Questions on Magneto P-Lead signals and Hall effect sensors

[Saville]

Ok so after some investigation with the cowling off my RV-8, I discovered why I can only check one magneto (Left) during the Run Up:

A p-lead wire from each Slick Mag goes to the instrument panel and the Left-Right-Both-Start key.

There is also a wire coming from the Left mag p-lead and is fed directly into my TruTrak Engine Data Module (EDM). The output of the EDM is sent to the TruTrak EFIS for display.

There is only one tach input to the EDM. So the builder must have decided to use the "listen for a problem" when running on the right mag only during run up. WHY there is only one input I do not know.

I'm given to understand that when the p-lead of a mag is grounded (to test the other mag) the impulse data is lost.

I'm uncomfy with this and am looking around for a solution.

One idea is to install a Hall effect sensor into my Slick Mag. UMA makes one. I can then hook that up to the one TACH input on my EDM. My understanding is that the HE sensor will always give the rpm impulse info whether or not that mag is grounded.

However I have no idea what the signal coming out of that Hall Effect sensor is and whether or not the EDM would understand it. Does it look like a P-lead signal?

The other idea I had was:

since the two P-Leads do come through the firewall, I could get a digital tach and tap off those wires. But that's a complex solution.

I suppose I might use the Lycoming Tachometer drive but that, too, would need a signal that the EDM can understand. Are signals from Tach drive senders the same as p-leads?

Am I missing a simpler solution here?

Thanks!

 

[jabarr]

I have a tach selector switch on my subpanel. I normally just listen to the other mag but I have the option of selecting either mag for tach readout.

 

[Saville]

I have a tach selector switch on my subpanel. I normally just listen to the other mag but I have the option of selecting either mag for tach readout.

Did you wire that up by providing a "T" on the two p-leads going to your start switch?

 

[jabarr]

The 2 p-lead wires go to the switch terminals. I added the 2 tach wires to the ignition switch terminals and they go to the tach select switch. There are inline resistors in the tach wires to prevent tach problems from killing the mag. I don't remember what the resistor values were--you can go to perhaps your EDM manual or I know the values are in the Dynon installation manual which is available online.

 

[Ironflight]

You know how you currrently have a single wire coming from one mag and going to the the Tru Trak? Just duplicate that wire from the other mag, and run it to one side of a two-position switch. Take the currrent wire and run it to the other side. Then take the common lead output of the switch and run it to the Tru Trak. Now you can select either input.

Frankly, the "listen to it" is awfully reliable to see if you have a fouled plug, or the mag is sick. Most of the current EFIS's, however, have gone to two inputs (one for either side), because many folks want to look at the RPM on either one. You could always change the EIFS!

 

[Saville]

You know how you currrently have a single wire coming from one mag and going to the the Tru Trak? Just duplicate that wire from the other mag, and run it to one side of a two-position switch. Take the currrent wire and run it to the other side. Then take the common lead output of the switch and run it to the Tru Trak. Now you can select either input.

This seems like the simplest solution. However I will have to drill a hole through the firewall to run those 2 wires to the instrument panel switch, and also run the common wire back out, as the TruTrak EDM tach input is on the engine side of the firewall.. So that means I need to learn about drilling and sealing holes in firewalls - which is not a bad thing to know.

It also means that mag checking is a 2 step process - starter switch to Left; SPDT switch to Left; starter switch to Right and SPDT switch to Right.



Frankly, the "listen to it" is awfully reliable to see if you have a fouled plug, or the mag is sick. Most of the current EFIS's, however, have gone to two inputs (one for either side), because many folks want to look at the RPM on either one.

Having been brought up on spamcans, it came as something of a shock to learn that people use listening as the second mag check.


You could always change the EIFS!

Future project. The TruTrak EFIS is no longer sold and the Garmin 496 that the whole setup is wired for is old as well. With the ADS-B 2020 thing, I figure 2018/2019 will be the time to do a full panel upgrade. In which case this issue will go away.

 

[N941WR]

Am I missing a simpler solution here?

Thanks!

Install two P-mags and an EICommander.

 

[Saville]

Install two P-mags and an EICommander.

I wouldn't exactly call that a "simpler solution". But as I said above, the 2020 thing will likely cause me to redo the instrument panel with updated avionics...avionic which will sport 2 tach inputs.

 

[Saville]

Just checking:

Am I correct when I think that:

1) the engine RPM data comes along the p-lead and

2) When you ground that p-lead to test the other mag, that engine rpm data, on the grounded p-lead, is lost?


Thanks!