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Thread: Diode board grounding on airheads - '78 R100/7

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    Diode board grounding on airheads - '78 R100/7

    Though I am not having a problem, I have heard that there is a grounding problem with the diode board and some have avoided it by going to solid metal mounts rather than the rubberized mounts that were on originally. Is that true?

    Also, while looking through my system up under the front cover, I noticed an unused threaded hole in the engine block just under the diode board. Would there be a benefit of running a ground strap from there to one of the 4 mounts for the diode board to make sure that there would never be a ground problem?

    Or...is this unnecessary?

  2. #2
    Administrator 20774's Avatar
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    I think it is necessary. I bought solid mounts and a grounding harness from Motorrad Elektrik many years ago...can't say as it made a dramatic change maybe because I didn't have any real issues...couldn't hurt though. As a side note, one thing that should be checked is the 3-wire cable going from the alternator to the back of the diode board. I noticed the diode board side of that connector had quite a bit of corrosion on it. It's tucked behind the board and the OEM cable is a shroud-type end which traps moisture. Also from Motorrad Elektrik, I bought three separate cables...those good in wiring can probably easily make their own.

    Snowbum has a good discuss about ground the diode board:

    http://bmwmotorcycletech.info/diodebds&grdgwires.htm
    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!

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    Thanks. I will call Mororad today!

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    Registered User lkchris's Avatar
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    At one point BMW produced a service bulletin and a retrofit wiring harness to improve grounding for R100 diode boards.

    R100s stretch the design beyond its original parameters and they run with slightly more vibration than the smaller engines and the designers determined the boards should be rubber mounted on these models (only). I'd think the solution to grounding problems is not to eliminate the rubber mounting but rather to add the grounding harness.

    Nonengineers seldom have the solution when engineers make mistakes.
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    Administrator 20774's Avatar
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    I don't think the vibration was that big of a deal...after all, these are smooth running BMWs, right? From what I've read, it was more of a heat issue and going to solid mounts will help pull heat away from the diolde board. Some of the period boards, Wehrle, had poor soldering which was futher damaged by the extra heat. Plus the additional ground can't hurt. My 0.02.
    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!

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    Registered User godfather's Avatar
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    Something to add to my list....thanks
    Attitude is everything!
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    Sir Darby Darryl Cainey's Avatar
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    I run extra grounds as well as these solid mounts.
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    Registered User godfather's Avatar
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    Quote Originally Posted by Darryl Cainey View Post
    I run extra grounds as well as these solid mounts.
    I just checked the Motorrad Elektrik web site and they state


    These all-metal replacements for the original type of rubber diode board mount will absolutely insure that the rubber mounts can never de-bond from the fasteners and allow the board to be damaged by contacting the engine case. This usually ruins the diode board instantly. Proper grounding of the diode board is assured, with no need for the typically inadequate external wire grounds. Poor grounding is a chronic reason diode boards may have reduced efficiency and short life from overheating.

    So...I'm confused
    Attitude is everything!
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    Registered User krpntr's Avatar
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    The rubber mounts break.

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    Quote Originally Posted by lkchris View Post
    At one point BMW produced a service bulletin and a retrofit wiring harness to improve grounding for R100 diode boards.

    R100s stretch the design beyond its original parameters and they run with slightly more vibration than the smaller engines and the designers determined the boards should be rubber mounted on these models (only). I'd think the solution to grounding problems is not to eliminate the rubber mounting but rather to add the grounding harness.

    Nonengineers seldom have the solution when engineers make mistakes.
    I think per Kurt's direction above, that there were several service bulletins with errors addressing this problem from the engineers at BMW. Is it possible that they could have made more than one mistake on this ONE issue? Go figure!

    Apparently the solid "non-engineered" mounts have worked without fail.

    Has anyone had a failure due to the solid mounts? I haven't heard of one. Perhaps engineers aren't gods.

  11. #11
    Administrator 20774's Avatar
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    I think the only "failure" you'll have with the solid mounts is getting the backside nuts tight enough. IIRC, the upper left one (as you face the rear of the bike) is a bit difficult to get to. I had to bend a spare 8mm open end wrench to get to the nut. Getting it started is another story...I've heard people using superglue on their fingers to hold the nut until it can be started. It's important to not let these little pieces fall down into the case somewhere...plug all holes at the bottom with rags just for that purpose.

    You can never have too much grounding. I say do it...it's hidden and can't really detract from the setup.
    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!

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    Quote Originally Posted by 20774 View Post
    I think the only "failure" you'll have with the solid mounts is getting the backside nuts tight enough. IIRC, the upper left one (as you face the rear of the bike) is a bit difficult to get to. I had to bend a spare 8mm open end wrench to get to the nut. Getting it started is another story...I've heard people using superglue on their fingers to hold the nut until it can be started. It's important to not let these little pieces fall down into the case somewhere...plug all holes at the bottom with rags just for that purpose.

    You can never have too much grounding. I say do it...it's hidden and can't really detract from the setup.
    About 30 years ago, I purchased one of those "finger" things that has a set of 4 "fingers" running through a cable outer and a slide button on the end. When you push it with your thumb, the end pokes out the 4 fingers and they can grab little things. I found one along the road later. I use them often holding nuts, retrieving things I've dropped, etc. Can't live without them!

  13. #13
    Day Dreaming ... happy wanderer's Avatar
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    Quote Originally Posted by lkchris View Post
    Nonengineers seldom have the solution when engineers make mistakes.
    That's a hilarious (and pretty judgmental) remark. Well, to me anyway. My team and I came up with solutions to engineering mistakes every day for many years. In field service and tech support that is what we did for a living. And it was anything but seldom believe me!

    I had an R75/5 diode board fail on me. Diode to board solder joints failed due to vibration. I remain unconvinced the rubber mounts would solve this problem because that board was rubber mounted and it failed anyway. Using new diodes I left a longer lead length in the holes and built up a nice solder cone for better support. Re-soldering the joints this way after a thorough cleaning of the board I never had another problem.
    MJM - BeeCeeBeemers Motorcycle Club Vancouver B.C.
    '81 R80G/S, '82 R100RS, '00 R1100RT

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    Quote Originally Posted by Krpntr View Post
    The rubber mounts break.
    They sure do. I checked mine a couple weeks ago and three of the four had come apart...the board end screw had detached from the rubber. It didn't appear to have moved any, luckily. It nows has nice new solid mounts. It was a lot easier job than I anticipated. Removing the tank & starter cover gave pretty easy access to the rear nuts. A magnet is nice to fish the nuts & washers out when (not if) you drop them. I even found a couple extras left behind from a previous repair job (or a BMW assy line worker). Haven't noticed any changes but it's one less thing to worry about.
    1983 R100RS (Sold)
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    Quote Originally Posted by 20774 View Post
    I think it is necessary. I bought solid mounts and a grounding harness from Motorrad Elektrik many years ago...
    BTW, When I called (and ordered solid mounts) Motorad, he said that he didn't have a grounding harness.

    I am simply thinking of making my own and running from underneath the board off one of the studs of the hard spacers, and then down to the timing cover case where there is a convenient threaded hole already. Soldering the connectors, of course.

    I also don't think you can have enough grounds!
    Last edited by jimmylee; 10-03-2013 at 01:50 PM.

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