EIS 4000 tach signal

GRT's install info shows a tach input wire connecting to the P-lead contact on the mag with a 27k ohm resistor in series. Seems like any tach signal needs to be a pulsed signal proportional to engine RPM. Why would a connection to the P-lead contact work? I thought the P-lead received a steady 14V through the ignition switch. What am I missing here?

Rupester said:

GRT's install info shows a tach input wire connecting to the P-lead contact on the mag with a 27k ohm resistor in series. Seems like any tach signal needs to be a pulsed signal proportional to engine RPM. Why would a connection to the P-lead contact work? I thought the P-lead received a steady 14V through the ignition switch. What am I missing here?

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Terry,

There is no 14V input to the mags through the ignition switch. It is a ground lead the kills the mag when it is closed.

That P lead does provide a signal to EIS 4000 for rpm but only if the mag is running. If that mag is switched off, like when doing a mag check, its signal to EIS drops the rpm indication to zero.

So, to make the rpm indication work when doing a mag check, you have to have a signal from each P lead to a switch to select the opposite mag when doing the check. My switch is labeled left mag or right mag. Its a pain in the butt compared to not having to do that, but it does work.

There are automatic electric circuits that eliminate the need for the switch spitting the signals but I don't have one.

So this means there IS a pulsed signal from the mag on the P-lead when it's running. Big surprise to me. How does that compare to having a tach sender that goes in the vent hole ? (on a Slick mag). Is one waybetter than the other?
I have a keyed ACS ignition switch, so the EIS4000 (which has two tach inputs) automatically displays whichever tach source is functional, per GRT. When I do a left mag check, it will display the RPM signal coming from the LSEI on the right, and vice versa.

Rupester said:

So this means there IS a pulsed signal from the mag on the P-lead when it's running. Big surprise to me. How does that compare to having a tach sender that goes in the vent hole ? (on a Slick mag). Is one waybetter than the other?
I have a keyed ACS ignition switch, so the EIS4000 (which has two tach inputs) automatically displays whichever tach source is functional, per GRT. When I do a left mag check, it will display the RPM signal coming from the LSEI on the right, and vice versa.

Click to expand...

I don't know which mag signal source is better.

I've added a toggle switch and label, Left/Right. On mag check, I use the switch to change my rpm output to my EIS4000.