IO-360M1B Tachometer sensor with Skyview EMS

[aerovin]

For anyone who has installed the IO-360M1B with the Skyview EMS:

The Skyview installation manual instructs that for those engines (using conventional magnetos) that do not have a dedicated tachometer transducer (and it would appear that this Lycoming engine does not), then the tachometer input comes from wires with resistors added that run from each of the two ignition switch P-Lead terminal to the EMS harness. The instructions specify to install the resistors as close as possible to the P-Lead pick off point so as to minimize the length of wire that carries high voltage spikes.

So, two things: does this wire to the EMS pose any potential grounding issues for the magneto whereby the mag is not grounded when you think it is (or it is grounded when you think it isn't)? Also, if the resistor is added right off the P-Lead terminal on the ignition switch, is this sufficient to avoid electrical noise presuming unshielded wire is used for the EMS sensing?

Thanks much for any input on this.

 

[airguy]

Good questions. I'm about a month from making those connections myself, and just recently read up on that connection.

The electrical noise is a product of frequency and amperage in the wire. In this case the frequency is definitely there to produce the noise but the current flow is very low thanks to the resistor (30k-ohm minimum, if I recall, can be up to 150k-ohm). The same principal applies to resistor spark plug wires - if you knock down the peak current you knock down the electrical noise. If you suspect noise from that wire you can always install a shielded wire there, it's relatively easy access in the airplane.

As far as grounding the mag, you'll need to pass a lot more current to ground to do that. You'll need a separate wire (no resistor) to the mag switch for grounding. The EMS is a high-impedance voltage sensor only, just looking for that voltage spike that comes from the P-lead on the mag. The situation is very similar to the oil pressure guage and restrictor fitting on the engine. The gauge (EMS) functions perfectly well through the restrictor (resistor) as long as the hose (wire) is not damaged. If the hose (wire) is cut (grounded) then the restrictor (resistor) prevents massive oil loss (grounding the mag).

 

[flyinga]

I used shielded wire for the p-leads and soldered the resistors ro the wire to the EFIS and attached it to the same pole of the mag switch as the p-lead. Works fine and no noise.