[Beejer]

Got an old O-320 engine off of a Cessna (probably) with the dreaded dual mag off of one mechanical output. That’s really not the problem though (though it will be solved in a future update when I try something really dumb… ).

The problem is my wiring. Messed up putting my new panel in and wired them to the positive… bad Ben. Now I have them sorted properly to the ground as the great googlie mooglie in the sky intended. BUT they still won’t turn off. I can ground both of them… engine starts. Ground one… engine starts. Turn them both on… engine starts. It’s a good problem I guess that the engine wants to always keep running but it does seem like a safety factor in case I bump into the propeller too hard or something.

Wiring and switches are all continuous and working. Each mag feeds into its own 12v switch and the starter is on a separate push button switch (I lose keys like crazy so nothing in my life has them).

So did I cook them into an always on fail state? Do I have to say some magic words to make them turn off? I’ve said a few words already… they might not be magic ones though.

As a bonus I’ll pre-ask this… can’t get my alternator to energize. It’s wired right now to complete the out and back loop. But if someone has some words to describe the correct wiring as if you were describing it to a small child…

I thank anyone who takes mercy on me in advance.

 

[BobTurner]

Got an old O-320 engine off of a Cessna (probably) with the dreaded dual mag off of one mechanical output. That’s really not the problem though (though it will be solved in a future update when I try something really dumb… ).

The problem is my wiring. Messed up putting my new panel in and wired them to the positive… bad Ben. Now I have them sorted properly to the ground as the great googlie mooglie in the sky intended. BUT they still won’t turn off. I can ground both of them… engine starts. Ground one… engine starts. Turn them both on… engine starts. It’s a good problem I guess that the engine wants to always keep running but it does seem like a safety factor in case I bump into the propeller too hard or something.

Wiring and switches are all continuous and working. Each mag feeds into its own 12v switch and the starter is on a separate push button switch (I lose keys like crazy so nothing in my life has them).

So did I cook them into an always on fail state? Do I have to say some magic words to make them turn off? I’ve said a few words already… they might not be magic ones though.

As a bonus I’ll pre-ask this… can’t get my alternator to energize. It’s wired right now to complete the out and back loop. But if someone has some words to describe the correct wiring as if you were describing it to a small child…

I thank anyone who takes mercy on me in advance.

Is the mag itself grounded? Often done by using the braid on a shielded P lead.
What alternator? Internal or external regulator? Either way, there is usually a ‘field circuit’ that needs to be energized (12 volts) for the alternator to work.

 

[newt]

The problem is my wiring. Messed up putting my new panel in and wired them to the positive… bad Ben. Now I have them sorted properly to the ground as the great googlie mooglie in the sky intended. BUT they still won’t turn off. I can ground both of them… engine starts. Ground one… engine starts. Turn them both on… engine starts. It’s a good problem I guess that the engine wants to always keep running but it does seem like a safety factor in case I bump into the propeller too hard or something.

If you haven't already, disconnect all the plug leads. This is a serious safety issue on the ground, and there's no need for an engine to be in an operational state while you're trying to fault-find bad magneto wiring.

The path from the p-lead terminal on each magneto to the crank case should be open circuit when the ignition is on, low resistance when the ignition is off.

With all the plug leads disconnected, run a wire to short the p-lead terminal to the crank case. You shouldn't get spark when you turn over the engine on the starter. If you do, your mag probably has an internal fault and should not be used until it's fixed.

You should have a nice hefty ground wire from the engine case to the airframe. The ignition switches probably won't work if the magneto is electrically isolated from the rest of the aircraft: Shorting the p-lead to the earthing block behind the instrument panel isn't going to shut off the mags if the earthing block isn't at the same potential as the engine.

For god's sake, whatever you do, disconnect all the plug leads until this is dealt with. Don't get yourself killed trying to chase a wiring fault.

- mark

 

[Ralph Inkster]

Scary situation you are describing!
If you think the mags are live, take precautions against accidental engine start.
Stay clear of the prop! Prevent anybody else going near that prop! Solidly secure the plane from moving if it does accidentally start! Put signage on the plane advising all PLANE UNSAFE, KEEP CLEAR!
Sorry for all the exclamation marks but in this case they are warranted.

ASAP- disconnect all spark plug wires till you figure out the P-lead switches.
Sounds like you have 2 toggle switches for your mags, each switch should have a wire (P-lead) going to that terminal on that respective mag, the other wire off the switch should go directly to GROUND.
Mags have nothing to do with 12volt power at all.

 

[Sam Buchanan]

Got an old O-320 engine off of a Cessna (probably) with the dreaded dual mag off of one mechanical output. That’s really not the problem though (though it will be solved in a future update when I try something really dumb… ).

The problem is my wiring. Messed up putting my new panel in and wired them to the positive… bad Ben. Now I have them sorted properly to the ground as the great googlie mooglie in the sky intended. BUT they still won’t turn off. I can ground both of them… engine starts. Ground one… engine starts. Turn them both on… engine starts. It’s a good problem I guess that the engine wants to always keep running but it does seem like a safety factor in case I bump into the propeller too hard or something.

Your mag toggle switches should be mounted "upside down", in other words, when the switch is flipped "up" to turn on the mag the switch is electrically open (actually in the "off" position) so the mag is ungrounded. When the switch is "down" to kill the mag the switch is closed (actually "on") which shorts the P-lead to ground and prevents the mag from firing. This is a HUGE safety issue, be absolutely certain you understand how this system works and should be wired!

I don't understand your "12v" comment........

 

[Beejer]

So as to the 12v comment, I meant that they fed into 12 volt rated switches, thinking that maybe they have too much resistance to ground the system. Would this matter?

Okay so dumb question time… the P-leads are the things that come off the back of the mags that are supposed to be grounded to turn off the mags?

And where is the engine supposed to ground from and too? And if I’m just grounding the mags directly, where does that attach to the mags? My mags are attached directly to the engine so I would think the engine ground would ground them but… something is not right somewhere.

As to how to make this safe… disconnecting the battery won’t do it as mags are self energizing? So yes… spark plugs out is the only way correct?

Concerning thing most to me is this is how the airplane came into my possession. I am simply re-hooking old wiring thinking I was safe as I hadn’t changed anything on the engine directly. Guess it really is better to just start from the ground up and build it yourself.

Thanks for the concern and advice. I will endeavor my best not to get dead or un-alive anyone else.