Mag switch question

[wadem]

Hey guys:
Was at the airport today and was discussing a problem a friend had on his 7 regarding mag switch position. This may be a silly question but here goes:
When the key is set to the left or right position, does this mean that the selected mag is the one that is firing? So on left, left is active right is dead?
or is it the other way around, i.e., left selected, the right mag is firing? thus the switch kills the selected mag. I thought it was always firing on whatever mag is selected. My friends 7 seems to be wired the other way, thus the query.
Also, my mag switch on the RV-4 is the same one as my friends, and that is the one that is on the Vans site. With these switches, does it automatically shut down the non impulse coupled mag so that it starts on the impulse mag only?
The vans switch is marked....OFF-RIGHT-LEFT-BOTH-START.
And finally, my mags are the bendix S4LN-20 and 21. How do I tell which one has an impulse coupler?

Thanks guys

Wade

[Mike S]

Originally Posted by wadem

With these switches, does it automatically shut down the non impulse coupled mag so that it starts on the impulse mag only?

How do I tell which one has an impulse coupler?

Thanks guys

Wade

I recall that the switch has the ability to shut down one mag, using a jumper.

Read the instructions

With the mags turned OFF, slowly turn the prop. If you have an impulse coupler, you will hear a click/snap sound as you pull the blade through, happens every fourth blade.

[DaAV8R]

The -21 is the impulse. Another way to verify the impulse mag is that the impulse mag has an aluminum spacer between the mag and the accessory case about 3/4" thick give or take. The non-impulse does not have this spacer.

[DanBaier]

OFF-both mags are grounded - neither fires (hopefully)

RIGHT-left mag is grounded, right mag is hot (so it fires)

LEFT-reverse of above

BOTH-neither mag is grounded, both fire

START-room for variation here. Most of the time, a Lyc with two mags will have an impulse coupling on the left mag and so when the switch goes to start, it not only engages the starter, but it also grounds the right mag so only the left fires. The impulse coupling fattens up and delays the spark so it's better matched to a slow turning engine (driven by starter motor). When you back off the Start position, the right mag becomes hot again. Permutations of this are controlled by a jumper that is installed on the back of the switch.

See http://www.aeroelectric.com/articles...tchOptions.pdf

Dan

[plehrke]

Originally Posted by DanBaier

OFF-both mags are grounded - neither fires (hopefully)

RIGHT-left mag is grounded, right mag is hot (so it fires)

LEFT-reverse of above

BOTH-neither mag is grounded, both fire

Dan

It depends on if it was wire correctly. If correct, see Dan's response. If wired incorrectly (or switch is not operating correctly as there are ADs on the switches), then all bets are off and I would do a mag check the next time the engine is started to especially make sure OFF is actaually OFF.