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Thread: 3 position master switch? on/off/on

  1. #1

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    Default 3 position master switch? on/off/on

    I don't see any difference between the two on positions, anyone know what it might be? I actually haven't looked close at the ammeter while switching between the two.. will try that and see if I notice anything.
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  2. #2
    pa20's Avatar
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    Default Re: 3 position master switch? on/off/on

    Drawing #13466 on the SWP CD's has a schematic which plainly explains the differences in the two "on" positions. You do have the drawings, correct?
    Mark Ohlau
    PA-20 N7744K 2020 Donation Paid

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    Default Re: 3 position master switch? on/off/on

    Quote Originally Posted by pa20 View Post
    Drawing #13466 on the SWP CD's has a schematic which plainly explains the differences in the two "on" positions. You do have the drawings, correct?
    Afirm, I have the drawings.... that's why I asked the question. but thank you.

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    Default Re: 3 position master switch? on/off/on

    Quote Originally Posted by j3doug View Post
    How old are you?
    Huh? Old enough to comprehend the advantages to owning the CD's for maintaining these classic aircraft, I guess. I take it that you fly a J-3, so I would expect that you too are fully aware that there is not much info out there on aircraft of our vintage. The CD's available from the Short Wing Piper Club are a godsend for troubleshooting, repair, and restoration of our SWP's.
    Mark Ohlau
    PA-20 N7744K 2020 Donation Paid

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    Default Re: 3 position master switch? on/off/on

    Quote Originally Posted by 70659 View Post
    Afirm, I have the drawings.... that's why I asked the question. but thank you.
    Awesome! Check drawing #13466, on the lower left part of the drawing (you will have to zoom in) You will note that the Master Switch has two poles, both protected by 30A fuses. My Pacer had been modernized and does not have this type of switch. But, I think it is safe to assume that either position accomplishes the same thing, and it wired this way for redundancy.
    Mark Ohlau
    PA-20 N7744K 2020 Donation Paid

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    Default Re: 3 position master switch? on/off/on

    70659,

    The referenced schematic is fairly rudimentary, as are our 50+ year old aircraft but the switch is simply providing redundant 30 (35?) fuses to power the 12 volt buss. The problem is that if you really have short circuit that blows the first fuse, it will blow the second, unless you take action prior to switching. A good bunch of our aircraft have had "upgraded" circuitry added to "improve things", alternators, continuous duty main relays etc. However, if you keep things simple, the original system has worked for a long time!

    Enjoy the day
    john
    Arizona Pacer

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    Default Re: 3 position master switch? on/off/on

    I thought it was redundancy, thanks.

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    Default Re: 3 position master switch? on/off/on

    John,
    I just went and looked at the schematic again, and it definitely referenced a 30A fuse. However, I am looking at the PA-20 drawings, so could there be a difference between what I have and the "modern" SWP's? Either way, I agree that this arrangement provides multiple opportunities for a light show in the event of a direct short!
    Mark Ohlau
    PA-20 N7744K 2020 Donation Paid

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    Default Re: 3 position master switch? on/off/on

    Hi Mark, my Pacer had 35's in it, but that doesn't' make it "correct", ha ha.!! Now has master relay & key switch for mags / start function. Kept the original master switch, just no fuses! One feature of the On - Off - On master switch, it provides two positions to leave on & drain the battery!!

    John

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    Default Re: 3 position master switch? on/off/on

    My 1950 PA 20 only has the two pole switch. The AFM supplement for the 125hp PA 22 states " on the three pole master switch, the UP position selects the main fuse, NEUTRAL is off, and DOWN is the spare".

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