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  1. #1
    Veteran Member Three Rings QuattroBucc's Avatar
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    Mar 06 2020
    AZ Member #
    541263
    My Garage
    B5A4 1.8TQ g25-660, MK4 GLI K04
    Location
    Santa cruz, CA

    Alternator Failed or Voltage Regulator failed

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    Car is a 98 pre-facelift AEB 1.8t

    Yesterday and today I noticed my battery light came on, monitoring my voltage it would drop to 11v, randomly come back up to 14V for a bit, then to 12v etc. It jumps all over every 15-30 seconds it seems like.

    I barely made it home tonight as it stayed below 12v and got all the way to 8v by the time i got home.

    I went and grabbed my digital multi-meter to test the battery/alternator. To my surprise the car started and didn't need a jump.

    Testing the battery and output from the alternator is generally the same readings, the voltage is bouncing all over the place, from 9v to 14v, every second bouncing to a different voltage.

    I'm pretty sure its the voltage regulator that failed after some research, I can't find the specific regulator replacement though, if anyone has a link to the correct one that would be great

    If anyone has any knowledge about this I would appreciate it, I'm no electrical guy

    I just pulled the alternator off the car to see the brand/amps. Looks like its a bosch re-manufactured valeo alternator

    https://imgur.com/a/7v93zED Here are pictures of the brushes and slip rings. top brush is more worn than bottom one. The slip rings look scratched, and the top one half of the circle is very dull.

    Do i just need a new regulator with new brushes? Do i clean the slip ring surface with light sanding? Or do i have to replace the whole alternator unit
    Last edited by QuattroBucc; 12-06-2024 at 12:33 AM.

  2. #2
    Veteran Member Four Rings walky_talky20's Avatar
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    Jun 30 2008
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    30427
    Location
    Erie, Pennsylvania

    Brushes look used up (too short to make full contact). I believe the dull one is from the poor contact and some arcing happening. It could probably be cleaned up pretty easily.

    I would use a multimeter in ohm mode across the 2 rotor ring contacts and make sure the rotor is not open.

    If it looks good, I would try to just replace the brushes (ie: desolder them from the regulator and put new ones on, I did a DIY guide on this).
    ^Don't listen to this guy, he's not even a mechanic.
    2001 Laser Red A4 1.8TQM, 5-Speed Swapped, 4.11 Final Drive, APR 93, 2.5" Exhaust, ST Coilovers, 034 RSB, A8 Brakes Front & Rear
    2006 Passion Red Volvo V50 T5 AWD 6MT
    2000 Satin Silver Passat 1.8T FWD Wagon, Slippy Tiptronic, 15" Hubcaps
    2001 Aluminum Silver Metallic A4 Avant 1.8TQM (winter sled)

  3. #3
    Established Member Three Rings adm750's Avatar
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    Aug 16 2023
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    2016 A4 Allroad, 08 BMW X5 3.0, 06 Grand Marquis
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    def make sure those aren't 'open'..... but absolutely rebuild that thing.....easy peasy and not spend so much cheesy... (that was cheezy...my bad..it's been a long week today...)

    get some crocus cloth and run it over those rings to clean 'em up a bit. let me see if I can find the place i got the parts for my BMW X5, it's a Valeo. these guys were fantastic....

    found it -- Maniac Electric Motors

    https://www.maniacelectricmotors.com/

    kit came with regulator, hardwear, bearings, all that stuff. I think at the time is was less than $50 but that was about a year and a half ago...
    highlightable former rides:
    03 Euro BMW 525 M-sport Touring, 97 BMW 528i, 85 528e, 79 Euro 635csi (dirty motor/dog-leg 5 speed) 89 gsxr750 (traffic knife), 89 Corvette (wifeys former daily), 79 RX7, 90 K1500 shortbed 350/5-speed), 83 RX7 tubeframe SCCA GT3 car, so many others....

  4. #4
    Veteran Member Three Rings QuattroBucc's Avatar
    Join Date
    Mar 06 2020
    AZ Member #
    541263
    My Garage
    B5A4 1.8TQ g25-660, MK4 GLI K04
    Location
    Santa cruz, CA

    Quote Originally Posted by walky_talky20 View Post
    Brushes look used up (too short to make full contact). I believe the dull one is from the poor contact and some arcing happening. It could probably be cleaned up pretty easily.

    I would use a multimeter in ohm mode across the 2 rotor ring contacts and make sure the rotor is not open.

    If it looks good, I would try to just replace the brushes (ie: desolder them from the regulator and put new ones on, I did a DIY guide on this).
    I believe you're right, i came across your guide last night it is very good.

    Before I got these replies...this morning I cleaned up the rotor ring contacts, and freed up one of the brushes that was a little stuck and re-installed the alternator, Output from the alternator was now very high, fluctuating from 17-20v while idling for 10 min or so.. Isn't it supposed to be a constant 14.4v?
    I don't want to fry anything electrical by overcharging so I'll wait for the regulator I ordered last night. But the fact it's outputting that voltage would mean the rotor isn't in open circuit correct?

  5. #5
    Veteran Member Four Rings walky_talky20's Avatar
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    Jun 30 2008
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    Location
    Erie, Pennsylvania

    Yeah. It does sound like your regulator has failed. The fact that it is outputting means the rotor is good.
    ^Don't listen to this guy, he's not even a mechanic.
    2001 Laser Red A4 1.8TQM, 5-Speed Swapped, 4.11 Final Drive, APR 93, 2.5" Exhaust, ST Coilovers, 034 RSB, A8 Brakes Front & Rear
    2006 Passion Red Volvo V50 T5 AWD 6MT
    2000 Satin Silver Passat 1.8T FWD Wagon, Slippy Tiptronic, 15" Hubcaps
    2001 Aluminum Silver Metallic A4 Avant 1.8TQM (winter sled)

  6. #6
    Veteran Member Three Rings QuattroBucc's Avatar
    Join Date
    Mar 06 2020
    AZ Member #
    541263
    My Garage
    B5A4 1.8TQ g25-660, MK4 GLI K04
    Location
    Santa cruz, CA

    New voltage regulator worked like a charm. very easy to install with the alternator in the car also

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