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I have a 1992 Tecumseh 5 hp motor that will not start. What is the function of the magneto and how does it work

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Hi and welcome to FixYa I am Kelly. The magneto works like this:

1. Inside the flywheel there is a magnetic portion in one segment. As that magnetic segment passes over the coil or electronic ignition module it generates a field in the primary coil winding that via inductance is transferred to the secondary winding. I.e. it is essentially a "step up" transformer. The breaker points take open and close to control the output of the coil / ignition unit so that the timing is exactly at the Top Dead Center (TDC) point when the piston is all the way to the top of the cylinder head. At which point the spark plug transmits the Spark into the cylinder for combustion.

There are a few safety devices that control the output of the ignition coil by grounding the breaker point wire or electronic ignition module wire.
1. The engine kill switch
2. The low oil sensor
3. On most models the handle control switch i.e the bar you press down on at the handle. That switch is there so that if the user lets go of the handle the engine will shut off.

Any one of the 3 can cause the engine not to have spark.

Other things that can cause it are
1. a failed ignition module.
2. a grounded high tension lead (Plug wire)
3. any chassis ground on any leg of the safety circuits.
4. the actual spark plug having a cracked ceramic dielectric or internally grounded.
5. on a newer chipper / shredder there is a chute safety switch that must be made when the chute is installed.

Most all of the above are "ONE wire circuits and each of them are there for user safety. Any ground any where will cause it not to start.

If you get to the point you think the oil safety switch is the problem disconnect ONE of the output leads and check for spark.

Most often the problem for these engines is they have sat idle off season and when the use tries to start it for the next seasons use they are fussy.

What happens is when unit was stored if the piston was not at TDC one of the valves stays open and then corrosion attacks the valve stem and it sticks open with an end reault of little or no compression.
Also what happens is the last seasons fuel can dry out and turn into an almost varnish like substance blocking carb fuel or air passages.

If you need help with "how to" clean the carb passages correctly just respond here and I will pass that along also.

Thanks for using FixYa,


Posted on Apr 26, 2010

  • Kelly
    Kelly Apr 28, 2010

    Hi again, I am just doing a follow up. Were you able to get your engine started? If not is there anything I can help you with? I will be watchng for any response and will gladly assist you iff you need it.





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