How to check for a bad magnatron?

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How to check for a bad magnatron?

Postby petep » Wed Oct 11, 2017 9:16 pm

Anyone know a good way to check for a defective magnatron. Having trouble with a my first start attempt and want to verify the magnatrons. I read some where that you should test from the spark plug end of the spark plug wire to the body of the magnatron with an ohm meter and see a reading of 2.5K ohms to 5K ohms. I am reading like 5.25 Meg ohms and wondering if I do not have bad magnatrons.
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Re: How to check for a bad magnatron?

Postby n502pd » Wed Oct 11, 2017 10:38 pm

Since these are basically lawn tractor items, my t-shooting technique may help diagnosing, but may not actually repair. The plug wire is molded/glued into the actual magnet portion of the device, and if they were abused by pulling on the plug wire, the wires may have come disconnected internally, as with the plug end connection. I dont remember if the plug wires are copper or carbon resistance wires. I think they are copper but I may be wrong. With the ohm meter connected to the case lug of the magnetron (the lug that should be grounded at installation) and the plug end, try pushing the plug wire INTO the magnetrom body while watching the ohmmeter. You may discover less resistance and that probably indicates a broken connection internally. One cant readily fix that. Then at the plug end, inspect the wire crimp to the actual metal clip within the angle boot. Slide the boot back to inspect this. use a bit of silicon spray to assist the slide. Ohmmeter as before, but this time wiggle firmly...dont pull...while watching the meter. If you find this changes the resistance, try reterminating that end. Auto supply stores have the clips and tools to do the job if you need that. Otherwise, you may have a bad unrepairable magnatron. What does the second magnatron deiplay for its resistance? Lastly, are you sure your ohm meter, leads and alligator clips are in good condition and are not loose? Short the two meter leads together and move everything...meter should constantly read zero, or very close to it, and more importantly not change while you move things. After 60+ years messing with electronics that is the first thing I do with an ohm meter and leads, even the meter that I have used one hour ago!!

Please let us know what you find, and what the fix was!
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Re: How to check for a bad magnatron?

Postby wlarson861 » Wed Oct 11, 2017 11:47 pm

I know of no way to check the magnatrons other than running them with a spark checker. As for being a culprit in failure to start they wont be the problem. In order to start the engine you need to insure the electronic ignition is working correctly. The electric ignition provides a hot enough spark to start the engine. The magnatrons don't usually produce enough energy at cranking speed to fire the plugs. above 300 rpms the mags will produce spark. Once the engine is running you can check the mags by turning off the electronic ignition. If it dies then both are bad, if it runs but is very rough then one is ok and the other is bad. I just had a mag die and bought a replacement on Amazon.com for around $20.
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