D120 tachometer connections to magneto

[contryflyr]

I'm not sure how to connect the D120 tachometer inputs to the magneto. I the picture in the manual leads me to believe it is wired from the ignition switch. It also states to put the resistor as close to the magneto as possible. Can I wire both the right and left tachometer inputs to the P-lead on the magneto and not go through the switch? This would also allow me to put the resistor close to the magneto. Will this cause me any problems like a hot mag?

 

[dynonsupport]

Specifically, you connect to the p-lead, which runs between the ignition switch and the magneto itself. You can connect anywhere along it, but connecting at the ignition switch means one less wire to pass through the firewall.

The resistor provides protection against that wire being shorted to ground. That's why you want it as close to the point that you connect to the p-lead as possible (which is not necessarily the magneto itself).

 

[dynonsupport]

You don't hook the EMS tach pickups through the switch, you hook them up at the switch.

A mag switch in an airplane grounds the p-leads when that mag is off. When the mag is on (or hot), that p-lead wire is ungrounded. So there's nothing on the other side to hook to, since the other side is ground. So you always hook to the wire that comes out of the mag. When the airplane is "on" this has pulses on it for the ignition, when it is "off," it is just ground.

Also, we don't suggest you hook up as close to the mag as possible. If you re-read the instructions, what is says is:

"It is important to connect each resistor as close as possible to the spot where you tap into the P-lead."

You can tap the P-lead anywhere from the mag up to the switch. At the switch is usually easiest, and closest to the EMS, but anywhere works. Wherever you do tap the P-lead wires, put the resistor right there.