced the VR and still running strong!

Therein lies the problem. I thought it was probably the VR, but none of the 4 dealers in the Dallas metroplex stock just the voltage regulator. That's why I settled for the off-brand unit a week ago, and unfortunately that means I don't have my original unit, as it was turned in as a core. But as long as it sounds like an alternator/VR problem to you as well, I'll go ahead with replacing the alternator again. Thanks for the help!

Originally Posted by nathanmeh

Therein lies the problem. I thought it was probably the VR, but none of the 4 dealers in the Dallas metroplex stock just the voltage regulator. That's why I settled for the off-brand unit a week ago, and unfortunately that means I don't have my original unit, as it was turned in as a core. But as long as it sounds like an alternator/VR problem to you as well, I'll go ahead with replacing the alternator again. Thanks for the help!

No problem. They never stock the VR because they want to make more money on the repair. Will a full alternator replacement, they get a higher markup on parts and a guaranteed repair if by chance the bearings or stator/windings are bad. Faulty alternator bearings on the E46 is pretty rare.. although I have seen it here and there. If you get an alt from a junkyard with low miles and put a Genuine BMW VR in it, it will last a very long time.

New BOSCH alternator fixed the problem. An excellently regulated 14.0V was the result. So lesson learned, no more off brand **** for the Bimmer.

Thanks for reporting back!

Thanks for letting us know the fix....

Reminds me of this quote -

"There is hardly anything in the world that some man cannot make a little worse and sell a little cheaper, and the people who consider price only are this man's lawful prey."
John Ruskin

And this one-
"The bitterness of poor quality is remembered long after the sweetness of low price has faded from memory."

Hi, hope this thread is still active, I appear to have exactly the same problem, i started having issues with the battery not charging and eventually completely running out of juice.

I recharged the battery and checked the voltage and it was around 12V with the engine running (which is basically just the battery voltage). I bought a cheaper alternator to completely replace the bosch unit that was presumed faulty.

I have had nothing but trouble, it appears to charge at about 13V tops and it switches during idle between 12.5V and 13.4V, however when the engine is idling you hear the engine struggle as if its under extreme load for a second or so then recovers, its like the aircon being switched on but much worse!

The battery was only 8 months old, ive had it tested also and all looks good so im very reluctant to change it again. from my experience the battery charging voltage is far too low. I cant understand it surely these alternators must work!?

you can charge the battery out of the car and it will last around 7 - 10 days then the car will fail to start. after reading this post im very tempted to re-build the oem bosch unit with new bearings and a VR. I'm in the UK hopefully the dealer will be able to order me one in!

Excellent information! I would just add that when it comes to electrical problems always start w/ the battery, because 50% of the time that is the problem. A lot of times the battery might test OK but it actually isn't. Just my two sense.

Sent from my DROID2

Originally Posted by scroades

A lot of times the battery might test OK but it actually isn't.

Exactly correct.

SJ 24,
" I'm in the UK hopefully the dealer will be able to order me one in!"[/QUOTE]

you probably only need #2.
order it on-line...ECS tuning sells them...
Use realoem.com to look up your part number for either the Bosch or Valeo brand VR.

RealOEM.com Home => Select another car => Engine electrical system => Alternator, individual parts

No. Description Supplement Qty From Up To Part Number Price Notes
01 Pulley alternator D=55MM 1 12317501754 $32.10
02 Voltage regulator 1 12317559183 $124.68
03 Cap 1 12317501755 $24.40
04 Hex bolt M10X150 1 07119904677 $3.82
05 Hex bolt M10X125 1 07119900630 $2.78
08 Cap nut 1 12427565287 $3.23
09 AIR CHANNEL UPPER 1 12311432465 $7.82
09 AIR CHANNEL LOWER 1 12311712932 $11.30
For vehicles with
Aerodynamics package
S716A=Yes
09 AIR CHANNEL UPPER 1 12311432465 $7



http://www.realoem.com/bmw/showparts...84&hg=12&fg=22

Hi QAfred,

Thanks for the quick response! Ive stripped the old VR off the original alternator, i was considering stripping, sandblasting and fitting new bearings etc... but the bearings are perfect so I dont think ill bother!

Cool I will cross reference the BMW and bosch part numbers and give the dealer a call in the week, luckily a friend of mine works there so he can bring it home for me!

i am still concerned there is some other underlying problem, im still finding it difficult to believe even a new crappy replacement alternator is actually no good out of the box!

There isnt a lot else there could be, im 99% sure the battery connections and engine earth strap are good, I completed an engine conversion some 18 months ago and its been perfect up until about 8 weeks ago.

Anyway I will get the VR replaced and hopefully that will cure this very annoying problem! cheers again for the info!

I'm having the same issue as well. Car idles up and down when i come to a stop. You can see the headlights flash slightly along with the interior lights. Question is should i return the alternator to autozone to have them replace it since its life time warranty but will same issue occur again? I hear everyone talk about replacing the voltage regulator but will this fix the problem. Seems like everyone that purchased an autozone alternator has the same issues. are those alternators that bad or im i just another person that received a faulty item.

Hi guys, just an update, I was probing around with the voltmeter again last night, I had to get a jump at work because the car wouldnt start!

Anyway, after getting home I checked the charging voltage, it was sat at 12.53V, way too low! I believe there needs to be a 12V feed to the alternator to allow it to work, i removed the connector and sure enough the voltage was present. However removing the connector made no difference to the battery voltage!

After removing the battery and charging it up (for an hr and a half), i re-fitted it and again checked the voltage, this time its sat at around 12.5V but after about 1 min after engine start it drops to 11.67V! i monitored the voltage again for the next two mins or so, and when the dip in revs and load on the engine happens at idle the voltage creeps back up to 12.5, stays there for 5 - 10 secs then slowly drops back to 11.6V!

I have read on other forums if the voltage is too low it could also be the stator windings inside the alternator? this annoying problem is driving me mad, do I bite the bullet and just get a re-con bosch alternator? or do i potentially waste £90 odd pounds and try the VR?

Probe directly at the alternator with both multimeter probes. Are you getting 13.4V at the regulated output? Do you have any voltage loss when using the chassis for GND instead of the alternator casing? You could have a duff earth strap in the engine bay or battery area. Any voltages measurable between the grounds?

Have you checked the calibration on your multimeter? Sounds like it is giving good data but some have been known to read a volt or two out after a while, leading to much anguish and replaced alternators.

alright.... i just went through the same thing you are going through....

I replaced 5 alternators int he last 6 months, 3 new batteries and the problem still exists..... took the car to local bmw dealer, they replaced the alternator with a new oem (problem was still there), replaced the battery with a new oem (problem was still there), replaced the belts and problem was still there.... only other thing that could be would be the DME..... now thats 2 people that are bmw techs tell me that i need to replace the dme.... so just to be sure, i took my car to the mechanic, they replaced the alternator AGAIN with a brand new bosch unit (so were the last 4) and he comfirmed that it would be the DME, he would check a few other things today and would get back to me..... im trading the car in next month due to all the headache it has caused me and all the money out of my wallet.... hope that helps you out

Check the bat cable yet? This caused slit of problems for me. Corroded wire.