yep exactly as bigted says. that will be your prob. sometimes the connections on the g3 are dirty or corroded or missaligned and dont touch when closed. have a look at this diagram 27 and 28 , 29 have to touch when closed so it earths to body.
it could be a break in the wire but i doubt it. try what bigted says first. pull off the wire from the g3 and earth it to the body whilst running . it should stop the mower.if it doesnt stop it then its not getting a proper ground.
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