Quote:
Originally Posted by Berkut  When you switch to L you are only running on the left magneto.
When you turn the key to L, the ignition switch connects the right magneto's P-lead to ground, preventing the right mag from firing. This leaves you just running on the left mag. Since the right mag is no longer firing, you get an RPM drop.
Now consider if the right magneto has a broken P-lead. When you turn the key to L, the switch grounds the right mag's P-lead. But since the P-lead is broken it can't ground the right mag, so it keeps firing and you don't get a drop.
The easiest way to check for a broken P-lead is to switch the ignition to OFF while the engine's running. The ignition switch will try to ground both the left and right mags, but if one has a broken P-lead it will keep firing and the engine will still run.
Here's some ignition switch trivia for you: You ever wonder why an aircraft ignition switch has the R and L positions backwards, like OFF-R-L-BOTH? Normally a switch is configured so that it completes a circuit to turn the device on. But in the case of an aircraft ignition switch, we complete the circuit (P-lead to ground) to turn the magneto off. So R is on the left because that position actually completes the circuit for the left mag, turning it off. Same thing for the L position. |
Great explanation!
Also we should point out the main reason for getting it fixed as you might know is that it could actually start up if someone moves the prop even though the ignition is off.