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Series D / BT-H mag

macvette

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Non-VOC Member
I fitted a new BT-H magneto to my bike and am using a kill switch. Is there a way of using the ignition switch as the kill switch?
Mac
 
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BigEd

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VOC Member
VOC Forum Website Moderator
I fitted a new BT-H magneto to my bike and am using a kill switch. Is there a way of using the ignition switch as the kill switch?
Mac
The black wire from the magneto stops the engine by being connected to earth through your kill switch. You have a magneto so you are no longer using your ignition switch to connect voltage to an ignition coil. It should be a simple matter of connecting your black magneto wire to the input side of your ignition switch and the output side to earth. (The" ignition switch" should no longer have any connection to the battery or you will get molten wires. :eek:)
 

macvette

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Non-VOC Member
I can see how that would work. Am I right in thinking that the ignition switch would have to be in the off position for the bike to run and that the red light would no longer work? I was hoping there was a way to keep it switch and light functionally standard and thought of putting a relay in controlled by the switch so that when the switch was "on", the kill switch connection to earth was opened by the relay and closed when the switch was "off ". I haven't tried this because I think the BHT cut out works on the high voltage side and I'm not sure that a 6v relay would stand up to that and may damage the mag. I am talking about the new BHT which is really a cdi system.
Mac
 

BigEd

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VOC Member
VOC Forum Website Moderator
I can see how that would work. Am I right in thinking that the ignition switch would have to be in the off position for the bike to run and that the red light would no longer work? I was hoping there was a way to keep it switch and light functionally standard and thought of putting a relay in controlled by the switch so that when the switch was "on", the kill switch connection to earth was opened by the relay and closed when the switch was "off ". I haven't tried this because I think the BHT cut out works on the high voltage side and I'm not sure that a 6v relay would stand up to that and may damage the mag. I am talking about the new BHT which is really a cdi system.
Mac
People have wired their BT-H magnetos through a relay as you have suggested and this is a neat way of doing it. You could wire your existing switch to open a normally closed relay when you turn the switch on. You could also have the switch turn on a warning light.
The down side is that if you have a flat battery there is no power to pull the relay contacts to the open position. Your magneto will thus remain earthed so you will not be able to start the engine.
The black wire on your BT-H magneto earths low tension in the CDI circuit and this seems to work with a relay without problems. The relay does not have to cope with much in the way of current so a fairly low rated one should be OK.
 
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Chris Launders

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VOC Member
My friends early 80s Ducati had exactly this system, the Ducati Electronica system had to be earthed to stop so had a normally closed relay operated by the ignition switch, the amount of power required to operate a relay is minute but if your worried about a flat battery stopping you simply install a male/female connector in the earth wire (female end towards mag) so in an emergency you can disconnect the earth, just make sure it's out of sight.
Chris.
 

macvette

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Non-VOC Member
Tried a 6v relay wired from the ignition. Ignition off bike doesn't start, ignition on bike starts but doesn't stop when the ignition is turned off:eek:. Ammeter shows charge/ discharge but the relay stays in with then gone running and works perfectly off the bike . It is series D with the red dot ammeter and I 'm wondering if there is a back feed through the charging / light circuit. Gone back to kill switch.
 

macvette

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Non-VOC Member
There is back feed from the ignition light. I connected the relay coil to the ignition switch terminal which supplies the ignition light, leaving the kill switch functional so that I could check the relay. Started the bike, let it settle to a steady idle. Turned the ignition switch on, relay latched in. Turned the ignition switch off, relay stayed latched in. Checked the voltage on the live side of the ignition light ( same as at the ignition switch) and it showed around 3.5 volts at tick over. Disconnected the ignition light at the switch leaving the relay connected. Started the bike with the ignition on, relay latched in, switched ignition off relay latched out. Connected the magneto kill wire to the relay dispensing with the kill switch. Ignition off bike won't start, ignition on, bike starts, ignition off bike stops.
I'm not bright enough to work out why there is a back feed so I think I'll try reconnecting the ignition light to the switch and connect a diode in series with the bulb to stop the back feed but still allow the feed to the bulb from the switch.
Mac
 

highbury731

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VOC Member
I fitted a new BT-H magneto to my bike and am using a kill switch. Is there a way of using the ignition switch as the kill switch?
Mac

Some Japanese bikes (eg Honda 250RS) have what is effectively a magneto, and kill the ignition by earthing it in the 'off' position of the keyswitch. In the 'on' position, ignition is not earthed, but the electrical circuits are connected in stead.
 

macvette

Well Known and Active Forum Website User
Non-VOC Member
Hi,
I've got the relay to work through the ignition switch by inserting a diode in the feed to the ignition light so now I can stop and start the bike using the ignition key( and kickstart ) and also have a working ignition warning light as a standard series D should have although the light comes on because of the load on the relay when the bike is not running and goes out when the engine starts. The ammeter works as normal. I'm going to try to put a photo here but am not sure how to do it. The relay on the right operates the lights, the one on the left puts the magneto kill wire to earth when the ignition switch ( at 12 o'clock) is off and to open circuit when the ignition switch on. power to the ignition switch is the brown/white wire on the right, the black wire on the right of the switch is the supply to the relay coil and the white wire on the same terminal is the supply to the warning light ( at 10 o'clock ). You can see the diode in the black shrink wrap. Mac
001.JPG
 
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