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PostPosted: Tue Aug 23, 2016 3:51 pm 
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Joined: Sat Jan 04, 2014 1:07 am
Posts: 17
Hello,

I am troubleshooting the ignition on my 1974 MX250. I need to know the correct interpretation of the white/red to black resistance measurement in the table provided in the service manual.

Magneto model M100-08:
Brn to Blk= 780-800
Rd to Blk= 85
Rd/Wht to Grn/wht= 50
rd/wht to blk= 0

The rd/wht and grn/wht are the two wires from the pulser coil. Black is apparently ground. Brn and red are the charge coil outputs.

Does "zero" mean that statically rd/wht connects to GND or does the "zero" mean open circuit? The former was my expectation until I was told that zero can mean open circuit (for a coil isolation test).

Incidentally, there is a figure in the service manual used to describe the ignition's theory of operation. In the figure, one side of the pulser coil connects to GND, but perhaps the figure is not to be taken literally as a schematic. One side of the charge coil connects to GND in the figure, too, but neither has a zero resistance spec value.

Thank you,
Ben


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PostPosted: Sun Sep 04, 2016 7:53 pm 
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Joined: Fri Jan 24, 2014 12:39 pm
Posts: 51
When you hook the tester leads to the coil wires and the meter does not change you have an open circuit. Could be a broken wire or a bad coil. If the readings on the meter change and go to zero,that would be a bad coil. Kinda depends on how the meter works really. Either way you have an issue. Main thing to remember is if the readings are not the correct ohms there is a problem. None of the coil wires should have any resistance to the black wire.


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PostPosted: Sun Sep 04, 2016 8:03 pm 
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Joined: Sat Jan 04, 2014 1:07 am
Posts: 17
Thank you for your reply.

Should there be about zero ohms, that is, a low resistance connection from w/r to GND?

Or can "approximately zero ohms" from the service manual somehow mean open circuit for the w/r to GND combination?

The pulser coil (w/r to w/g) resistance was in spec at about 50 ohms.

Thank you


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