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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
    Join Date
    Sep 2005
    San Antonio, TX
    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 kill switch on the /5, right?

    Snowbum has a couple of articles on the starter:

    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
    Join Date
    Jul 2004
    Charlottesville, VA
    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
    Join Date
    Sep 2013
    central Illinois
    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
    Join Date
    Jan 2010
    Pismo Beach, CA
    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 >

    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

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