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cant wire p-mags using a single switch?

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Erich_Weaver(at)URSCorp.c
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Posted: Tue Sep 26, 2006 3:49 pm Post subject: cant wire p-mags using a single switch?

This was posted on the Vans airforce forum recently by an individual that

could not get his p-mags working.  After having sent it back to the

factory, this is what he reports:

"My dead P mag worked great back at the factory.

Then I had a brain wave.

I had tried to combine the off-power fail test-Run modes onto one DPDT

switch.

Unfortunatly there is a momentary change in state when you go from OFF to

one of the other states.

What happens is the Pmag becomes ungrounded before it gets power.

This does not work...You have to have a seperate switch for the PMag power

fail.Preferably a normally closed momentary switch"



After a request from a reader asking for further elaboration, this was the

reply:



"Its not when you lose ground...The P lead is UNgrounded in flight.

Its no big deal in terms of flying. I.e when your cruising along the PMag

will be ungrounded....If it then loses power it will operate as normally.

I.e not an issue.

The problem is if when starting you Unground the Plead before you apply

power...The Pmag then won't work...At least mine didn't.

As this will never happen in flight its a non issue.

The only reason it was an issue for me was that I combined the OFF-TEST-Run

modes onto a single switch. You cannot do this because no matter how you

wire the switch you will always unground the Plead before you apply power.

Long story short you need a seperate switch to test the power fail

mode...or pull the CB or whatever.

Right now I just have the power connected permanently...I.e no way to test

it (unless I pull the fuse) .

I want to find a normally closed momentary switch that I can't leave in the

OFF postion...Haven't found one yet."



As I have wired my p-mags using a single switch per Bobs Aeroelectric

Connection, but have not start tested the engine yet, I am concerned.  Does

the above make any sense?  Surely someone has successfully followed the

'Connection schematic before and NOT had this problem ?



Erich Weaver

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nuckollsr(at)cox.net
Guest

Posted: Tue Sep 26, 2006 5:49 pm Post subject: cant wire p-mags using a single switch?

At 04:47 PM 9/26/2006 -0700, you wrote:



 	  Quote: 	
		  

This was posted on the Vans airforce forum recently by an individual that

could not get his p-mags working.  After having sent it back to the

factory, this is what he reports:




	


   <snip>

 	  Quote: 	
		  As I have wired my p-mags using a single switch per Bobs Aeroelectric

Connection, but have not start tested the engine yet, I am concerned.  Does

the above make any sense?  Surely someone has successfully followed the

'Connection schematic before and NOT had this problem ?


	


   The anecdote quoted doesn't offer much clarity for the

   issue because there are no schematics offered nor part numbers

   of the switches used.



   The switches shown in the Z-figures are progressive transfer

   devices that offer specific performance in terms of what happens

   each time the toggle is moved. Further, the schematics I've

   published are CONTRARY to factory recommended p-mag wiring

   which is discussed on page Z-6 of:



http://aeroelectric.com/articles/Rev11/AppZ_R11J.pdf



   The reason for the variance is that from the pilot's perspective,

   I want to know that a P-mag's internal alternator is

   functioning during run-up so moving the p-mag switch from

   full up to mid position needs to interrupt battery power.



   Wiring as shown does NOT offer an ability to power up

   a p-mag with the ignition function disabled for the purpose

   of using built in timing tones or setting the prop for

   hand cranking. From the systems designer perspective, I

   chose to favor the pilot and add the third maintenance

   switch so that rarely conducted operations that use

   battery power with the ignition function disabled may

   be accomplished.



   Bob . . .

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