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Thread: 1971 R75/5 Generator Light Question

  1. #1

    1971 R75/5 Generator Light Question

    Hi all

    Iíve looked at other posts but donít believe this situation has been covered.

    I was riding my R75/5 the other day and noticed I was getting an intermittent generator light. About 3 minutes later it came on permanently. As I was only 3 miles from my house I rode it back. I turned it off and checked the voltage at the battery it was 12.6. I started the bike with the starter and with the engine running it read 13.2 Volts.

    My questions are thus: is 13.2 normal voltage and my problem is likely a wiring issue in the headlamp? Or do I need to look elsewhere first?

    Thank you

  2. #2
    Liaison 20774's Avatar
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    What was the engine RPM when you read the voltage at the battery? Idle or low RPM will result in low voltage at the battery. If the RPM is above say 3000, you should be seeing around 14 volts at the battery.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  3. #3
    Registered User STEVENRANKIN's Avatar
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    light

    Hello, 12.5 volts at the battery posts sounds good. The fact you can crank the engine over with the starter kind of tells the battery is most likely OKAY.

    As for voltage when the bike is running, keep in mind at idle speed to about 1500 RPM BMW systems don't charge very much or at all. So, to get the proper charge voltage, RPM should be about 2K RPM. Look for a range of 13.8 to 14.8 Volts.

    A generator light staying on past idle means two things, one, there is no charging taking place, this can be tested by a voltmeter at the battery. Second, a diode or diodes in the diode board has broken down or failed. This is something that would have to be checked with a bit more complicated test method and the diode board off the bike. The reason for the light to come on in this instance is because the Alternator produces AC current. This current is passed through the diodes in the diode board to convert it from AC to DC current. Now in the generator failure indicator light system, there is also a diode which blocks current flow in one direction. Under normal operations the charging current runs the bike and charges the battery, no current flows from the battery to the system. When the system fails, current reverses and starts to flow from the battery instead of into the battery it is now out. This reverse flow lights the indicator light in the dash.

    In the case of diode board failure, AC current defeats the diode in the warning light system and the bulb lights when it is not supposed to.

    I know, a long note here. First thing to check is voltage as I listed. If your voltages are correct, it is possible to have proper indicated charging voltage at the battery at the correct RPM but, it may not be DC charge due to a failed diode board.

    This only touches very briefly charging issues. IF the charging voltage is too low, there is a few methods needed to find the cause involving brushes in the alternator and the voltage regulator. A search on line or a visit to Snowbum's site will most likely turn up a step by step method to trouble shoot in this case.

    After my long drawn out post, I guess the bottom line is if the charging system have the proper voltage and the charging light is still steady past idle, there is a good chance the diode board is faulty. If the voltage is low, something else is faulty.

    Good luck. St.

  4. #4

    Update

    So after further examination I found that as RPMs increased so did the voltage I stopped revving when it showed no sign of stopping at 15 Volts. My meter registered DC voltage but when switched to AC showed no voltage.

    So I replaced the voltage regulator. Voltage is now a steady 13.8 Volts.

    I also removed the diode board and tested it for resistance and appeared to get the proper readings that I had seen elsewhere.

    BUT I still have the charging light on. Only now at start up and idle the light is off/dim and it brightens as the RPMs climb. Is this a sign that the diode board is shot? Could I have reconnected the 3 pin connector on the back of the diode board upside down?

    Thanks again.
    Last edited by PeteM1965; 07-26-2021 at 07:44 PM.

  5. #5
    Registered User STEVENRANKIN's Avatar
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    When in doubt

    When in doubt recheck your work.

    There is a way to test a diode board, I am sure snowbum or someone has a method for doing it. The light should go on at idle and probably will stay on until about 2000 RPM or thereabouts.

    I may be wrong but if the light is staying on past this RPM, it is possible the diode board is bad. I say that taking into the voltages you are getting at the battery when raving the bike past 2000RPM.

    15 volts may be a tad high, I can't remember if the indicator light will come on for over charging or not? To be honest, I haven't ever seen an overcharging airhead.

    Again, check Snowbum's site or one of the other guys such as Anton Largidare, or Boxer two Valve, they hay have information. St.

  6. #6
    I can't remember if it's hard to re-wire the diode board wrong...but Euro Motoelectrics has a pretty good knowledge base of articles on their site (Technical Tips section) as well. I have a /5 and had some charging issues - i used their diode board test white paper and it was pretty simple and clear and showed that my diode board was not working properly. Electrical / charging issues can be tiresome but the systems are somewhat simple and easy to test. Good luck, Chuck
    C R NY
    1972 R75/5
    1969 2002
    1982 528e

  7. #7
    Quote Originally Posted by crouthier View Post
    I can't remember if it's hard to re-wire the diode board wrong...
    Reversing the brown and black or blue wires at the brushes will cause the light to stay on.

    As for the three AC legs alternator to diode board, the order of the wiring doesn't matter.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://web.bigbend.net/~glaves/

  8. #8
    Registered User Guenther's Avatar
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    Is the blue wire from the diode board's D+ connected to D+ at the voltage regulator?

    The voltages at D+ and B+ at the diode board should be about the same with engine running.

    You may have to use a Y-spade connector at the D+ and run a wire from behind the diode board to have an accessible measure point for D+ when the diode board is installed.

    Where did you get the readout for the 13.8V? At the battery?

    /Guenther
    2017 F700GS - I wish she had a drive shaft

  9. #9
    Quote Originally Posted by Guenther View Post
    Is the blue wire from the diode board's D+ connected to D+ at the voltage regulator?

    The voltages at D+ and B+ at the diode board should be about the same with engine running.

    You may have to use a Y-spade connector at the D+ and run a wire from behind the diode board to have an accessible measure point for D+ when the diode board is installed.

    Where did you get the readout for the 13.8V? At the battery?

    /Guenther
    Yes, at the battery.

  10. #10
    At this point I have ensured all wiring to the diode board and under tank is correct. Battery is charging at 13.8V DC. Could this be a loose wire in the headlight causing the bulb to stay on?

    Thanks for the help so far

  11. #11
    Registered User STEVENRANKIN's Avatar
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    Wire

    Yes, a loose wire in the system could cause a problem. Also, as I said, the bulb system in the stock BMW instrument cluster is a weak spot. Check it and all connections for loose, or crud. St.

  12. #12
    Registered User Guenther's Avatar
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    The blue wire from the diode board goes to the starter relay and from there - still blue color - on to the ignition light bulb.

    Verify that there are TWO blue wires well seated into the starter relay and the spades are clean.

    /Guenther
    2017 F700GS - I wish she had a drive shaft

  13. #13
    Registered User STEVENRANKIN's Avatar
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    The Chase

    The blue wire from the diode board goes to the starter relay and from there - still blue color - on to the ignition light bulb.

    Verify that there are TWO blue wires well seated into the starter relay and the spades are clean.

    /Guenther


    A good point, if you don't have a wiring diagram handy for your bike, chasing electrical problems is like trying to navigate in a big city you have never been in. We and Gunther, can suggest wires and connections to try and trace but having a diagram, studying it, and knowing how each component works, makes far more sense in finding a problem.

    Rick at Motorad Electrik had a good little book printed on Boxer charging systems. Last I knew it was out of print in the US. I managed to snag a copy from the UK at Motobins. Rick does not know when it will be back in print. EME sells very good laminated color wiring diagrams, the best I have seen so far.

    Good luck. St.

  14. #14

    Final update

    So after retesting the diode board I found that it had a faulty diode. I purchased a replacement from Euromoto Electrics which finally fixed the problem.

    So to sum it all up I got a flickering Charging Light that went to solid on

    Checked battery and it was charging at idle but as idle increased so did voltage up to 15 volts whereby I stopped revving so as to not destroy battery

    Replaced voltage regulator which fixed voltage issue but light remained on.

    Tested, and retested, diode board and found a faulty diode.

    Purchased and installed a replacement diode board which solved the problem.


    Thank you much to all the people who go commented here.

  15. #15
    Liaison 20774's Avatar
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    Thanks for the summary!
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

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