How does a /5 starter button work?

Discussion in 'Airheads' started by deafanddiabetic, Feb 27, 2018.

  1. deafanddiabetic

    deafanddiabetic Would rather be wrenching.

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    My /5 starter button stopped working this summer, I just kicked it for months but it sucks to do after awhile.

    I took the switch apart this last week, cleaned all the contacts, resoldered some frayed ends, etc. I know the starter works great because I can simply ground the wire from the headlight bucket and it fires right up. So the problem is the switch or the wire from the switch.

    But I'm wondering how that button works, anyway? I couldn't determine how pushing it in makes any kind of contact happen. Is it supposed to push in "deep" enough to touch the mount screw on the handlebar switch assembly and that is how it finds its ground?

    My button still doesn't work and I wanna make that magic happen again.
    #1
  2. PaulBarton

    PaulBarton Long timer Supporter

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    have you checked the starter relay and all connections to/from? Under cold starting and low voltage situations the relay will make a "cricket" sound and fail to engage the starter. If you are simply getting nada when you push the button it could be corrosion at the relay contacts or something in the wiring between the starter button/relay/headlight bucket switchplate.

    To address your actual question, you should have a plastic switch in a metal housing. I believe the set screws in the metal housing help ground the switch through the handlebar. That is the case with the horn switch. I will try and dig up the wiring diagram for reference.
    #2
  3. Big Bamboo

    Big Bamboo Aircooled & Sunbaked Supporter

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    New ones are still available from Max for $91
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  4. PaulBarton

    PaulBarton Long timer Supporter

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    Here are a couple of different versions of the /5 wiring diagram. The first one shows the switch grounded to the frame via a wire (brown) through the switchplate in the headlight bucket.

    https://www.dropbox.com/s/5sljaxopi46hugn/72 BMW r75 fused.jpeg?dl=0

    https://www.dropbox.com/s/u1t42390gr1rsxe/bmw r75slash5 Wiring diagram.docx?dl=0

    Here is a pic of the bottom of the starter relay (located under the tank mounted to the spine).

    https://www.dropbox.com/s/r0uwhej8e2vc3bv/2216-new-old-starter-relay.jpg?dl=0
    #4
  5. Warin

    Warin Retired

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    Umm which wire did you ground? Arr the brown/black ? Ground that and having the starter work would indicate that the starter system works fine - so no need to check relays etc.

    There will be a single brown wire coming from the starter button - that should be ground. I'd check that in the headlight bucket.

    Then check the wires (both brown and brown/black) have not been cut between button and headlight.

    Then your left with the button .. should connect the brown and brown/black wires together.
    #5
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  6. Pokie

    Pokie Just plain Pokie.

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    It's been a while but if I'm remembering right,.... The most common place that trouble starts is the mounting of the switch itself. If the switch has been crushed on the side from the screws being forced in and the holes not lined up, the ground tab on the side of the switch will no longer ground. The switch will have a brown with black stripe wire running up from the headlight. When the button is pressed, the contact with the brown with black stripe wire will contact the brass tab on the side of the switch. The brass tab "should" contact the screw on the side of the switch mount to complete the circuit. The wires and tab may seem small, that's because all they do is switch the starter relay, the starter relay switches the big power.
    #6
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  7. cycleman2

    cycleman2 Long timer

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    The starter relays are a problem area in these old bikes. Make sure all the contacts are good. You should be able to check the continuity of the wire from the switch to the relay at the relay. ie: when you push the button it closes the circuit.
    #7
  8. bmwrench

    bmwrench Long timer

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    That's my memory, too.
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  9. deafanddiabetic

    deafanddiabetic Would rather be wrenching.

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    I appreciate all the replies, maybe I should've provided more information.

    Anyway, my starter system is fine. Relay is fine, it's aftermarket from Rick. My entire system is a standard 74 /5 wiring system, every component and wire is the same. I can put a jumper cable from the brown black in the headlight bucket (that goes to the starter relay) and put it to any bolt and the starter spins and the bike starts right up.

    Basically, the push button worked for like a year, then slowly got more finicky, now it doesn't spin the starter anymore.

    When I took the switch apart, there was that three nippled copper ring and it was pretty corroded, I cleaned it nicely, cleaned all five wire contact points, resoldered the ends of two of them that were very frayed and put some electro grease on there and put it all back together. I thought that would be enough to work again.

    If I have to buy another switch I can, but at this point I'm almost more apt to make a simple push button mechanism that'll ground a wire against my handlebars or the frame to use for the starter. I have no starter protection circuit because they don't make an aftermarket that functions in that same way (that I know of), but I guess I coudln't figure out HOW the brown/black touches anything grounded within that whole plastic piece.
    #9
  10. fxray

    fxray Long timer

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    d&d, Exactly what bike do you have? You said it is a "standard 74 /5 wiring system". Do you mean that you have a 1974, which would be a /6 model, with the carry-over /5 handlebar switches? If so, then that is like my 1974 R90/6 which has the /5 style controls. I can tell you how mine works.

    With the ignition switch key turned "on", there is battery power to the starter relay. Pressing the starter button provides a ground path for the other side of the starter relay. This completes the circuit inside the starter relay, energizing its internal, electromagnetic coil. This creates a magnetic field and pulls in the contacts to power the starter solenoid.

    When you said, "I can put a jumper cable from the brown black in the headlight bucket (that goes to the starter relay) and put it to any bolt and the starter spins and the bike starts right up," what you are doing is providing the ground path for your starter relay that is normally provided by pressing the starter switch button.

    Some other things enter in, these being the clutch switch (found on the left handlebar perch), and the neutral switch, found under the transmission.

    Pressing the starter switch button completes an electrical path from the ground side of the starter relay, through the now closed starter button switch, and onward to the clutch switch. If the clutch switch is open (stuck, corroded inside, or simply the clutch lever is not pulled), that is the end of the electrical path. Assuming the clutch switch is in good condition, when you pull the clutch lever you are allowing a little spring loaded plunger on the clutch switch to extend and turn on (close) the switch.

    So now, let's assume that you are squeezing the clutch lever and, with that lever held in, you press the starter button. You will now be providing an electrical path from the ground side of the starter relay, through the now closed (because you are pressing it) starter button switch, through the now closed (because you are squeezing the lever) clutch switch, onward from the other pole of the clutch switch via a brown wire to ground.

    But I mentioned the neutral switch as well. That comes into play because the same wire that runs from the starter button switch to the clutch switch is also connected (through a diode) to one pole of the neutral switch on the transmission. The other pole of the neutral switch is connected via a brown wire to ground.

    So, let's assume that you are squeezing the clutch lever and, with that lever held in, you press the starter button as before. So long as your transmission is in neutral (and you have a properly functioning neutral switch), your clutch switch could be a frozen lump of crap and you would still be providing a ground path for your starter relay. It would be the complete path from the relay, through the starter button switch, through the diode, through the neutral switch, and through the brown wire to ground. Ground in this sense of the word is simply the frame of the motorcycle, which, in turn, is connected to the battery's negative post.

    You know from running a jumper wire to provide a ground path for your starter relay that the supply side of the circuit is working. Your relay, your solenoid, your Bendix, your starter motor all work just fine. The fault is somewhere in the ground path that is supposed to be provided by some combination of the starter button switch, along with either the clutch switch or the neutral switch, or both.

    On my R90/6 with the /5 handlebar switches, I can test my starter button and its related wires by locating the little harness from the RH handlebar switch where it comes into the headlight bucket. In that harness, there are seven wires. Among these, the brown/yellow stripe wire, and the blue/yellow stripe wire are tied to the starter button. With these two wires unhooked inside the bucket, I can use a continuity tester of some sort. When the starter button is pressed, these two wires should have continuity. When the button is not pressed, they should not have continuity. Any other condition indicates a wire that is open or shorted, or a defective push button switch.

    I can also test my clutch switch and its related harness by locating the little harness from the LH handlebar switch where it comes into the headlight bucket. In that harness, there will also be seven wires. The two wires of interest are blue/yellow stripe, and brown. With the clutch lever released (not squeezed), there should be no continuity between these wires. With the clutch lever squeezed, there should be continuity between these wires. Any other condition indicates a wire that is open or shorted, or a defective clutch switch.

    If your neutral switch works, then when the transmission is in the neutral position, and the blue/yellow stripe wire is unhooked from the circuit board inside the headlight bucket, you should find continuity from the blue/yellow stripe spade of the circuit board to ground (a brown wire inside the bucket).

    Do you pull in your clutch lever when you try to start the bike? If not, it is a good practice. Try it and see if that makes a difference.

    On my R90/6, I can verify all of this by checking these conditions:
    • With the bike in gear and the clutch released (not squeezed), pressing the starter button does nothing.
    • With the bike in gear and the clutch lever squeezed (disengaged), pressing the starter button engages the starter and fires up the engine.
    • With the bike in neutral, pressing the starter button engages the starter and fires up the engine regardless of whether the clutch lever is squeezed or released. (It is better practice and less load on the starter motor to always squeeze the clutch when starting, but the bike will start regardless, so long as the transmission is in neutral.)

    To summarize, if you have a 1974 /6 bike, with /5 style switches, there are other possible causes for a non-start condition, aside from a defective switch button on the RH handlebar. If you truly have a /5 bike that follows the diagram posted above by PaulBarton, then diregard all of what I said here.

    If there are errors in anything I have written here, you must hold me harmless. I wrote this lengthy post free gratis, and you must remember that you often get what you pay for.

    Ray
    #10
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  11. Carott

    Carott Adventurer

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    fxray has written an excellent diagnostic for the 1974 bikes.

    One additional item to check is the diode on the headlight circuit board.

    On my 1974 R90/6 (with the /5 handlebar switches) the diode had failed open (no continuity in either direction).

    This caused a situation where the starter button would only function if the clutch lever was pulled in. Putting the bike in neutral with the neutral light on, the starter button did nothing.

    Rather than remove all the headlight wiring to access the back of the circuit board I wired a replacement diode on the front using spade connectors.

    Snowbum has all the information on his site. (Sunglasses and a fresh pot of coffee recommended)

    Brett
    #11
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  12. fxray

    fxray Long timer

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    Good point, Carott, and that was a brilliant repair that you made.

    I suspect that, since deafanddiabetic mentioned earlier that he could short his black/brown wire to ground and make things work, he really does have a /5, so all that tripe I wrote is nonapplicable. Oh well, it made me stop and remember how the '74 /6 works anyway. :D
    #12
  13. deafanddiabetic

    deafanddiabetic Would rather be wrenching.

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    fxray, I appreciate the information, I actually was not aware that the neutral switch worked alongside or in sync with the starter button. I might take your paragraph and put it in my general airhead notes as I always run into something useful when putting junkers back together.

    I built a slash 2 conversion. I used stock /5 wiring harness, /5 switch gears, /5 tail light switch, and a /2 key ignition assembly which for all intents and purposes is a /5 assembly, there is no clutch switch.

    My starter relay is an aftermarket from Motorrad Elektrik, my reg/rec also. My wiring is stock according to an unfused 74' /5 wiring diagram, only I added a 7amp fuse on my green 30 wire coming off the ignition board and a 30amp coming straight off the battery to red 15 wire.

    Regardless, according to the info I've received in here, it's my switch not making ground contact.

    I have one unused hole in my headlight bucket that I put a blue LED in that was supposed to be my turn signal indicator, but since I have handlebar turn signals, there's no need to have a warning light for them. I'll use that hole and put a $4 momentary push button in place of my non-functioning handlebar button.
    #13
  14. fxray

    fxray Long timer

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    That oughta work, and it might make the bike harder to steal too. Of course, now all of us know where the starter button is. :augie

    It sounds like you have a pretty cool machine.
    #14
  15. heiko's garage

    heiko's garage n00b

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    I am so glad that I am not the only one who as a 74 /6. I just bought it as a basket case and I am now trying to make sense of all the wiring. It is completely apart and I can't find a wiring diagram that matches what I see on the bike. non of the diagrams I have seen have the brake light switch on the master cylinder listed.
    I appreciate any input.
    #15
  16. WRC51

    WRC51 Long timer

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    Very informative thread, I learn something everyday. If Iam reading and understanding fxrays post should the clutch switch keep the starter from engaging if the engine is running and the clutch lever is not pulled in? My bike (76 R90/6) does not have the clutch switch and I have hit the starter button a few times when trying to switch the turn signals, Thanks.
    #16
  17. Cookiedad

    Cookiedad Adventurer

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    I have had that starter/TS switch apart on 2 /5 bikes. Yes there is a 3-sided contact that works the turn signals that must be clean & bright. There are also 2 small contacts on the bottom of the push button. One is connected to the brown/black wire, and the other is connected to the small tab at the side that is touched by one of the mounting screws (this is from memory and it may connect to a brown wire). Those contacts touch (contact) a small washer that sits on a foam-like cushion under the washer. When starter button is pushed, the contacts touch the washer making connection between the brown and brown/black wires. On my bikes the cushion was old and disintegrating not allowing the washer to be in position to contact the 2 contacts on the button. I was able to fashion some foam to hold the washer in position so that the contacts touched it and completed the circuit (starting the bike). I also cleaned the contacts and the washer. It may just be easier to just buy a new switch, but I am retired former Navy electronics technician and my time is free. I also like a challenge.
    #17
  18. Warin

    Warin Retired

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    Really old bikes had no safety things... if you pushed the starter button the starter would try to turn, even if the clutch was not pulled in and a gear was engaged.. some times resulting in the bike charging of the stand into the distance. :wave

    For safety:
    An interlock with the neutral switch could be made that resulted in the stater motor only working when the gear box is in neutral

    An interlock could be made with the clutch (usually made at the clutch perch similar to a front brake light switch) so then the starter could be used if the clutch was pulled in (no matter what gear).

    If both the above were installed then you have one of the more modern setups. None of these interlocks sensed if the motor is running or not, that is still left up to the 'operator' (at least for the present).
    #18
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  19. spo123

    spo123 Man About Town

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    ONLY the later /5 BMW motorcycles (up to 750cc) had fuses. ONLY the last 1/2 year of the 1973 /5 BMW motorcycles (up to 750cc) were of the Long Wheel Base variety. There were no /5 BMW motorcycles manufactured in the model year 1974......The BMW /6 (up to 900cc) always had FUSES. The BMW /6 began to be manufactured in model year 1974.
    Please do be aware that a BMW model year begins in September, fyi
    #19
  20. WRC51

    WRC51 Long timer

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    Great info on this subject...but I am not reading/understanding the reply's correctly sorry. So lets say on a stock 1976 R90/6 if a person was riding the bike and went to make a lane change and hit the starter button by mistake would the clutch switch prevent the starter from engaging? I have done this a few times my clutch switch was removed by the PO and I know it cant be good for anything in the starter system, I thought about trying to move the blinker switch over to the left control perch but would rather replace the clutch switch if that would prevent the carnage to flywheel and starter Bendix, Thanks and please use small words as I am Old..:ear
    #20
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