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Need help understanding my wiring diagram

Posted by mustangm NJ (My Page) on
Fri, Jul 30, 10 at 13:06

Thank you for taking time to help me understand a bit more how my mower wiring works. I have a Murray model 42544x8B with a Briggs engine model 28E707-0121-E1 code 9802112E. The wiring diagram is attached below. I have many questions, but let's start with the following:

1) I understand from reading other posts that 12VDC travels from the + terminal of the battery to a terminal post on the starter solenoid. A control wire travels from the ignition switch to the starter solenoid and signals the starter solenoid to transfer 12VDC to the starter which should crank the engine. Please correct me if anything I state is not correct. I really want to understand this stuff.

2) The cranking engine turns the flywheel magnets past the magneto, which should generate a spark at the spark plug. From a mownie post I understand that the "Briggs magneto ignition system uses a completely self contained spark ignition system that DOES NOT use any external source of power." The post also states "To kill a Briggs engine spark (and that kills the engine), you must GROUND the coil kill wire circuit."

My first question is to understand in more detail the kill wire circuit. In my wiring diagram I see a wire leaving the ignition switch terminal 4 (hard to see in diagram - this is the top wire @ 12 o'clock position, labeled YELLOW). This wire travels into a connector and then over to the magneto. I also see what looks like a wire from the magneto to ground. How do these wires cause the magneto to kill the spark?


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Follow-Up Postings:

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RE: Need help understanding my wiring diagram

You got it right on Paragraphs One and Two.

The yellow wire coming out of the ignition switch is grounded to frame in the OFF position and open circuit in the RUN and START positions. This way the engine will run when being cranked (START) and continue when key is released (RUN). When you switch to (OFF) ignition is grounded and motor is killed.

The Seat Switch (10), PTO Switch (4) and Clutch Brake Switch (also 4) are wired in parallel with the ignition terminal. If any of them faults, the motor stops.

The seat switch has additional contacts to bypass when you stop mower and disconnect PTO so you can dismount without killing engine.

Hope this helps.


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RE: Need help understanding my wiring diagram

Thank You ggoyeneche, that does help a bit. I am still unclear about this "open circuit' that you mention. When the magneto generates the electric pulse, does this pulse somehow pass thru all the safety switches and then back to the spark plug? If you can clarify the path that the electricity flows, I think I would better understand. Thanks so much!!!


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RE: Need help understanding my wiring diagram

When the connection to the magneto (armature) is unconnected or open, the magneto will spark. If you ground that lead, it kills the spark. Therefore that terminal provides a means of remotely stopping the engine with out using the old fashioned method of a shorting strap on the sparkplug.

Since the safeties are electrically chained together with the ignition switch, any one of them can prevent the magneto from firing is they are closed.


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RE: Need help understanding my wiring diagram

So if the wire from the magneto is closed (connected) at the ignition switch, then that wire provides a path to ground at the ignition. Is this correct?

Likewise if the wire from the magneto is closed (connected) at a safety switch, then that wire provides a path to ground at that safety switch. Is this correct?

On the otherhand, if the ignition is at (RUN) and the safety switches are all open (unconnected), then the path of least resistance is to the spark plug. Is that correct?

Is there a path to ground at each safety switch or is there some common grounding point?

Can I assume that the wire circuit used by thes safety switches to stop the engine are not at all connected to the 12VDC battery?

Thanks for putting up with my questions.


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