Briggs and Stratton Ignition Coil - Can Anyone Explain This?

minuteman62-64

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I'm working on a 10 HP B&S motor. The ignition coil is the "Magnetron" (solid state, no points) type. Here's what's confusing me. Based on my readings, in a good coil, the resistance between the kill switch lug and the coil frame should be 0 ohms. When installed, the coil frame is grounded to the motor. The function of the kill switch is to ground the kill switch lug to the motor.

How can the motor ever start if the kill switch lug is always grounded to the motor? And, of course, it does start. What's happening here?
 

Grub54891

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Are you talking about the ign switch or one of the kill switches? In both cases,the switch is ungrounded when in operation. It's when you turn off the ign switch it grounds out. The kill switches do the same thing,ground out when you get off the seat.
 

gm280

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On lawn mowers, yes the ignition "off" position does short the primary side of the coil to ground and stops further spark. But on some OB engines the kill switch can either ground out the ignition primaries in the same way, or shorts two primary sides of coils together stopping the spark. But in both cases, spark is stopped and the engine stops...
 

minuteman62-64

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I'm talking about the switch that turns the engine off. What mystifies me is that the coil appears to be shorted to ground regardless of the position of the off/on switch. How does it start?
 

rbh

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If the switch is showing a closed circuit or open circuit whether on off or on, the switch is hooped.
(The older machines with points grounded to kill the spark, not sure on the new ones if its the same????)
 

gm280

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minuteman, yes the primary side of the coil has to have one wire going to ground. Because how else can it transfer the high voltage to a spark plug that is turned/screwed into the engine with one wire to the spark plug tip to fire. So trying to read the primary side will look shorted to ground because of that.

images

As you can see in this schematic, the black wire and the green wire makes up the primary side of the coil. And the high voltage output and the green wire makes up the secondary side of the coil. This is not a schematic diagram of your B&S engine, just a diagram to show why reading the coil can be misleading and look shorted to ground...
 

bigdee

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If grounding the kill lug does not stop the engine you need to replace coil. I just replaced one on a Kohler last week. They can be pricey, found one on ebay for $6.....I had to drill new mounting holes so it would line up with flywheel but saved $$$.
 

generator12

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Not sure what the "kill switch lug" is, but I can offer this: The primary of the coil may have a very low resistance - a resistance lower than the resolution of an analog VOM. If that's what your using, you might recheck it with a digital VOM. I've seen many instances over the years where this has occurred. Not with a B&S coil to be sure, but enough to make me cautious about a "zero ohms" reading with an analog multimeter.
 

minuteman62-64

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Not sure what the "kill switch lug" is, but I can offer this: The primary of the coil may have a very low resistance - a resistance lower than the resolution of an analog VOM. If that's what your using, you might recheck it with a digital VOM. I've seen many instances over the years where this has occurred. Not with a B&S coil to be sure, but enough to make me cautious about a "zero ohms" reading with an analog multimeter.

That might be it. Or, I was thinking, maybe something in the transistor circuitry grounds it during the non-firing cycle. In any event, it works (just wanted to know why :) ).

The kill switch lug is the lug on the coil that goes to the On-Off switch. In the off position, it is grounded to the motor. Hence my confusion.
 
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