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Dual P-Mags, Single Power Wire?

Toobuilder

Toobuilder

Well Known Member
OK, did a search but gave up after a while...

I plan on installing a pair of Pmags on the -8 and I intend to follow the basic wiring as called out in the manual (P leads, run off the key switch). Concerning the 12v power lead however, I'm wondering if there is anything wrong with running a single lead off the main buss (fused) to a normally closed (test) switch, then to the first Pmag, then "daisy chain" it over to the second?
 
I think the intent is to have each P-mag powered by ship's power (internal power is only a backup). So if you daisy chain them, and you blow a fuse or fry a wire or something, then both P-mags are running off internal power. You're not giving yourself any redunancy in that regard. Both my P-mags have their own circuit to the Main Buss with their own fuse.
 
You're right... I thought the Pmags came off the buss when they came up to idle speed, but they don't. The internal alternator is in fact described as "emergency".

A single feed does up the risk a little, but it's still a redundant system. I'll have to think about this one.
 
i don't think this is correct. At some low RPM (1200ish) the internal generator powers the Pmag exclusively at least that is how my recent instructions read.
 
RV8R999 said:
i don't think this is correct. At some low RPM (1200ish) the internal generator powers the Pmag exclusively at least that is how my recent instructions read.
Click to expand...
It depends...
113 Series self power when they lose ship's power and are spinning above around 800 RPM.

114 Pmags self power any time they produce enough power to do so, around 800 RPM.

Also, do not forget to run independent grounds to the engine case. I have seen some grounds run to the forest of ground tabs on the firewall. Not only that, they were on the same tab.
 
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I believe it depends what model P-mag you have. I know the most recent use their internal alternator as the primary power source once they are above the critical RPM.

erich
 
Alright, I give up!

I thought that I remember reading that the internal alternator is "primary" and the ship's power was for starting and "backup" to the internal alternator. However, the instructions have a heading called "Emergency Power", and it describes the internal alternator...

I think I'll just sit this one out until this aspect is resolved. :confused:


EDIT. I see it depends on the model... I have the 113's
 
Pmag self power testing - Caution!

If you want to test the self power feature of the P-mags, follow these steps:

1. With the engine running about 1200 RPM, turn off on ignition, leaving a Pmag still firing.
2. Slowly reduce power and note when the engine dies.
DO NOT shove the throttle back in to keep the engine running. Let it die.
3. Turn off the remaining ignition and Master
4. Wait long enough for any fuel in the intake tubes to evaporate. DO NOT fire the engine up right away. Remember, the E/P-mags fire two cylinders at the same time, one on the compression stroke and one on the exhaust stroke. If you try starting the engine with fuel still in there, it will backfire on the exhaust stroke.
5. Fire the engine up in the normal manor, after waiting for the fuel to evaporate.

YMMV.
 
Hmmm...

I think I'll test by:

1. Run engine to above 1200RPM
2. Turn off one ignition p-lead
3. Turn off master
4. If engine is still running the internal Pmag generator is working on the Pmag with P-lead on.
5. Reverse process for other P-mag.

I guess this would be for 114s though?
 
Ken,

That will work to tell you if the internal power is working or not there are a few problems with it. One is that you could harm your alt. by turning the power back on.

For a preflight test I would just drop the power to each Pmag, one at a time.

The test I outlined above is not a preflight test but rather a test to find out at what RPM they will drop off line. I see doing this just once, when you are in Phase I.
 
RV8R999 said:
Hmmm...

I think I'll test by:

1. Run engine to above 1200RPM
2. Turn off one ignition p-lead
3. Turn off master
4. If engine is still running the internal Pmag generator is working on the Pmag with P-lead on.
5. Reverse process for other P-mag.

I guess this would be for 114s though?
Click to expand...

I've thought about it and this is my plan as well. Since I have the 113's and their "primary" mode is the ship's power, I'll run a fused dedicated power wire to each ignition, and use the master switch as the "test" switch. Since the ignitions are the only electrical components on line at engine start anyway, I'll do this on each engine start before bringing anything else online. I also understand that this test is not the same as the internal alternator drop test, which really only needs to be done once or so.

Bringing the main buss on and off line is no problem if you kill the alternator field and shed the electrical load as well... It's the same as bringing the alternator on line at engine start.

Thanks for the info everybody! :D
 
Yes, we should do this check right after engine start prior to powering up any avionics or PFD systems, that way nothing is lost in the run-up just prior to T/O.
 
I really didn't want to add another switch in the system just for this check so that is why I like the use of the master better...
 
Ken,

Take a look at the bottom of my electrical page to see how I wired up my 113's and perform the pre-flight check.

While that pre-flight check won't work for the 114's, it might be worth the effort to create a similar sheet for your POH.
 
Thanks for the idea...I need to start putting my POH together pretty soon. Although it is something I dread doing!
 
N941WR said:
It depends...
113 Series self power when they lose ship's power and are spinning above around 800 RPM.

114 Pmags self power any time they produce enough power to do so, around 800 RPM.

Also, do not forget to run independent grounds to the engine case. I have seen some grounds run to the forest of ground tabs on the firewall. Not only that, they were on the same tab.
Click to expand...

Bill,
Why do the ground wires "have" to go to somewhere on the engine ?
If you run the ground wires to a forest of tabs mounted on the firewall and there's an engine ground strap going from engine to the tabs/firewall wouldn't that work just fine ?
 
Wicked Stick said:
Bill,
Why do the ground wires "have" to go to somewhere on the engine ?
If you run the ground wires to a forest of tabs mounted on the firewall and there's an engine ground strap going from engine to the tabs/firewall wouldn't that work just fine ?
Click to expand...
The short answer is:
That's what the manual said.

The long answer is:
You avoid two additional connectors that could fail by going to a case bolt. A ground stap failure would take out both E/P-mags.

Also you shorten the wire run by going to an engine bolt close to each E/P-mag, which I hope reduces the chance of them snapping and the odds of both failing on the same flight should be rather high.
 
yes but if the Pmags are grounded to the engine case and the ground strap failure occurs then the Pmags are no longer grounded (except to the engine) resulting in loss of A/C power to the Pmag but not the internal generator which will still be functional. Am I wrong?
 
RV8R999 said:
yes but if the Pmags are grounded to the engine case and the ground strap failure occurs then the Pmags are no longer grounded (except to the engine) resulting in loss of A/C power to the Pmag but not the internal generator which will still be functional. Am I wrong?
Click to expand...
Ken,

You are correct, but let me restate it. If the P-mags are wired to the ground tabs and the ground strap fails with no backup ground path to the engine, you just became a glider.

With each P-mag wired directly to the engine case, should the ground strap break (most likely at a ring terminal) the engine will continue to run.
 
Isn't the electrical ground of the aircraft the battery gnd and not the engine? Hence the reason we have to connect the engine to the airframe which in turn is connected to the battery gnd? If the engine ground strap breaks you have just isolated the Pmags and the Alt from the airframe. If you run the Pmag ground leads to the airframe (or even better, directly to the battery gnd terminal) they will remain operational regardless if the engine is grounded or not. Although I'm not entirely sure if the plugs will fire unless they are grounded as well...maybe they are grounded to the Pmag through the case and therefore will function normally. Make sense?
 
Mike,

The idea is to have two completely independent ignitions. While putting two on one breaker may work, it is not advisable. It doesn't get much simpler than that.
 
Best Practice

After talking to a few people, I feel that the best reason is, "its best practice" to isolate each P-mag with its own breaker. Its only a cost difference of one $20 breaker, a few feet of wire, and a square inch of panel space to isolate something so vital as the dual ignition system.
 
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