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Thread: Alternator exciter wire issue I think

  1. #1
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    Alternator exciter wire issue I think

    I have a 90 325 and it died due to no charge issue.

    I tested the voltage coming out from the alternator and just saw the voltage drop.

    I figured, 25 year old alternator.....change it out.

    So, new alternator is in and doing the same thing....so i then check the exciter wire on the back of the alternator and it has 1.5v.

    So.....off to Google and I see a few posts talking about this so I do the following:

    Check the battery light in my cluster and the bulb works.

    I put 12v to the back of the alternator and it starts putting out 14v and charging and my battery light goes out in the dash.

    Any ideas on what I should look for???

  2. #2
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    Since the alternator/battery warning bulb is good, I see two possibilities. One would be a problem with the wiring from the cluster to the alternator exciter terminal. The other would be a problem with the regulator in the replacement alternator.

    Using the wiring diagrams from http://wedophones.com/BMWManualsLead.htm you can ohm out the exciter wire after disconnecting the cluster and the exciter wire on the alternator. Also check for a short to ground.

    If, as I suspect, you used a generic (inexpensive) reman the odds are good that this unit is simply bad. The generic remans are iffy at best and have been known to not work out-of-the-box or fail shortly afterwards. While helping a friend we went through 4 of the generics (free swap for a bad unit) before I convinced him to return the generic reman and spend the extra money and get a factory rebuilt. It worked out-of-the-box and is still working 11 years later.

    For many parts I will only use OE or OEM parts, even though they cost a little more. They almost always work out-of-the-box and can be expected to have a life similar to the original parts the car was built with.

    If the bearings in the original alternator are still good, there is a good chance that replacing the regulator & brush assembley is all that is needed.
    The car makes it possible, but the driver makes it happen.
    Jim Levie, Huntsville, AL

  3. #3
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    I have not pulled the cluster out...but this is what I have found so far.

    If I supply 12v to the exciter wire the car starts charging and works fine. Yes, it is a non-OEM....but it is working.

    I have tested ground from alternator body to car body and it ohm's out.

    On port 12 I get 12v with key on.

    If I take the exciter wire off I get 12v.

    I ohm tesed fro port 12 to exciter wire and it test fine.

    Put the exciter wire on the alternator and I get 1.5 volts.

  4. #4
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    That makes me suspect a bad regulator.
    The car makes it possible, but the driver makes it happen.
    Jim Levie, Huntsville, AL

  5. #5
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    If your alternator is bad will it start the car and in the middle of driving, the power dies? Would that be an alternator problem?

  6. #6
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    Son of a B.... I put in the old alternator in and took the VR from the new alternator.......and it is working now.

    I guess I am sticking with OEM pairs from now on

    Now I just need to order a VR so I can return the POS alternator.

  7. #7
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    You can get an OEM (Bosch) regulator from rmeuropean.com for $39. It will be a bit more from a dealer.
    The car makes it possible, but the driver makes it happen.
    Jim Levie, Huntsville, AL

  8. #8
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    Took a ride to Bavarian Autosports $36 Installed and working.

    I assume this is normal, 12.6-12.7 at idol.....13.5 the minute I touch the gas.

  9. #9
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    See Alternator diagnostics post in http://www.bimmerforums.com/forum/sh...0-Jedi-Master)

    The voltages you measure are low as you will see when you read that post.
    The car makes it possible, but the driver makes it happen.
    Jim Levie, Huntsville, AL

  10. #10
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    thanks.....after charging the battery that was very low, I am now getting 13.75 out of the back of the alternator at idle.

  11. #11
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    Okay that is better. Do you see about 14v at 2500rpm as I described above? If the battery was really low, your idle voltage would be affected. Which is why I say to run the tests on a fully charged battery.
    The car makes it possible, but the driver makes it happen.
    Jim Levie, Huntsville, AL

  12. #12
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    yes, when I rev it up it does hit 14v

    At least I can say now that I can change an e30 alternator in about 5 minutes

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    Hi old thread here.

    I may have a similar issue and I was wondering how to test that the exciter wire is shorting to ground. What I'm doing now is unplugging the dash cluster completely, then removing the exciter wire from the D+ pole of the alternator, and connecting it to ground and then to battery b+ to see if there are continuity readings.

    Am I doing it correctly ?

    Is the exciter wire at the alternator, also connected to the starter at a certain point ? Does that need to be disconnected to do this test properly ?

    Would greatly appreciate a reply - am struggling with this issue

    Thomas

  14. #14
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    No e30s, again :(
    you need the wiring diagram for that. I don't have enough experience on this issue to help you over the computer. maybe someone else will come on that does but for now, I would get a diagram and start tracing the wires
    No e30s again.

  15. #15
    richardodn's Avatar
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    You can get the wiring diagrams at http://wedophones.com/BMWManualsLead.htm

    With that said, the exciter wire circuit basically looks like this:

    Battery >> C100 post inside the fuse box >> Ignition switch >> cluster* >> C101 > D+ terminal on alternator

    *Inside the cluster, early models had only a lamp. When it burned out the alternator stopped charging. Later models had a resistor in parallel with lamp so it would continue to function with the lamp out.

    There is also a tap on the exciter wire that leads to the ABS computer. ABS uses this to monitor system voltage.

    So to answer your question, to test if there is a short to ground on the exciter with the cluster unplugged and the D+ disconnected you do a resistance/continuity check between the wire and chassis ground.
    "Howdy, Folks!"

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  16. #16
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    Richard you're a big help. Thank you. Unfortunately, I do have continuity to ground on the exciter wire with the cluster and wire unplugged. However, i don't get a continuity tone on my dmm (digital multimeter) but i get readings. When I touch the two dmm leads together, I get the tone. So does this mean that I have short, just not a significant one ?

    Second question, just off the top of your head, will the ABS stop functioning if the exciter wire is disconnected?

    Third question, to resolve my current short, I intend to do a direct connection between the B+ terminal/post and the D+ post on the alternator, with a one way diode in between (with current going from battery to alternator but not backwards). This would be easier than fixing my wiring harness entirely. I got that idea from this video : https://www.youtube.com/watch?v=e9knTSJA1eo

    If this works I'm going to route it through the key or one of the relays connected to the dme so that it automatically switches off when I shut down the car (so that I don't need to wire a separate switch in).

    Just off the top of your head, do you see any problems with this idea?

    Thanks !



    Thomas

  17. #17
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    The ABS light might come on, because you have low voltage then when the alternator is malfuctioning. Low voltage will cause problems to DME, IHKA, heating, AC, memory for seats, steering, transmission etc and other modules if it drops below 11V for more than 90seconds.
    Once explained on another forum by Ronald Trochelmann from Performing Imports = now Noelle Motors USA, a BMW Master with 3 PhD's from Germany in Engineering and he know these systems better than almost anyone. He did 12 years of R&D on these system for a manufacturer in Germany.
    Shogun tricks and tips for the E32 series are HERE!

  18. #18
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    To be absolutely sure you have a ground fault, measure again with the ABS computer, cluster and D+ wire disconnected. This should isolate that wire from the rest of the car. Do a resistance (ohm) reading between D+ and chassis ground and see what that comes up as. If you're seeing infinity (usually displayed as exactly 1) for the value, reconnect your cluster and D+ and leave the ABS computer disconnected. See if the system now charges. If you did see a fault, disconnect the C101 connector (main engine harness on firewall) and measure again. If you still see a short to ground, inspect the wire and harness as the problem is under the hood. If it goes away, then the problem is under the dash.
    "Howdy, Folks!"

    1986 Delphin 528e - Roof rack equipped lumber hauler.
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    1998 Avus Blau 328iC - Someone else's project now
    2008 Platinum-Beige X3 3.0si - Current project
    2012 Alpine White X3 xDrive35i - My new snowmobile.
    2020 Estoril Blue 440i xDrive cabrio - This car is a blast to drive.

  19. #19
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    Quote Originally Posted by richardodn View Post
    To be absolutely sure you have a ground fault, measure again with the ABS computer, cluster and D+ wire disconnected. This should isolate that wire from the rest of the car. Do a resistance (ohm) reading between D+ and chassis ground and see what that comes up as. If you're seeing infinity (usually displayed as exactly 1) for the value, reconnect your cluster and D+ and leave the ABS computer disconnected. See if the system now charges. If you did see a fault, disconnect the C101 connector (main engine harness on firewall) and measure again. If you still see a short to ground, inspect the wire and harness as the problem is under the hood. If it goes away, then the problem is under the dash.
    Thank you Richard. But can I bypass all that with a direct connection between Battery+ and D+ with a wire with a diode worked in ? And then disconnect the original exciter wire ? Thanks.

  20. #20
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    richardodn is offline Old Guy BMW CCA Member
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    As Shogun said, no. It will mess up the ABS and potentially some other cluster lights that ground through the alternator.
    "Howdy, Folks!"

    1986 Delphin 528e - Roof rack equipped lumber hauler.
    1989 Zinnoberrot 325iX Sedan - I miss this car. (Deceased)
    1998 Avus Blau 328iC - Someone else's project now
    2008 Platinum-Beige X3 3.0si - Current project
    2012 Alpine White X3 xDrive35i - My new snowmobile.
    2020 Estoril Blue 440i xDrive cabrio - This car is a blast to drive.

  21. #21
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    Thank you. I have more results to share. Disconnected the cluster, abs module, ecu and the exciter wire. Exciter wire shorts to ground. Also shows continuity when tested against battery positive.

    - - - Updated - - -

    But interestingly, with everything disconnected (dme, abs ecu, dash cluster), even the dme, fuel pump and o2 sensor relays disconnected, I flipped the key to no2 and the continuity between B+ and the unconnected exciter wire disappeared ! It went to "1" immediately. I find this perplexing. With everything disconnected, shouldn't the key not activate anything?

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