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Thread: Alternator help

  1. #1
    Join Date
    Jul 2011
    Location
    Austin, TX
    Posts
    2,180
    My Cars
    S54 E36 M3, E32 740il

    Alternator help

    Ok, need some alternator help (electrical is definitely my weak point):

    18 months ago, original alternator died at 210k miles. 8 year old battery survived, died 6 months later.

    A couple months ago I noticed the "new" alternator would make too much voltage over 4500 rpm - indicator lights, headlights got brighter.

    This alternator died on Wednesday. Battery got pretty low Wed night, but was ok to start the car without a jump or charging after I replaced the alternator the next morning.

    However, voltage was low - both before I started the car and while running the half hour to work. Was a little higher on the way home from work Thursday night, but I put it on a trickle charger and it was fully charged (and everything seemed fine) this morning.

    However, now the new alternator is exhibiting the same overcharging behavior above 4500 rpm. Except sometimes it just cuts out completely over 4500, voltage drops, and the battery light comes on. Revs drop, everything goes back to normal. (I should note it behaved normally when I first put it in - I checked. It wasn't until the trip home on Thursday that it started acting up again).

    Ideas?

    I wonder if the original alternator damaged the old battery, then the old battery damaged the voltage regulator in the new alternator trying to charge that old / damaged battery. Then rinse and repeat - the replacement alternator finally dies, and since I didn't charge the new battery, the second new alternator fried its regulator trying to get the battery charged. I had the battery tested tonight and it's perfect still.

    So maybe I just fried this new alternator, and a new one will have me back to normal? Am I on the right track, or did I burn up something somewhere?

    -Josh: 1998 S54 E36 M3/4/6 with most of the easy stuff and most of the hard stuff. At least twice. 271k miles. 1994 E32 740il with nothing but some MPars. 93k miles.

  2. #2
    Join Date
    May 2018
    Location
    Anchorage Alaska
    Posts
    93
    My Cars
    1997 M3 Sedan
    At idle how many volts do you read at the battery?

  3. #3
    Join Date
    Jul 2011
    Location
    Austin, TX
    Posts
    2,180
    My Cars
    S54 E36 M3, E32 740il
    Quote Originally Posted by ArcticM View Post
    At idle how many volts do you read at the battery?
    I'm using an adapter with torque, which seems to read low. It read 12.0 with the car off, and when I took the battery in to be tested it read 13.2 - totally healthy.

    Took the car to O'Reilly's this morning, alternator' regulator tested good, but failed when I revved it.

    Got a replacement, that one didn't have the bottom mount threaded, but I swapped regulators.

    When I did that, I noticed the alternator shaft wasn't pressed in far enough. The top brush on the regulator didn't engage all the way. The other new (non-threaded) one did engage all the way. Fit the same after I swapped them, leading me to think it was a shaft issue on the one I had installed.

    If anything, the new one fit even worse. After reinstallation, it was the same - but failed at slightly lower rpm now. Seems like this may have been the problem.

    I'll have a new (threaded) one Tuesday or Wednesday. I'll be checking the regulator before I install it.

    Anyone have any thoughts about all of this?

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