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Thread: 1972 R75/5 Voltage to Ground Question

  1. #16
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    Doh, I think I understand now. The diode chassis is actually in 2 halves. The lower half has the B+ terminal for power and the lower mounting holes are insulated from the engine casing. The top half of the chassis is grounded through the upper mounting holes and connected to the lower half of the chassis through the diode circuitry.

    So if the diode board is letting 13+V go to the engine casing when it is bolted into position - Does that mean the board is fried?

  2. #17
    Administrator 20774's Avatar
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    There are methods for testing the diode board...Snowbum has some discussion on his website. Another guy, Robin Frankham has procedures, but I can't seem to get to his website now. Rick Jones from Motorrad Elektrik also sells a very nice, handy book for testing all the components of the Airhead charging system. Check the Resources and Links thread for other websites that describe the charging system and diode board.

    In the end, the best thing is to substitute a known-good board. If the problem goes away, you have your answer.
    Kurt -- Forum Administrator ---> 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. #18
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    Part of the problem with the /5 generation of airhead was that fuses where never incorporated into the electrical system which was a mistake. Makes it difficult to figure out which circuit is causing the problem.

    I've been putting a /5 back on the road and have been thinking about not only moving some of the main wiring connects but also putting a modern fuse board into the setup.
    1971 R50/5 SWB with R75/6 drivetrain
    2013 DL650

  4. #19
    Registered User lmo1131's Avatar
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    Part of the problem with the /5 generation of airhead was that fuses where never incorporated into the electrical system
    My '73 R75/5 has fuses.... two of 'em.
    "It is what you discover, after you know it all, that counts." _ John Wooden

    Lew Morris
    1973 R75/5 - original owner

  5. #20
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    /2 fuses

    Quote Originally Posted by Lmo1131 View Post
    My '73 R75/5 has fuses.... two of 'em.
    Likewise my'72

  6. #21
    Unfunded content provider tommcgee's Avatar
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    Quote Originally Posted by 20774 View Post
    There are methods for testing the diode board...Snowbum has some discussion on his website.
    All you need is a multimeter to check diodes. Unpowered, out of circuit, they should read low resistance one direction and high the other, or if you test them under power, they should drop about .6 volts each.
    Salty Fog Rally 2007, 2009, 2011, 2012, AND LOOKING FORWARD TO 2014!

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  7. #22
    R100GS, '89 Guenther's Avatar
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    I think the older diode boards had all mounting holes isolated and rubber type standoffs were used to bolt on. This required a wire (brown) which connected the top diode plate to chasis ground. As I said "I think" because I have never "touched" an old airhead.

    The lower plate of the diode board has to be well isolated from ground because the fat red wire from Batt+ is connected there. If there would be a "short" you only would see a spark once and it became "isolated" (=non-functional) all by itself.

    Since you didn't see any obvious malfuctioning of the electric system I doubt there is a problem with the diode board/connection.

    The amperage disappeared once I pushed & pulled the ignition "key" a few times.
    If there wouldn't be that strange reading of 13V between Batt- and chasis - which is a mystery to me - I would look at the ignition key for the cause of the leackage.

    I still don't know where you measure the 13V between Batt- and "FRAME". Where do you tap in on the frame?

    /Guenther
    Last edited by Guenther; 10-03-2012 at 05:36 PM.

  8. #23
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    Thanks - put my multimeter on the diode board and it's fried.

    Here's a useful link for bench-testing the board:
    http://jhau.maliwi.de/mot/r-elec.html

    The board has conductivity almost everywhere it shouldn't, which explains why it's grounding the the positive feed from the battery - through the chassis of the board- to the engine casing - to the tune of 13+V.

    The 6 large diodes appear to be OK. The three smaller ones on the board (BYW72) appear to be kaput. The two BYW72s on the end, straddling the board & chassis, seem to be OK. I'm going to try a repair with NTE580 from RadioShack (~$4 a piece).

    N.
    Last edited by NigelS; 10-03-2012 at 06:13 PM.

  9. #24
    MonoRT MonoRT's Avatar
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    Based on what youÔÇÖve found, it looks like $12 worth of D+ diodes might fix your diode board. Snowbum talks about how to bend and solder diode leads if you are interested. However, if all three D+ diodes have failed so that they are allowing some current to flow through them in reverse, youÔÇÖve got to wonder what caused that?

    While youÔÇÖve got the patient open, you might want to trace the D+ lead from the diode board up to the voltage regulator and from there, back down to the alternator brushes. ThatÔÇÖs all positive wiring, so any bare spots could cause nasty shorts to ground that might have killed those diodes. Or, weÔÇÖve all heard stories of diode boards getting fried when the front cover was being removed or replaced with the battery connected. Apparently, you can use the cover to short the positive parts of the board to ground. Perhaps a front cover accident fried those diodes?

    If you do have to replace that diode board, IÔÇÖd advise against another Wehrle. I don't think BMW switched from Bosch to Wehrle because they were looking for a better quality component. You can still get brand new Bosch boards if you look around. Or, a cheaper and perhaps better solution would be a board made by Thunderchild or Emerald Island (sold by Motorrad Elektrik). Those units are just car alternator diode packs attached to circuit boards made to fit an airhead ÔÇô they seem to be pretty reliable.

    My ÔÇÖ85 Wehrle board has 2mm wide traces to the two small Y centertap diodes. 2mm is too thin for the amount of current flowing there and those traces burnt right off my board. I donÔÇÖt know if Wehrle has addressed that issue yet.

  10. #25
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    Quote Originally Posted by MonoRT View Post
    Or, weÔÇÖve all heard stories of diode boards getting fried when the front cover was being removed or replaced with the battery connected. Apparently, you can use the cover to short the positive parts of the board to ground. Perhaps a front cover accident fried those diodes?
    I never heard of frying the diode board until the internet - and my newest (2002) Clymer manual. My 1978 Clymer makes no mention of such a catastrophe happening, and I have never experienced such an occurrence. But then, my R80/7 is a 1978 model and my actions are not sloppy or uncoordinated when removing the front cover. Eventually, I intend to investigate if shorting the diodes is even possible on my R80/7. I tend to think (without some real conniptions) it is hard to do - and who wouldn't notice all the sparks? I tend to think of the diode shorting like the $2k o-ring. But, other airheads may have this problem and it can be made to happen.

    That aside, it looks like the NTE580 diodes are a better part, provided they fit OK. Also, you may want to get an adjustable voltage regulator from Euromotoelectrics.com before that component causes more grief. Compared to the mechanical, the electrical systems are disappointing on BMW airheads IMO. Good luck!
    Stan

    AH# 13238

  11. #26
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    All good comments and suggestions gentlemen. Disconnecting the battery is a must before removing the front engine cover. Getting the cover off, past the horn, almost inevitably means it's going to touch some part of the diode board.

    The second problem with durability is the location of the board - subject to a fair amount of heat and vibration.

    For those of you looking for an alternative to the OEM diode board - here's a nice looking option:
    http://www.ascycles.com/detail.aspx?ID=2107

    N.

  12. #27
    R100GS, '89 Guenther's Avatar
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    Reading through the threads again I realized that the 13V was measured between a DISCONNECTED batt- and some frame/chasis point. So no mystery here.

    Diodes typically when "fried" have no conductivity left in both directions.

    /Guenther

  13. #28
    MonoRT MonoRT's Avatar
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    Sorry ÔÇô I should have known better than to bring up those stories about front covers and fried diode boards. IÔÇÖve never had a problem with that either and IÔÇÖm a convicted klutz. At any rate, when you look at a Bosch style diode board, it appears that the only ÔÇ£hotÔÇØ part available for shorting when the bike is off is the aluminum heat sink/bus that ties the positive power diodes to the B+ terminal. If you manage to short that heat sink to ground with the cover for an instant, you should get sparks, but nothing much else ÔÇô the current is flowing from the heat sink, through the cover and to the case ÔÇô no current should be flowing through any diodes ÔÇô seems like youÔÇÖd have to maintain the short long enough to get things pretty hot before you could do any lasting damage.

    IÔÇÖm still wondering about the original problem. The bike slowly drains its battery when it is not being run ÔÇô presumably a short somewhere is bleeding the battery down. The three small diodes that supply positive power to the D+ circuit were identified as bad. However, when the bike is off, barring any other problems, the D+ circuit is isolated and should not provide a path between battery positive and ground. Even if the three D+ diodes are damaged so that they will allow reverse current flow, they should be isolated from B+, so they should not be able to provide a path between B+ and ground. YouÔÇÖd expect poor charging from bad D+ diodes, not battery drain in the off state. If the diode board is providing a path between B+ and ground when the bike is off, IÔÇÖd think that one or more of the plus power diodes is allowing current to leak through. Or, that the board or the lower mounts are structurally compromised.

    If the B+ lead is disconnected, does the power drain stop?

    For the record, IÔÇÖm a software guy, not an electrical guy, so IÔÇÖm just throwing out ideas that some of the guys with training and experience might want to comment on.

  14. #29
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    Quote Originally Posted by MonoRT View Post
    For the record, IÔÇÖm a software guy, not an electrical guy, so IÔÇÖm just throwing out ideas that some of the guys with training and experience might want to comment on.
    Oh, but I think you earned your wings with your troubleshooting here a while back! You definitely have a ground-up understanding of the system...people should listen.
    Kurt -- Forum Administrator ---> 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!

  15. #30
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    Quote Originally Posted by MonoRT View Post
    IÔÇÖm still wondering about the original problem. The bike slowly drains its battery when it is not being run ÔÇô presumably a short somewhere is bleeding the battery down. The three small diodes that supply positive power to the D+ circuit were identified as bad. However, when the bike is off, barring any other problems, the D+ circuit is isolated and should not provide a path between battery positive and ground. Even if the three D+ diodes are damaged so that they will allow reverse current flow, they should be isolated from B+, so they should not be able to provide a path between B+ and ground. YouÔÇÖd expect poor charging from bad D+ diodes, not battery drain in the off state. If the diode board is providing a path between B+ and ground when the bike is off, IÔÇÖd think that one or more of the plus power diodes is allowing current to leak through. Or, that the board or the lower mounts are structurally compromised.
    I agree the problem is a mystery. Bad diodes should cause the charging light to stay on, which should be easily noticed. The basic problem could be in the wiring.
    Stan

    AH# 13238

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