Mags always grounded?

[odens_14]

I was wiring my mags today on a brand new IO 360 M1b engine from vans. Well when I finished both mags were always grounded, I figured the shield must be touching the center conductor, but when I checked with the voltmeter the wire is good. Now this has me really confused, with all the wires disconected from both mags they are both still grounded??? I tested this by using my voltmeter from the p-lead to the firewall. can someone please tell me what I'm missing, I can't have 2 bad mags on a brand new engine can I?

[Mel]

The mags are grounding through the points. If you turn the prop, you will find a point where they are ungrounded when the points open, but this is not a safe practice with the spark plugs in the circuit as the mag will fire.

[odens_14]

*sigh of relief*
Thanks, Mel, I was hoping it would be something simple like that. I'll pull the leads off and turn the prop a bit.

[Bob Kuykendall]

Originally Posted by odens_14

...can someone please tell me what I'm missing...?

I think that just about every conductor inside the magneto is connected to ground either directly or through either the primary or secondary coil. I think that if you check closely you'll find at least some resistance on the P-lead terminal, that being the resistance of the wire through the coils.

Here's an article with a typical magneto schematic.

As I recall, the P-lead grounds the end of the primary coil that goes to the condenser and to the points. So you'll find near zero resistance at the P-lead terminal when the points are closed, and a bit more resistance when the points are open.

Thanks, Bob K.

[aerhed]

Also, There's probably a spring steel tab your P-lead cigarette screws in against. If the P-lead is unscrewed this springs against the case & grounds.

[az_gila]

Originally Posted by aerhed

Also, There's probably a spring steel tab your P-lead cigarette screws in against. If the P-lead is unscrewed this springs against the case & grounds.

...mags, not Slick ones, correct?

[Tom McCutcheon]

Also, if you are trying to check with the key in the off position all the mags are grounded.

[aerhed]

Gil, right-o. Bendix does that.