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P-lead for the Tach

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ChangDriver

Joined: 15 Sep 2007
Posts: 266

Posted: Sat Oct 10, 2009 12:12 pm Post subject: P-lead for the Tach

Bob:



When wiring a tach that uses the p-lead for its signal, can you help me with:



1) would it be best to run new shielded wire from the mar to the tach vs trying to tap the p-leads off the mag switch?



2) is 20 AWG shielded wire the recommended wire size for the p-lead (that is what I see in your book)?



Thanks in advance,



Craig

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nuckolls.bob(at)aeroelect
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Posted: Sat Oct 10, 2009 6:09 pm Post subject: P-lead for the Tach

At 03:13 PM 10/10/2009, you wrote:

 	  Quote: 	
		  

<capav8r(at)gmail.com>



Bob:



When wiring a tach that uses the p-lead for its signal, can you help me with:



1) would it be best to run new shielded wire from the mar to the 

tach vs trying to tap the p-leads off the mag switch?


	


   You can tap the signal at either end but at the mag switch

   terminal end makes the most sense. Your installation instructions

   for the tach may also recommend putting a resistor in series

   with the signal lead at the switch so that a faulted wire

   in the tach wont kill the mag.



 	  Quote: 	
		  2) is 20 AWG shielded wire the recommended wire size for the p-lead 

(that is what I see in your book)?


	


   20AWG or larger is the general rule for any wires that run

   out to the engine nacelle . . . for mechanical robustness.

   22AWG or even 24AWG would be electrically capable of controlling

   mags . . . but I don't think I've ever seen them wired with less

   than 20. Some older airplanes even had 18AWG.



   22AWG shielded between the mag switch and tachometer will

   be quite sufficient.

        Bob . . .



         ---------------------------------------

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

         ---------------------------------------

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