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Discussion Starter #1 (Edited)
I have a 2003 325i with 156k miles on it. A few months ago I started having electronic issues with the car not starting in the morning and the radio shutting off. I noticed the battery was quite old so I replaced it with a OEM battery from a E90 dated 11/17. The car daily drove for a few more weeks with no issues until my radio and electronics stopped working while driving. (again) I did some voltage checks via the dashboard while driving and the voltage was low 11.5 - 12.5. A few times my voltage dipped so slow it triggered the transmission safe mode on the way home.

After replacing the battery the issues persisted, I was instructed to replace my alternator. Last week I picked up a 120A Valeo unit from LKQ that was bench tested and told it was good. My old alternator had no labels or stickers of any kind. I popped it into my car thinking it would fix the issue..

I trickle charged the battery overnight and it read 12.6v via the dashboard test. I started it up today and the voltage while idling began at 11.9v. (It was a little bit higher than my old alternator's idle, however..) I drove it around the block a few times and slowly the car started dipping down to 11.6 with lights and heater on. I turned on the xenon lights and the voltage instantly dipped down to high 10's and slowly picking back up to 11. The battery light started to flicker so I took it back home. Now that the car is resting in the driveway the dash says the voltage is at 11.6v and I cannot start the car unless I trickle charge the battery again. This is the first time I've seen the battery light flicker so much.

I'm guessing my replacement battery is bad due to all the times it was discharged thanks to the old alternator? I'm going to take the battery in as soon as I can to get it tested.

I'm clearly not an expert with this stuff so I'd appreciate any advice anyone has to offer. Thanks in advance.

EDIT: I replaced the alternator with a new Bosch 120 unit from FCP Euro and went ahead and put a new battery in. Same problems, car ran great voltage for a few days.. battery light returned and the battery keeps getting discharged to 11.8v. Haven't had time to really investigate with it being so cold outside..
 

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Also don’t forget there’s a few battery connections to check.

Positive: battery terminal, jump position under hood, starter pole and alternator
Negative (ground): battery lead to terminal and body, engine ground to engine and body

Any bad connection to any of the junction points above will lead to a bad charge.
 

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Discussion Starter #3
Also don’t forget there’s a few battery connections to check.

Positive: battery terminal, jump position under hood, starter pole and alternator
Negative (ground): battery lead to terminal and body, engine ground to engine and body

Any bad connection to any of the junction points above will lead to a bad charge.
Am I looking for anything in particular? I don't see corrosion of any kind.. that was my first guess.
 

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Old alternator could have messed up your new battery, and that could have burned out your new alternator. Lots of rebuilt alternators are already garbage anyway. Take it back and get the battery tested too
 

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Despite the bench test, I think your replacement alternator is bad. The battery light operates by comparing the alternator's output to the battery voltage (the output feeding the light is past a diode, so battery voltage doesn't make it to the battery light). IIRC if the alternator output is less than half of the battery voltage, then the light turns on.
 

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Discussion Starter #8
Despite the bench test, I think your replacement alternator is bad. The battery light operates by comparing the alternator's output to the battery voltage (the output feeding the light is past a diode, so battery voltage doesn't make it to the battery light). IIRC if the alternator output is less than half of the battery voltage, then the light turns on.
I knew I should have bought a new one.. I wanted to go used to save money. :/
 

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never ever get a rebuilt alt , it's just not worth it

i've seen them bad the next day, or test good but really be bad

new is the only way to go

the napa gold new is not a bad choice

other than that it's prob gonna be oem
 

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Am I looking for anything in particular? I don't see corrosion of any kind.. that was my first guess.
Make sure your battery terminals are flush with the battery posts. If you can't seat them properly then back the clamp screws off fully and open them up with a flat blade until they do.

But I don't think it's your battery.

Understand that the only time you should be drawing from the battery is when you start the engine. Once the engine starts the alternator is responsible for providing all of the car's electrical supply needs no matter what. That's running the engine and all of the auxiliaries PLUS pumping charge back into the battery.

You dashboard voltages are way low. It should be between 14 and 14.5 volts at idle without auxiliaries on and 12.5 to 13.5 volts with all of them on. If you have those kinds of numbers you are good.

That's where the red light comes in. It tells you that the current through the light is going from battery to the car, which is the wrong direction, instead of alternator to battery.

When you rebuid an alternator there are certain parts that can be replaced quite easily. Voltage regulators and / or brushes. Bearings and rotating element. I don't think they replace the stator or the diodes. That probably requires a little too much effort to make the rebuilds commercially viable.

So, it is possible for a rebuild alternator to go bad because the diodes are the original and they do go bad. Their job is to allow current to flow out of teh alternator in one direction only. From alternator to car. When they fail they start allowing bidirectional flow throught the alternator and that can give you a red light.

You can easily check the diodes using a multimeter with the unit in or out of the car. Here is how. If the diodes have failed then a new alternator is required. You don't need a diode option on your multi. You can do the same with a simple connectivity test. If you get a reading in both directions you have faulty diodes.

If you do this with the alternator on the car make sure you disconnect the battery first. POSITIVE TERMINAL. Very important.

https://www.youtube.com/watch?v=oUhgyqm0wro
 

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rebuild alternators sometimes don't have new VRs, so that might be the issue as well... just went through this fiasco on my M3, ended up buying new from fcp and problem fixed.

anothe issue might be your belt slipping, that being said have someone rev it and look to see if the belt travels smooth or the tensioner is making it skip or might be loose in general. I call alternator.
just get a new one.
In canada Valeo alternator is $500+ unlike in US, $210. the markup here up north is stupid insane.
 

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Also, I did get my $ back from the rebuilt failed unit.. you might get your $ back as well. Technically they give you 12month warranty.
And when I ran the on board diagnostics the old was dropping below 12v at 5k+ rpm.
New alternator is at 14-14.1v at idle and drops only slightly when under load to about 13.6.
Just for reference.
 

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that's how you tell what is on your car, altho, that amazon alt has a pulley that looks not like a bosch so, who knows

get off amazon

buy it at fcp or ecs
 

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I concur with those that talk badly about rebuilt alternators.
There was a time when a Bosch reman alternator was as good as/better than a new one. It also carried a better warranty with towing coverage.

Some 5 or more years ago, Bosch sold ALL of their "Rotating Reman" (That's starters and alternators just to name a couple) to the Chinese.
Since that time it's a crapshoot if you'll get a good one. We've seen failures right out of the box, failures just a few miles later and a host of other issues such as noisy bearings.
Back in the day....many German cars & parts were built to the 150th. percentile. Alternators could be rebuild 2 sometimes 3 times. Now they're made cheap & light. Once they fail they're usually no good for rebuilding any longer.

I had a friend that went through 5 Bosch reman alternators on his vintage M3 before he landed a good one. Unfortunately it was his only option.

We always use new. when we can. Rarely there is no other choice.
 

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Discussion Starter #18 (Edited)
Well.. I don't know what to say but the replacement alternator decided to work! I ran the car for more laps around the block and now I'm running a healthy 13v. Not sure why it decided to work but i'll be ordering a new one just in case. Thank you all for the help.
 

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Well.. I don't know what to say but the replacement alternator decided to work! I ran the car for more laps around the block and now I'm running a healthy 13v. Not sure why it decided to work but i'll be ordering a new one just in case. Thank you all for the help.
13 volts isn't healthy.
 

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Well.. I don't know what to say but the replacement alternator decided to work! I ran the car for more laps around the block and now I'm running a healthy 13v. Not sure why it decided to work but i'll be ordering a new one just in case. Thank you all for the help.
13 volts isn't healthy.
Agreed and emphasize!

Every document I've ever seen, or tested myself, about 12V systems is that a healthy alternator should be putting out at least 14V, plus a little more.
 
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