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Thread: New alternator, newish-battery - voltage drops when engine is warm

  1. #1
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    New alternator, newish-battery - voltage drops when engine is warm

    2003 540i/6 with 137k miles on the clock. Two weeks ago, my original alternator died (dash battery light on solid and test 9 showed less than 12v, with no change in voltage with engine speed. The battery is an Interstate MTP-49 that was replaced in April.

    I pulled the original Bosch unit and replaced it with a Genco remanufactured unit.

    When the engine is cold, I consistently get 13.4-13.8v via test 9, varying with engine speed (13.5v @ 1000rpm), but once the engine is warm, the battery light flickers, then goes solid and the voltage starts dropping (11.8-12.2v, varying with engine speed). I'm honestly afraid to drive more than a few miles. Voltage at the battery, after being on the a tender, overnight is 13.06v. There is no voltage drop when comparing the engine jump post and the battery terminals.

    I'm admittedly not thinking straight -- I was laid off in August and my wife was diagnosed with cancer on Friday -- so, money is too tight to take this to the dealer and I'm bereft of ideas of where to look for the issue.

    The Bentley says that to check the alternator, I need to pierce the harness, but I don't have a piercing probe and the harness connector looks different than the illustration (I'm wondering if the I6 and V8 harnesses are different?)... Can AutoZone, etc actually test the V8 alternator? (The closest auto parts store -- an O'Reilly -- couldn't bench test the old alternator; they said it had to be in the car and honestly, I don't trust them to competently hook up my car.)

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    Gut instinct is to wonder what exactly Genco does to their reman'd alternators. Are they actually replacing brushes? diodes? bearings? Or?

    The BMW documentation is interesting here. I think that the M62TU has one of them fancy alternators. BMW lists two fancy styles: "Compact alternator with multifunction controller (MFR) and start load response" and "Compact alternator with multifunction controller (MFR)". The start load response function reduces the alternator load/output for the first "few" seconds after starting the car. The MFR also controls the battery light (unlike on older cars where it might just be a simple circuit to the alternator). For the fancier alternators BMW says that the following conditions will set the battery light:

    • Failure of the belt drive (Ualternator = Uterminal 15 / no phase signal)
    • no charge due to a fault (Ualternator = Uterminal 15)
    • Interruption in excitation circuit
    • Overvoltage by a defective, conductive controller output stage (Ualternator > Usetpoint)
    • Break in charge line (Ualternator - Uterminal 15 >= 3 V +/- 0.5 V)


    The M62TU in your 540 is different than than the alternators used in the older V8s and the sixes in that it's water cooled. Dunno what differences there are beyond that. I think that the six got a fancy (but still air-cooled) alternator, and maybe the non-TU V8s got the "dumb" alternator?

    That said the big thing is check the wiring. No voltage drop is good, but how are the grounds? I'm assuming that when you check for voltage at the jumper post you're not grounding out through the same wire(s) that the alternator is using. My meager experience suggests that voltage regulators typically fail with excessive voltage (18V+) and brushes fail with low voltage.

    BMW suggests using DIS as a scope, and shows normal and failed diode waveforms -- which would indicate that you might be able to get away with a cheap k-line cable and something like Testo in lieu of a scope. Or just grab a multimeter and put it in AC mode. You should see 0 volts AC with the car running, if you see non-zero values your diodes are toast.

    Unfortunately finding parts for these alternators is an exercise in futility so pretty much anything other than borked wiring means you've got to replace the alternator.

  3. #3
    JimLev's Avatar
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    Sharkd, sorry to hear about your situation (car and wife).
    As blarf Sid your alt is water cooled and different that any of the other ones BMW has used.
    There is no ground strap on this alt, nor does it have any brushes.
    Ground is thru the alt case to the engine block.
    I'd say that the Genco alt is shot, can you have them send you a new one?

  4. #4
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    I just went through this exact problem only days ago. Your newish alternator is failed.
    https://www.bimmerforums.com/forum/s...-miles-(540I6)
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    Quote Originally Posted by JimLev View Post
    Sharkd, sorry to hear about your situation (car and wife).
    As blarf Sid your alt is water cooled and different that any of the other ones BMW has used.
    There is no ground strap on this alt, nor does it have any brushes.
    Ground is thru the alt case to the engine block.
    I'd say that the Genco alt is shot, can you have them send you a new one?
    Sure but the block's gotta ground somewhere. Also would be interesting to see one of these failed units taken apart -- could there be a bad ground internally? Are these smart VRs even being replaced?

  6. #6
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    Blarf, the ground to the chassis is on the pass side engine bracket to the car body.
    I posted pics and a bunch of info when I disassembled one of these units.
    I bought bearings and a VR from a place in Colorado, the VR didn't work. They replaced the VR and that one didn't work either.
    The VR box had a made in Mexico sticker on it. Under the sticker was made in China.

    AutoZone and O'Reillys don't have the setup to hold the alt in there tester or the correct connector. Been there, done that.

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    Where are you located?

    If you put a voltmeter on the batter when the car is running you should see 14v.

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    Quote Originally Posted by JimLev View Post
    Blarf, the ground to the chassis is on the pass side engine bracket to the car body.
    I posted pics and a bunch of info when I disassembled one of these units.
    I bought bearings and a VR from a place in Colorado, the VR didn't work. They replaced the VR and that one didn't work either.
    The VR box had a made in Mexico sticker on it. Under the sticker was made in China.
    Wonder if the Bosch remans are using the same Hecho in La Zona Guerra VR. When I said internal ground I figured there's gotta be some sort of ground strap or something to connect the guts to the case?

    Quote Originally Posted by JimLev View Post
    AutoZone and O'Reillys don't have the setup to hold the alt in there tester or the correct connector. Been there, done that.
    Well, yeah. I trust those guys about as much as tow truck drivers for mechanical advice.
    Last edited by blarf; 09-24-2018 at 01:42 AM.

  9. #9
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    Quote Originally Posted by blarf View Post
    Gut instinct is to wonder what exactly Genco does to their reman'd alternators. Are they actually replacing brushes? diodes? bearings? Or?
    They're aka BBB/VIP and a fairly large remanufacturer. I trust the parts shop I got it from (they're a local institution) and will see what they say about taking it back. At the time, it was that or wait for an alternator to arrive, delayed by the hurricane (which washed out the main road between my neighborhood and town), else I would have ordered from FCP Euro.

    I'll check the grounds, today. (Of course, in my distracted haste, when checking for a parasitic drain, I forgot to switch terminals and popped the fuse in my DMM, on Sunday… new fuses are on the way).

    Quote Originally Posted by JimLev View Post
    Sharkd, sorry to hear about your situation (car and wife).
    As blarf Sid your alt is water cooled and different that any of the other ones BMW has used.
    There is no ground strap on this alt, nor does it have any brushes.
    Ground is thru the alt case to the engine block.
    I'd say that the Genco alt is shot, can you have them send you a new one?
    I'm familiar with the alternator -- I pulled the original Bosch… and, given that this is the 4th time I've pulled the cooling system apart since buying the car in 2011 (three radiators and one water pump), I'm intimately familiar with that aspect of the car.

    Quote Originally Posted by Santaclaus4 View Post
    I just went through this exact problem only days ago. Your newish alternator is failed.
    https://www.bimmerforums.com/forum/s...-miles-(540I6)
    Yeah, I saw your post and was hoping mine wasn't the same, but figured it is.

    - - - Updated - - -

    Quote Originally Posted by RobertDenali View Post
    Where are you located?

    If you put a voltmeter on the batter when the car is running you should see 14v.

    I'm in NC (it's in my profile). As I said in the original post, I'm getting proper voltage at the battery and the jump post (grounding to a suspension stud) and via test 9, until the engine warms.

  10. #10
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    It's the alternator. I cleaned all the grounds and drove it 1.5 miles to O'Reilly. Right as I was turning in, the charging light illuminated and the voltage dropped.

    IMG_3046.jpg
    Test 9 voltage as I left the house.

    IMG_3052.jpg
    O'Reilly alternator tester results.

    IMG_3053.jpg
    Test 9 voltage as I returned to my house.

    The independent parts shop will take the failed alternator back for a refund. I've ordered a Bosch unit from FCPEuro. Thanks for all the advice.
    Last edited by sharkd; 09-24-2018 at 02:58 PM.

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    Quote Originally Posted by sharkd View Post
    They're aka BBB/VIP and a fairly large remanufacturer. I trust the parts shop I got it from (they're a local institution) and will see what they say about taking it back. At the time, it was that or wait for an alternator to arrive, delayed by the hurricane (which washed out the main road between my neighborhood and town), else I would have ordered from FCP Euro.
    Sure, however these newfangled voltage regulators are probably only made by one or two companies. It seems unlikely to me that Genco would be making their own VRs. Seeing two alternators from different (re)manufacturers with the same failure mode makes me wonder if Bosch has started sourcing them from the same subpar company that Jim's talking about. Hell, I wonder if they even replace the VR.

    I feel ya on pulling the cooling system apart. When I did the six speed swap I ended up forgetting a clip for that hose that runs parallel to the A/C belt. Guess what happens when the coolant heats up and the hose sags? Now I've gotta pull the damn thing apart again because the water pump is peeing. And I just drained/refilled the coolant a few weeks ago.... still haven't found a not-so-messy way of draining this damn thing either since it has a Zionsville radiator.

    The VR/diode tests are things you can do on your own with a multi-meter as I outlined above. For anyone else following at home, from your pictures I'd also expect that revving the snot out of the engine would cause the alternator to self-exite so that would potentially rule out the exciter circuit.

    Edit: Jim -- your pictures are 404 now. Any chance you could upload them somewheres else?
    Last edited by blarf; 09-24-2018 at 05:21 PM.

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  14. #14
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    Quote Originally Posted by JimLev View Post
    Hmm. It would be really neat if this parts shop would pull apart the failed alternator (or allow OP to) -- both to see if the VR was actually replaced (if so, with what?) and to see the internal wiring. Did any of the screw terminals break, or what about that metal tab at about 11:50.

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