Testing a John Deere E Mag

[jpasture]

Can someone share with me how fast the armature should turn to take a reading on a DC Ammeter? I have seen the Webster mag intrustion book and they recommend 500 rpm. It would be great to know just what the magneto is putting out and whether it needs to be charged.

And the second question is if the magneto is weak is it generally a low charged magnet?

Last would someone tell me which side the N marking goes when replacing the magnet? So far I have put them back on the way I find them - but have found them both ways! Maybe it doesn't matter?

Thanks for any help here - John

[Don C. Wiley]

John;

I'm assuming you're talking about an E John Deere gas engine. If this is the case, you must understand that the "magneto" on those engines, really aren't a magneto as most folks understand them.

The E John Deere gas engine "magnetos" are a low voltage alternator. If you want to measure the "output" it's not measured in amps, but in volts. And on these units you'll need to set your "voltmeter on AC, since it's an alternator.


I take the gear off and chuck the shaft in my cardless drill and with it set on the high speed setting I run it with the AC voltmeter connected with aligator clips so I can hold it down, I run it and it should produce between 5 and 7 volts or thereabouts.

Since you are working with an AC sine wave, there will be two points in each revolution of the armature, that it produces it's maximum voltage. There is also two places in each revolution that the voltage output is zero. This necessitates careful timing so your ignitor points snap open at the precise time that the alternator reaches it's peak voltage.

The alternator doesn't care which side the "North" pole is one. That just sets up a magnetic field so the armature "cuts" through that field to produce a voltage in the armature, which is a sine wave if viewed on an ociloscope.

If the magnet hasn't been removed, they don't normally get weak enough to seriously affect the output voltage. However after saying that, it wouldn't hurt to recharge them if you have a "good" charger and not the small home wound ones. Unless you have about five pounds of heavy copper wire wound on each pole of the charger, they probably won't significantly increase the magnetisim.

Having a lot of fun trying to help other collectors!

"DELCO DON" Southern Illinois

[Andrew Mackey]

Reversing the magnets will lead to an early decline on their staying power, as they will not be matched to the mag case. The magnets and the mag case are usually recharged as an assembly, for that reason. A simple test for a weak magnet - short the coil wire to the mag case, and attempt to turn the mag. A very noticeable stop pont should be felt - resistance to turning the armature. If there is no difference between grounded and open lead, then chances are the coil is bad.
Andrew