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Discussion Starter · #1 ·
I have a Craftsman Lawn tractor with a 18.5 Briggs and Stratton Intek OHV motor 31P777-0348-E1 that keeps killing the armtature Magneto.
After mowing the lawn last spring, I shut off the tractor to empty the bagger and when tried to restart it it would not start. The engine had no spark and I figured out the magneto was bad. I replaced the magneto and it started right up. I mowed the lawn as usual and shut it off to empty the bagger, and it was dead again. This time when I ordered the magneto P/N 795315 the part that came in was 799650. Was there as issue with the old design. Think that there was a design issue and the new design was the answer, I installed the new magneto and the tractor fired right up. It was a little rough when it first started, but after a few minutes it smoothed out and I mowed the lawn. When I was finished I shut it down to empty the bagger...and ....you guessed it, there was no spark again. I know it is the magneto again, but do not know why this is happening. I have tried a new plug, checked the seat, brake and clutch switches and they seem fine. I have inspected the wiring and there are no burnt or frayed wires. I am at a loss Please:help:
 

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Welcome to the forum Grumpy! :howdy:

Since it seems to happen when you shut it off, wonder if it could be a short in your ignition switch? Maybe it is intermittently shorting 12V into the kill wire? :confused:
 

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Discussion Starter · #3 ·
Thanks for your reply Mark. How would I go about testing the ignition switch ? seems a little more to it than the safety switches.
 

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Hmm, just looked at Sears parts and that switch and the ammeter are the only two of the electrical parts showing a current back order. Always makes one wonder if they are having a problem with them when you see that. :D

http://www.searspartsdirect.com/par...P0405132/00001?blt=06&prst=&shdMod=9172736401

Do you have an owners manual for it? If not you can download a .pdf file of it here:

http://www.managemylife.com/mmh/lis_pdf/OWNM/L0406036.pdf

Page 33 has the wiring diagram and shows in detail the ignition switch and also a table of the different switch positions and whether they should read with an ohm meter open or closed.

Your going to need a volt-ohm meter though to test it. The terminal M (one of the two black wires on the switch) should never have 12 volts on it. It will read an open connection to chassis ground unless one of the safety switches are activated or the switch is on the "off" position. In the "off" position or if a safety is activated when in the "on" position terminal M will read a short to ground killing spark. Make mental note of the bold & italics above as that will tell you if one of the safeties is shorted when the ignition switch is in any other position besides "off". ;)

Puzzling thing is if the ignition switch is even momentarily shorting 12 volts into the coil kill terminal one would think it would blow the fuse. Or if it does not blow the fuse at that time, any one of the safety switches when activated would blow the fuse.

But with a new plug there really is nothing else there that could damage the coil that I can see. Strange problem. :confused:
 

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Discussion Starter · #6 ·
Mark, I was talking with some friends about this today and I was thinking of what was happening when I shut the tractor off. One of my buddies ask if it backfired, I thought about it and it kinda did, not a load bang...but a little pop. So I decided to break out the impact driver and check the flywheel key. Guess What...it was sheared. Could this have anything to do with my magneto failing.
 

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I don't know how that would kill a magneto. But it would definitely change the timing of your spark and also keep the engine from running at all once it got far enough out of time.

Maybe that second replacement coil has not failed after all. Maybe after replacing the sheared key you will be back up and running. :cool
 

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I was wondering what the final answer was to this problem. I have the same motor and it just started doing the same thing only no backfire. Mowed the lawn twice with the new coil and it's dead.
 

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sears home repair technician ordered repair kit 71917192931 to solve known problem . will it solve problem ? i believe it is a 9 volt fuse kit
 
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