no spark on r75/5

Discussion in 'Airheads' started by chipsdad, May 6, 2012.

  1. chipsdad

    chipsdad Adventurer

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    I bought the bike with no battery a while ago and have been working on it since. I repaired all of the fried wiring as it should be. not only my motor is /6. my chassis is a /5. here are some vitals that you may ask in order to help me get spark.

    -tested from coil post to ground 12v
    -bike cranks over nicely
    -no spark between points
    - neutral light goes off and on properly

    I am thinking it is something improperly wired but I dont know what to check. I have cleaned all visible terminations and greased connections.,
    #1
  2. disston

    disston ShadeTreeExpert

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    So I'm thinking the wiring should also be /5? You say you have repaired wires already, did you use a wiring diagram or just replace one wire at a time?

    You are getting 12 volts to the coil. Is the black wire, on the right coil, going back to the condenser?

    It is often a perplexing problem to fix ignition when the short wire between the coils has a break.

    Points are new? Did you also replace the condenser with a known or new part?
    #2
  3. Wirespokes

    Wirespokes Beemerholics Anonymous

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    With the ignition on, spark plugs out but plugged into the plug wires and grounded to the head, touch the negative coil lead to ground. Let go. Do this quickly several times and you should see sparking at the plugs.

    This will simulate the action of the points.

    If there is sparking, it will tell you the points need attention. If not, there's something wrong with the coils.
    #3
  4. chipsdad

    chipsdad Adventurer

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    the negative coil lead?? that is the one that bridges the two coils or the other post. if it is the other post it will only fire the one plug correct?
    #4
  5. disston

    disston ShadeTreeExpert

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    The coil contact that has a black wire going to it. It's on the right side of the bike. The left side wire is hot when ignition is on. That wire is Blue/Green, I think.

    Leave the short wire between the coils alone except to check it for continuity. It is amazing that this short wire can be broken inside the insulation or have badly fitting crimp terminals and be the source of many a mysterious ignition problem. Common problem actually.
    #5
  6. chipsdad

    chipsdad Adventurer

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    My black post and both the cross over posts have 12 volts. Does this indicate a bad coil
    #6
  7. Plaka

    Plaka Brevis illi vita est

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    What the hell is a black post??

    Guy, I don't mean to be harshing on ya but if you don't know how to test a coil then you don't know how that system works. You're quite capable or you wouldn't have torn into the project. But hows about doing some homework? I don't see this as the place to spoonfeed someone Ignition 101. I'm sure not gonna, too many other people have written it out already---and you haven't read it.

    Mottorad Electric sells a booklet for cheap that takes you through the thing. Snowbums site has everything you want to know (tho' he is long winded, convoluted and visually annoying---a different price to pay for the info)

    The whole system has 5 components and some wire. Understanding what each does and how they work together is not difficult. Perhaps an hours worth of reading. It would take me two to write it out.

    Once you understand how it works you will also understand how to test each part. From there you know what is and isn't bad.

    if you come to some conclusions and want someone to check your work before you spend money, a forum can be valuable. If you get stuck it can also be valuable. You post up and say in detail what checks you have done and what the results are. You should have covered the bases. Then someone can advise you on some fine point you might have missed. But you probably won't need much if any help. It's really that straightforward a system.

    When, not if, it takes a crap on the road, you will know what to do to get going again.

    Or you could go the take-it-to-a-mechanic and always carry AAA route. Many if not most people do.

    But if you are going to roll your own, RTFM, do like it says, then ask questions.

    Cheers, and good luck with it.
    #7
  8. hwy61

    hwy61 Been here awhile

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    Gee Plaka, There are lots of places to look up what a black post is...
    #8
  9. disston

    disston ShadeTreeExpert

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    You are going to have to have a wiring diagram. They are in all the manuals that we constantly tell new riders they have to have. There are some wiring diagrams on the Web but you need the correct one. It does not help when we ask pertinent questions and get no answer because you don't think it's important. If you don't know what you are doing you also don't know what is important.

    Check out Snowbum's Web site. He has some of the wiring diagram's. His site can be not easy to navigate but the diagrams are there.

    http://bmwmotorcycletech.info/

    You have a /5 bike. I suspect you have /5 wiring irregardless of the /6 engine. Fortunately for the ignition the wiring is the same for /5 and /6 anyway.

    Here are some basics. You must have a battery large enough to crank the engine and run the ignition. Too small a battery does not work in this situation where you are trying to find a problem.

    When the key is on the left side coil has battery voltage, 12.5 volts minimum, at the positive post. The wire here may be a blue/green wire.

    The left side coil negative terminal has a short wire to the right side coil positive terminal. This short wire can be taken off and checked for continuity. It's resistance is not important but it should be very low. Bend the wire to see if it stays contingent. i.e. It doesn't loose connection. Clean these terminals and replace short wire.

    There should be a black wire attached to the right side negative terminal. Follow this wire to where it connects to the condenser. You can disconnect this wire at both ends and using a meter test to see it is the same wire from the right side coil to the condenser.

    Finally the ignition points are also connected to the condenser.

    If all of this wiring is in place and you get power to the left coil and the function of the points seems correct then it is time to spend some money.

    Buy a new set of ignition points and a new condenser.

    Instructions on installing and adjusting the ignition points are in the manual. You may need some further explanation but try to get the jest of the matter from the book. Maybe you have done this before? I mean ignition points? All cars and motorcycles used to have these. They started disappearing in cars in the mid '70's. They were gone from Airheads in 1981.
    #9
  10. disston

    disston ShadeTreeExpert

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    While you are at it measure the resistance of the plug wires. You need good plug wires and new spark plugs to get mystery bikes running.
    #10
  11. Plaka

    Plaka Brevis illi vita est

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    Yeah, it's the one where you always get smoke leakage. See, all the components are packed with a certain amount of smoke from the factory. If any of it leaks out you're in big trouble...
    #11
  12. Plaka

    Plaka Brevis illi vita est

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    New points are good to have even if they are not a problem. But go to an outo parts store and get a BIG Mallory condensor. Things last forever (100k + Miles) and you never have to worry about or replace the condenser again.

    Knowing how to work with the points is important to not frying them. So you want all that info before beginning. Slapping in some old fried points and filing them a bit is not a bad idea. You can still buy points files I imagine.
    #12
  13. disston

    disston ShadeTreeExpert

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    Plaka,

    Nice idea. Interjecting something like this is not helpful though. I suspect that Chips Dad is capable but not all that experienced. It would just be easier at this point to get it running don't you think?

    He has a VOM so I don't think he's hopeless.

    We often run into this problem of trying to help riders and we don't know how much experience this rider has. May be his second car is a 1951 Ford P/U. Maybe he knows a thing or two? Maybe the problem is that he's an Engineer? How often do we see NASA scientist types that can't get their Airhead running?

    So give the guy a break if you can. I know you're having a ruff day and want to take it out on some noobies but they do mean well, until proven other wise.

    Your Friend,
    Charlie
    #13
  14. Plaka

    Plaka Brevis illi vita est

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    I gave my answer more than a second thought...and I was having a decent day with some projects turning out well. i vacillated between not saying anything and giving the answer I did. I chose to answer because that is helping. Spoonfeeding the guy might look more helpful, but it the end you foster dependance rather than independence. I noticed the guy had a meter. Definitely a good candidate. But some attitude adjustment required.

    Working on electrics requires one to be in "Brain On" mode. I don't get the impression, based on his question, that our OP is there. I could write out so long bit on how it all works...but he hasn't bothered to read any of the treatises already out there. Why should he read mine? Just 'cause I wrote it out just for him all special like? if you read what's out there and don't get something, then ask. Show a willingness to educate yourself, help yourself and solve your own problems. Ignorance is fine, failing to exert yourself (beyond asking an ignorant question in a forum) is not fine. I chose to point this out.
    \
    I see these interminable threads centered round just this. The op gets barraged with questions and answers containing a mish mash of information, misinformation, truths and half truths---and none of it is hanging together on an armature of a a basic understanding of the system.

    An example below:

    "Bend the wire to see if it stays contingent" Um, no. See if it stays continuous.

    "Follow this wire to where it connects to the condenser." Not unless you plan on cutting the loom open.

    "You can disconnect this wire at both ends and using a meter test to see it is the same wire from the right side coil to the condenser. " Maybe, maybe not. The matter of shorts is ignored. You can get continuity in a shorted wire. You still get a bad circuit. indeed, if it's shorted it continuous to every ground wire on the bike. And of course if it's shorted the coils will never make spark.

    If you are thinking about the system as a whole then all this stuff is clear without asking.


    Last but not least, when you do your homework you take a seat at the table as an equal. You don't need to know it all or be the most experienced. You do need to pay some dues and show some self reliance. It's a 40 year old machine, well documented with a loose fraternity of owners who are enamored of the things. How you wear your hat kinda matters.


    Speaking of hats; to put on my psychologists hat for a moment...dealing with a problem that requires analytic thought AND dealing with analyzing a system you know little about is anxiety provoking, to say the least. Depending on personality, on way to reduce this anxiety is to retreat to a position of helplessness. (formally: a regressive defense). This of course gets you nowhere but you do feel better. Applying a 2x4 upside the head, or a boot in the butt, can provide a second anxiety provoking force...looking badly in front of peers. Now you don't feel better anymore. The defense itself is attacked provoking some other response. Some other defense can be invoked (when someone responds with rage, irrational hostility, etc, you're seeing it) or a more positive response that reduces the primary anxiety can be forthcoming. like reducing the ignorance about the system and thinking carefully about what is going on. The boot in the butt routine is a gamble, it can go either way. But if it goes right the recipient will gain confidence with each success. it ends up with someone you feel good helping rather than feeling used for helping them. This preserves your own head (integrity) and you can stay with a position of offering more, and more frequently.

    Personally I stay off the airheads list. I don't like too many of those people nor do I care for the so-called airheads ideal. They aren't simple by choice...they don't have a choice. But it can be a good resource, especially if you like being helpless.
    #14
  15. chipsdad

    chipsdad Adventurer

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    sorry, i didn't grow up with several machines in the front yard on blocks to sharpen my mechanical skills. i am not ashamed to ask questions that may seem a little easy for you to answer. I do have an in depth electrical understanding as I work in telecom. Granted there are different electrical characteristics that i pay attention to then what we see in automotive wiring. If you feel that it is below your credentials to answer a question that is too simple for you to answer don't answer. i find it puzzling that you spent as much time as you did forming your reply. I thank you once again Disston for your informative reply. I was refering to the black post as the post that returns to the condenser. I believe that where i am i need to look at the plug wires and the condenser. i checked the points with a test light and they appear to be working correct.
    #15
  16. Plaka

    Plaka Brevis illi vita est

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    mmm..guess I wasted my time. But then you didn't read (or understand) what I did write, so I would have been wasting my time anyway. Ah well...back to burning my fingers on my current project.
    #16
  17. chollo9

    chollo9 Screwed the Pooch

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    "What the hell is a black post?"

    I think post # 7 in this thread would be an excellent example.
    #17
  18. chipsdad

    chipsdad Adventurer

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    sorry for the confusion, i was referring to the post with the black wire. the one going from the coil to the condenser.
    #18
  19. disston

    disston ShadeTreeExpert

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    We don't usually test points for resistance. That might work but I don't do it that way and I've not noticed anybody saying they "tested my points and the resistance was over 5,000 Ohms" . I go by the appearance, severely pitted gets replaced. I also go by mileage, I forget how many miles we are supposed to be able to get but I don't get more than 10,000 out of a set of points. Maybe if I rode my bike more miles I would get many more miles out of them.

    The points are carrying the entire Voltage of the ignition system. This is not the 12-14 Volts that is the operating system of the bike. The ignition of a stock BMW sees Voltages in excess of 20,000 Volts. This is how the spark is made. The spark has to jump the gap in the spark plugs and the only way to do this is this high Voltage. Due to this all they don't last forever. (about now some older geezer than I will pop up and say he has the original points in his /5, :lol3). Make sure the plug gap is correct. New plugs are better. New points are better. If the points don't show a lot of metal transfer from one contact to the other and don't show a "valley" on the other contact then they may be usable. Part of the problem is then setting the gap because any "hill and valley" on the points makes it hard to set the gap with a feeler gauge.

    The condenser; I don't know of any way to test the condensers. Unlike the points where the appearance may be used to help decide if you need new ones a bad condenser will most likely not show any outward appearance of it's failure or failing. If the condenser is suspect it has to be replaced to see if it is bad. If this doesn't fix the problem then maybe it's not bad but if it does fix the problem then you have found a bad condenser. Condensers do go bad. Not all mechanics always replace the condenser with new points. Some replace it every other new set of points. Some rarely replace this part. And they end up on the side of the road with out a clue what is wrong.

    The condenser is also carrying the ignition Voltage of over 20,000 Volts.

    You should be able to test the black wire from the right side coil to the condenser. Unplug this wire at both ends and measure it's resistance. It should be very low to zero.

    BTW, I had a dead ignition recently because the plug wire was caught in the timing cover. After I fixed the mounting and got it all straight it would still not fire. The wire was broken inside the yellow cover on the points wire. Spliced it.

    There are tests, measurements for the coils themselves. They are mentioned in the manuals. Do you have a manual? I will look this up later and tell you how to test the coils if need be but I suspect your problem is 1..the black wire, 2..the condenser or points, 3..plug wires or caps.

    The coils are rarely bad on these vintage machines.

    BTW, I seem to burn my arm or fingers every time I work on my bike. That's what Aloe is for.
    #19
  20. chollo9

    chollo9 Screwed the Pooch

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    You're good, I was being a smartass, others here are used to it. So is my mother.
    #20