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Thread: 1985 K100RS sticking starter!

  1. #16
    I ran this by a friend of mine who designs high end electronic systems for racing cars (like 5,000 hp nitro dragster engines) and he immediately determined it is due to the relay being too small for the job. He designed a dual relay system, with the smaller relay triggering the larger one, so in the event the battery isn't strong enough the bike just won't start. Installed it today, in the original relay box, and it works.
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  2. #17
    Bluenoser
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    Not sure if there are anything specific to the relays in our old K bikes, like one way diodes and such, but I'm wondering why just installing a larger auto relay wouldn't work. I'm sure after all this time, somebody must have tried it.
    1986 K100RT

  3. #18
    Dances With Sheep GREGFEELER's Avatar
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    Quote Originally Posted by stevenm View Post
    I ran this by a friend of mine who designs high end electronic systems for racing cars (like 5,000 hp nitro dragster engines) and he immediately determined it is due to the relay being too small for the job. He designed a dual relay system, with the smaller relay triggering the larger one, so in the event the battery isn't strong enough the bike just won't start. Installed it today, in the original relay box, and it works.
    I'm no automotive electronics expert, so maybe these are the proverbial stupid questions, but here I go. The failure is that the high current points faces in the stock relay arc and stick when there is not enough current to fully engage them and they bounce causing the arcing. I can see how this double relay setup would save the original relay from damage, but doesn't this setup just transfer the problem to the larger relay? Being a bigger relay requiring a stronger current to engage, wouldn't the arcing just happen sooner (i.e. with a bit stronger but not strong enough battery)?

    Secondly, where did you find room to mount the second relay? If this works I would do it to both of my K75's and my K1200RS.

    I know that BMW finally supposedly fixed this problem in the last few years of the K1200 series. I believe starting in 2003, but would have to confirm, and I don't know what it was that they did.
    Greg Feeler
    Ambassador & amateur K-Bike collector, it seems
    1972 R75/5, 1990 K75, 1990 K1, 1992 K75S, 2003 K1200RS

  4. #19
    BMW addressed this by using a new relay with a built in circuit to prevent the problem. Unfortunately it isn't a plug and play solution. Apparently the smaller relay, if there is a low battery, will not have enough current to trip the larger relay, so there is no possibility of it sticking in the on position. I fit the two relays in the original relay compartment. Took a little finesse but it works fine.

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