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Thread: Starter Relay

  1. #1

    Starter Relay

    Just need help in locating the starter relay on a 1978 R100/7? Is it on the left side?

    Thank you

  2. #2
    Administrator 20774's Avatar
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    Yes, left side of the main frame rail. I don't have any pictures handy, but there are I believe two one inch square relays that plug into sockets. I found this older thread:

    http://forums.bmwmoa.org/showthread....al-connections

    And this is for a /6 but probably more or less the same, but there's another picture for a '78 R100RS:

    http://forums.bmwmoa.org/showthread....wiring-routing
    Kurt -- Forum Administrator ---> 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

    Relay

    Great Thank you!

  4. #4
    Quote Originally Posted by 20774 View Post
    Yes, left side of the main frame rail. I don't have any pictures handy, but there are I believe two one inch square relays that plug into sockets. I found this older thread:

    http://forums.bmwmoa.org/showthread....al-connections

    And this is for a /6 but probably more or less the same, but there's another picture for a '78 R100RS:

    http://forums.bmwmoa.org/showthread....wiring-routing
    Relay towards the front of bike is the horn relay. Be careful, as there are some always-hot wires going up there.
    "The difference between death and taxes is that death doesn't change every time congress meets." - Will Rogers

  5. #5
    Registered User lkchris's Avatar
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    Always hot wires are red
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  6. #6
    B Reams brook.reams's Avatar
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    Red Is Hot Except When Its Not

    Quote Originally Posted by lkchris View Post
    Always hot wires are red
    Except on British motorcycles where they are ground wires. I have to ask myself, "Which country?" these days as I work on vintage British bikes at a small independent shop on Mondays and Tuesdays and on my BMW projects the other days. And of course, they use Brown for the hot lead from the battery. But that's okay, because the hot lead is the (-) terminal and that's the ground for the BMW folks and they use brown ground wires on that terminal as well.

    It would not be good to forget what day of the week it is nor the nationality of the bike I'm working on. :-)
    Brook Reams - Arvada, CO
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  7. #7
    Quote Originally Posted by lkchris View Post
    Always hot wires are red
    The problem is that sometimes the color is obscured by dirt, wrapping, clutter, etc. I have seen people take a screwdriver to pry up the relay, and the screwdriver tip connects and grounds the hot wire! Sparks!

    Some people think that the only always hot wire on a bike is the one off the battery to the solenoid which is usually a very large gage wire, and the one that runs to the ignition switch. They think also that other hot wires are not "hot" until the switch is turned on (as is the case with many vehicles).

    I believe, that there is a hot wire (not turned off by the ignition switch) going to the horn relay even though one would think that it wouldn't need to be hot, but it is!

    Try to use plastic screwdrivers for that prying around
    "The difference between death and taxes is that death doesn't change every time congress meets." - Will Rogers

  8. #8
    Quote Originally Posted by SCBuckeye View Post

    I believe, that there is a hot wire (not turned off by the ignition switch) going to the horn relay even though one would think that it wouldn't need to be hot, but it is!
    Indeed that is the way relays usually work. A relay is just an electrically activated load switch. Power is directed from the battery to the load through a relay, with the relay turned on or off by current applied to a magnetic coil. This "signal" to the coil is controlled by a manual switch such as the ignition switch or the horn button, but the wiring that carries the current from the battery to the load is often unswitched from the battery.
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  9. #9
    Administrator 20774's Avatar
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    Since SCB and I have the same model, I thought I'd see what the diagram looked like. I'm using Mrs. Haynes for my diagram.

    - Battery -- two wires from the positive terminal; one red wire to 87a on the starter relay and one black wire to terminal 30 on the starter solenoid (BTW, this is how charging current gets back to the battery from the diode board)

    - Starter motor/solenoid -- red wire from terminal 30 to terminal 30 on the diode board (because this is hot, that's why one needs to remove the battery negative before removing the front engine cover)

    - Starter relay -- 87a and 87 are identical for the relay, connected internally. Snowbum suggests splicing these two together outside of the relay in case of corrosion on the relay terminals. Terminal 87 goes to the 1) ignition switch; 2) light relay; and 3) clock (if equipped).

    Haynes doesn't show a relay for the horn but does on the '77 R100RS. My bike has the dual FIAMMs so I suppose there's a relay somewhere. On the R100RS, the hot lead off the relay goes to the clock.

    So, it does appear there are other hot lines running around!!
    Kurt -- Forum Administrator ---> 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!

  10. #10
    Fortis Fortuna Adiuvat Omega Man's Avatar
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    Quote Originally Posted by SCBuckeye View Post
    The problem is that sometimes the color is obscured by dirt, wrapping, clutter, etc.
    Yeah, found some of all of the above this morning on a project
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  11. #11
    Administrator 20774's Avatar
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    Quote Originally Posted by Omega Man View Post
    Yeah, found some of all of the above this morning on a project
    That'll buff right out!!
    Kurt -- Forum Administrator ---> 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!

  12. #12
    MOA #24991 Pauls1150's Avatar
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    More Red Wires


  13. #13
    Also, relays CAN be dis-assembled (carefully), and cleaned and re-assembled. Usually, they get full of corrosion and gunk and will not make good contact. Also, being down there near the master brake cylinder doesn't help as when the cylinder develops a leak (as they can being old now) sometimes that brake fluid will seep into the relays.
    "The difference between death and taxes is that death doesn't change every time congress meets." - Will Rogers

  14. #14
    Quote Originally Posted by SCBuckeye View Post
    Also, relays CAN be dis-assembled (carefully), and cleaned and re-assembled. Usually, they get full of corrosion and gunk and will not make good contact. Also, being down there near the master brake cylinder doesn't help as when the cylinder develops a leak (as they can being old now) sometimes that brake fluid will seep into the relays.
    Good point

  15. #15
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    Indeed - my '78 R100/7 died on the road a few weeks back with "0" electrical power anywhere. It turned out to be that several years ago the leaking m/c dumped brake fluid down the frame and onto the relay & relay socket, collected in the socket terminals and started the corrosion process. I had cleaned up the leaking fluid where I saw it and repaired the m/c, but the ticking time bomb of corrosion continued on unseen down in the socket until I was riding last month checking to see that my push rod tube seals and head gasket replacement job was done OK. Some extra vibration from a couple miles on dirt roads pushed it over the edge, and there was nothing for it. I've cleaned the contacts, relays and terminals up but am going to replace the terminals and the block for better peace of mind.

    The main starter relay is the the one toward the back, on the left hand side of the frame backbone under the fuel tank. If you look straight down from the m/c - it is right there, waiting to get dumped on....
    BMWs in my garage: 1982 R65LS, 1978 R100/7

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