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  1. #1
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    Autozone says bad diode in the alternator

    So, I have had hard starting for a few days, so I went to my local Autozone and the battery checked out okay. Checking the alternator, he said the output was fine, but his little checker said an internal diode was bad inside the alternator.

    So I am guessing the voltage regulator is bad, and that it is inside the alternator. Do i just order a new alternator and replace the whole thing at once, or can the VR just be swapped out?

    From my searches, it seems like the '97 is a simpler swapout than later years, does anyone have a web writeup on the procedure? One post said to expect about 1.5 to 2 hours for the job.

    Thanks in advance!
    Michael1997 540i Automatic
    148k
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    The diodes and voltage regulator are 2 different (

    components. Your alternator has 6 diodes in it and generally not easily replaced by the home owner. Your alternator is basically a 3 phase generator with each phase being converted to pulsating DC by a pair of diodes. The regulator just controls the voltage level that the alternator outputs. This would be an easy fix for an alternator/starter repair shop.
    Euro had a post showing how to take an air cooled alt apart, replace the bearings, brushes, and regulator.

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    Thanks JimLev! Paging EuroDavid, alternator

    writeup please!

    Autozone will order me an alternator for $207 + $100 core. It is a 140 amp, so it looks to be the right one.

    Thank you,
    Michael1997 540i Automatic
    148k
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  4. #4
    Craig in Canada
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    Here they are

    Eurodavid's Valeo writeup (unfortunately all of his photobucket links are now broken):
    http://bimmer.roadfly.com/bmw/forums/e39/9383071-8.html

    Eurodavid's Bosh writeup:
    http://bimmer.roadfly.com/bmw/forums...347757-10.html

    We really need to do the PDF thing for writeups. This photo link stuff is getting "old". People like Euro go to all the trouble to do writeups and then changes at downstream sites like photobucket make it useless. And, until the software changes at least, Euro can't just edit the post and fix the photo links. He's got to repost the whole thing with new links.


  5. #5
    Craig in Canada
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    Bosh -> Bosch - oops


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    Jim, you're an unmasked EE!!


    Ed in San Jose. BMW CCA member since 1987 (Nr. 62319). Golden Gate Chapter. '97 540i 6 speed. Build Date 3/97. Aspensilber over Aubergine leather.

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    My college friends called it "Plug&Socket" school






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    Craig in Canada
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    Re: The diodes and voltage regulator are 2 different (

    I would like to know more about replacing diodes in a Bosch.

    I replaced my alternator in 2006 after mis-diagnosis by my indy (I had no time to deal with a noise issue myself at the time). I've noticed that the replacement rebuilt unit is often "weird". Flickering voltage, even at cruising highway speed, and nothing else seems to be to blame. I'm not positive that it's only the regulator and if the rectifier was modular it would be a no-brainer replacement. Even if they were independent heat-sinked parts that could be soldered in I'd do it - I'm an EE, I think I can handle it even though I don't drive a soldering iron any more.

    I'm hoping to use a Eurodavid writeup to rip it apart some time I'm ready. The bearings are still fine at this mileage, but... One challenge will be that now I can't use the model/age/VIN of the car to know exactly which alt I have. Since it's a rebuilt of "similar application" then there might be differences in the rebuild parts needed versus factory original.


  9. #9
    Eurodavid
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    Michael, I'm just the RF riffin photographer. Jim

    is the 'splainer. I didn't understand a word of his post to you except when I saw my name, and said "hey, that's me!"



    Eurodavid

    P.S. Gracias, Craig, for finding it ;-)

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    Craig in Canada
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    de nada


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    Yes | No

    1 hour job is more like it. I changed on the 528

    and it was less then an hour. The hardest part was to put the belt back alone. If you have a help, that should be a simple job too.

    I reused the belt (less then a year old) but I changed the alternator in search of a TransFailSafe mode.

    Alternator test in Autozone failed while in the car, but once I took it off and put it on the tester it was fine.




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  12. #12
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    Re: Autozone says bad diode in the alternator

    Okay, lots of info here. Thanks to all. I am going to replace vice rebuild because I don't have the time to let the car sit while I try to rebuild. $209 plus tax plus $100 core charge to Autozone and I will give it a go on Friday.

    The procedure is brief in the Bentley. Almost scarily so. It basically says to disconnect the battery, take off the serpentine belt, remove 3 bolts, unplug the harness and pull it out. Can't be that simple. This isn't my '65 Impala.

    One question, Bentley says to disconnect something on the positive jumper connection on the left side of the engine. If I disconnect the negative lead from the battery, why mess with the jumper connection? Or am I misunderstanding the step? It also says to remove the plastic cover on the left valve cover. Again, why?

    Thanks again.

    Michael1997 540i Automatic
    148k
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  13. #13
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    This has nothing to do with hard starting

    Bad alternator has nothing to do with hard starting, unless you mean the battery is dead and wont start

    Andreas
    Queens NY
    BMWCCA# 186796
    86 325es (wrecked by careless driver)
    87 325 (project for life)
    00 540is 6spd (daily driver)

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    Re: This has nothing to do with hard starting

    Well, yes I am blaming my hard starting on the alternator. Auto zone tester says the battery is fine. And the alternator has a bad diode. Ergo...

    I am still trying to figure out why Bentley says to undo the positive jump connector on the engine tho.

    Michael1997 540i Automatic
    148k
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    Update: maybe not the alternator?

    I went to pick up a new alternator, different parts store, and asked them the check everything before I bought it. Unfortunately, everything checked out okay. Battery, alternator and starter. So I went to another parts house and they said everything checked out fine. I even pulled the battery out and had them test it in the store test bin. Checked out fine.*

    So I did not buy an alternator, I just don't know if that is the problem or not. I am not sure what to do next.

    The only thing I can come up with is that we bought my wife a car a couple months ago. So now I only use my car for VERY short trips, and maybe I don't drive it long enough to keep a good charge on the battery. Not sure what I can do about that, but perhaps that is the problem.* Gonna take her out for a good, healthy drive every day for the next few days and see if the starting problem remains or disappears.

    Would love to hear some ideas. Thanks.
    Michael

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    Yes | No

    Put it all back together with clean connections (m

    at the alternator and battery. Check the battery ground cable connection to the body too. Then atart it up and read the voltage at the battery with the engine at a fast idle. It should read between 13.6 to 14.2 volts depending on how low the battery is and what accesories you have turned on. If your reading is good you could have a module not powering down and draining the battery or like you said you just don't drive it enough. A trickle charger will take care of that.

  17. #17
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    Re: Put it all back together with clean connections (m

    I wonder about a slow drain. I know of trickle chargers, but being in an apartment, it is not an option.

    When all put back together, initially, after starting, the alternator read 11.9 volts, but then slowly increased to about 14.2 or 14.3 volts. So it appears to be doing its thing.

    Gonna drive her a bit more today and tomorrow and see what happens.

    Thanks,
    Michael1997 540i Automatic
    148k
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  18. #18
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    Do you have a meter to measure the current drain(m

    when all the modules go to sleep after 16 minutes? It should be less than 30ma (0.030 amps)

  19. #19
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    PLEASE explain your hard starting

    When you say hard starting do you mean the engine CRANKS and doesnt not turn over OR do you turn the key and the engine does nothing?



    Andreas
    Queens NY
    BMWCCA# 186796
    86 325es (wrecked by careless driver)
    87 325 (project for life)
    00 540is 6spd (daily driver)

  20. #20
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    Re: PLEASE explain your hard starting

    A slow, half-second crank followed by a half-second pause, then another short crank, another pause, then faster cranking for about a second or 2, then she finally starts. It is not exactly the same every time, but close to this.

    Michael1997 540i Automatic
    148k
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