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Thread: '73 R75/5 LWB won't start, battery fine.

  1. #1

    '73 R75/5 LWB won't start, battery fine.

    Think it might be starter relay. How to bypass relay and use solenoid directly?
    If I'm persistent I can sometimes start up with kick pedal.
    Thanks for any help,
    Tom Carter

  2. #2
    Liaison 20774's Avatar
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    Tom -

    Welcome to the forum! Can you give some background? Did this start recently or has it been a long time problem? And what is happening...any sounds or are things deader than a doornail? Not sure I remember correctly...no kill switch on the /5, right?


    Snowbum has a couple of articles on the starter:

    http://bmwmotorcycletech.info/boschvaleostarter.htm

    http://bmwmotorcycletech.info/startingprobs.htm

    Be sure all the simple things are checked. Tight connections on the battery terminals. Good clean connection of the battery negative cable to the hollow bolt at the speedo boot...don't over tighten. The battery is fine...what have you done to confirm that? Have you put a digital meter across the battery terminals as you tried to crank the starter? Should move some but not go below 10-10.5v...any lower means a bad cell.

    Maybe your starter relay has corrosion on the terminals. Unplugging and replugging the relay a few times might clean them off, allowing the bike to start.

    Try the easy stuff and let us know how you make out!
    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
    Keep in mind that relays CAN be dis-assembled and cleaned. They are not supposed to be, but they can - I've done it many times.

    You just have to carefully pry apart the cover from the frame - make SURE that you don't let a thing like a screwdriver blade accidentally gouge into the insides and break the really fine wires that run from the inside contacts to the windings. All the points need to be cleaned with contact cleaner, and I usually also use wet-or-dry 400 paper to bring back the surfaces and then clean again. Clean out all the debris inside, corrosion etc. Then re-assemble (I like to put RTV sealer on the cover-to-base surfaces) - sometimes you have to re-bend the cover edges or I simply use cable ties to hold together.

    Also clean all the prongs on the outside of the relay so they make good contact when plugged into base.
    "The difference between death and taxes is that death doesn't change every time congress meets." - Will Rogers

  4. #4
    Registered User AntonLargiader's Avatar
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    Hi Tom,

    If you put power to the thin black wire on the starter solenoid (or the terminal that it connects to), you'll activate the starter.

    The /5 starter relay is a unique thing. I might have a new one at the shop... call me tomorrow afternoon. Pretty sure I have a /6 one, not sure about /5.
    Anton Largiader 72724
    Tech articles - YouTube
    Virginia Motorrad Werkstatt BMW motorcycle service and repair in central Virginia

  5. #5
    Airmarshal-IL James.A's Avatar
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    Like Anton and the others said, the starter relay on most or all airheads is the gate keeper for electrical potential to all of the bike's systems. I'd recommend reading snowbum's article on /5 starter relays. There is often a term, "cricket sound", associated with it. I have done snowbum's modification on my bike, and I am sure I have an un-modified relay around here somewhere. SCbuckeye's recommendation to open it up and clean it out is good advice also, but when I opened mine to do the work, it was pristine inside.

    The fact that you can kick start the motor tells me that the trouble lies with the starter solenoid or the starter motor itself.

    Lastly, I strongly recommend that you join the Airheads Beemer Club if you are not already a member, and when posting here, a general geographic location is helpful in locating hands on help.
    Best of luck, James A...
    1973 R75/5

  6. #6
    Registered User lmo1131's Avatar
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    I bought a "used" /5 starter relay on eBay with the intention of modifying it. When I got it the can had already been opened and the modification done ... it's worked fine for the past five years, and in fact works better than the original worked from day-one.

    The "cricket" sound mentioned (if you are knew to old airheads) is the sound of the starter solenoid plunger very quickly cycling in and out (much like the sound the starter makes when the battery is too low). It's an inherent flaw in the relay design.

    Here's the "cricket" fix write up > http://bmwmotorcycletech.info/slash5cricket.htm

    Getting the relay "can" open isn't a pretty ordeal, you have to pry the edge of the metal housing away from the fiber base (below).

    "It is what you discover, after you know it all, that counts." _ John Wooden
    Lew Morris
    1973 R75/5 - original owner
    1963 Dnepr

  7. #7

    Thanks to all of you who replied to my query about /5 starting issue

    Turned out be the starter relay, and with the kind guidance and patience of Anton Largadier, our local BMW wizard/guru in Charlottesville, I now have a new relay - not the official /5 one (too $$!) but a generic one. I'm anxious to install it but need advice on pinouts. I have the diagram of the old one, but the new one has pins labelled 30, 87a, 87, 85, and 86. I know the red wires go to 30, but not sure about the others. Suggestions? Anton said I will lose the /5 restart protection and that's ok. Thanks for any help!
    Tom Carter

  8. #8
    Liaison 20774's Avatar
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    The original relay was wired:

    - #15 -- green/black
    - #30 -- red
    - #31b -- brown/black
    - #87 -- black
    - #D -- blue

    What's the generic relay you are using? Snowbum has a section on this page about using generic relays:

    http://bmwmotorcycletech.info/slash5cricket.htm
    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!

  9. #9
    Registered User lmo1131's Avatar
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    Anton said I will lose the /5 restart protection and that's ok.
    It WILL be okay until you strap on a tank bag... just a heads up. The switch on my /5 intersects my tank bag perfectly.
    "It is what you discover, after you know it all, that counts." _ John Wooden
    Lew Morris
    1973 R75/5 - original owner
    1963 Dnepr

  10. #10

    Generic starter relay

    Quote Originally Posted by tom44 View Post
    Turned out be the starter relay, and with the kind guidance and patience of Anton Largadier, our local BMW wizard/guru in Charlottesville, I now have a new relay - not the official /5 one (too $$!) but a generic one. I'm anxious to install it but need advice on pinouts. I have the diagram of the old one, but the new one has pins labelled 30, 87a, 87, 85, and 86. I know the red wires go to 30, but not sure about the others. Suggestions? Anton said I will lose the /5 restart protection and that's ok. Thanks for any help!
    Tom Carter
    All it says on relay is the ubiquitous "China". I'll check out the link to Snowbum...many thanks!

  11. #11

    Would like to send your jpeg but can't paste it in reply!

    Quote Originally Posted by tom44 View Post
    All it says on relay is the ubiquitous "China". I'll check out the link to Snowbum...many thanks!
    So how do I paste a simple JPEG2000 pic to show you the pins on the generic relay I have?

  12. #12
    Liaison 20774's Avatar
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    Quote Originally Posted by tom44 View Post
    So how do I paste a simple JPEG2000 pic to show you the pins on the generic relay I have?
    One method is given in post #13 here:

    https://forums.bmwmoa.org/showthread...-the-MOA-Forum

    Another way is to use the small picture icon given in the post tools above the message area.

    PictureIcon.jpg
    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!

  13. #13

    Generic relay r75/5

    [ATTACH]68036
    Thank you. Now, how do these pins relate to the stock relay pins & wire colors?
    Attached Images Attached Images

  14. #14
    Liaison 20774's Avatar
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    This is where having the right relay helps...no thinking involved!!

    The relay consists of two sides - the contacts and the relay. In your cast, I would think that the contacts are #30 and 87. When these contacts are made, power flows through to the starter...this should be the heavy current the starter needs. The relay would then be contacts #85 and 86. The relay (internally it's a small coil) is powered by nominal 12v when you push the button to engage the starter...not high current, just enough voltage/current to cause the relay part to pull the contacts closed. From the looks of your relay, I would say that it's a normally open relay...meaning to me that the contacts remain open under normal conditions which is at rest.

    Using this information and what is on Snowbum's link given above, I think you can figure out which colored wires to attached to the new terminals.
    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!

  15. #15

    Thank you!

    I will "have at it" using your suggestions.

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