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doramsey(at)gmail.com
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 Posted: Tue Dec 13, 2011 8:59 am Post subject: starter switch wiring |
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I wired my starter switch and magnetos like Z-26 with the magneto wire shields both connected to the ground terminal on the switch and the other end of the shields connected to motor ground. Before installing the switch I confirmed continuity was correct in all switch positions. Why would one have continuity to ground from the magneto post even when the switch is in the "both" position. In fact there is strangely continuity from magneto post to magneto post. If it matters, the magnetos are Bendix and both have impulse couplings (no jumper required on the switch). I assumed that the magneto post is isolated from ground unless its wired to ground. Is that incorrect? [quote][b]
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racerjerry
Joined: 15 Dec 2009
Posts: 202
Location: Deer Park, NY
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| Posted: Thu Dec 15, 2011 6:56 am Post subject: Re: starter switch wiring |
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Most likely you are just reading the magneto internal coil resistance. Disconnect the “P” leads and re-measure. You will still see “continuity” between the magneto’s “P” lead terminal and ground with “P” leads disconnected.
The magneto coil’s primary winding consists of a small number of turns of wire wrapped around an iron core. One end of the primary winding is internally grounded and the other end is connected to the “P” terminal/stud. The resistance of the primary winding is very low (not much more than the meter lead resistance) and difficult to measure using a standard ohmmeter. Slick specifies primary coil resistance to be between 0.5 and 1.2 ohms (points open / disconnected).
With your ohmmeter, verify that each bare “P” lead (aircraft wiring) has continuity between each lead and ground when magneto switch is in the OFF position and “OPEN” (no continuity) when in the BOTH position.
If that works, reconnect everything, fire up the engine and do a normal magneto RPM drop check to verify all is well.
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