Ricksrv4

Ricksrv4

Member
Want to ask about putting the P-Leads together with other wires through the firewall? Should P-Leads for the slick mags be kept separate when penetrating the firewall? I have a stainless steel firewall pass through planning to use for most of the wires needing to go through the firewall.
 
You can definitely run the two p-leads through the same whole with any full voltage switch wires (like solenoid, etc), but don't run them with any low voltage signal wires (like EGT/CHT).

P-Leads are relatively high voltage and electrically noisy. Even if everything is shielded, it's an easy way to have a "ghost" reading problem you don't want.

Just my .02
 
When the engine is running the p lead is an open circuit. It is not an ignition lead. It is not carrying a spark. All it does is ground the mag to the case so the mag can't fire when the switch is "off". So you should be able to run it anywhere you like.
 
sblack said:
When the engine is running the p lead is an open circuit. It is not an ignition lead. It is not carrying a spark. All it does is ground the mag to the case so the mag can't fire when the switch is "off". So you should be able to run it anywhere you like.
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Correct me if I'm wrong but I thought that, when the engine is running, both P-leads carry pulses which can be used by an electronic tach.
 
They are called "P leads" because they are connected to the magnetos primary winding. It is true that when the mags are not switched off, these wires are part of an open circuit, but they are still electrically connected to the magneto coil primary winding circuit, and therefore do carry alternating current voltage. Grounding the P lead obviously shuts off the mags by routing all of the A/C current to ground and away from the coil. I would not reccom end result naming the P lead wires through the same feed through as instrumentation wiring, but it will be just fine if it is routed through a feed through along with the main buss and alternator control wires.
 
Mike H said:
They are called "P leads" because they are connected to the magnetos primary winding. It is true that when the mags are not switched off, these wires are part of an open circuit, but they are still electrically connected to the magneto coil primary winding circuit, and therefore do carry alternating current voltage. Grounding the P lead obviously shuts off the mags by routing all of the A/C current to ground and away from the coil. I would not reccom end result naming the P lead wires through the same feed through as instrumentation wiring, but it will be just fine if it is routed through a feed through along with the main buss and alternator control wires.
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Yup... even "on" or ungrounded this is basically an antenna and the wire inside is making an electromagnetic field that will carry through air to copper anywhere around it. The EGT/CHT/OT/OP signal is really tiny like .001v/degree. So even a stray random +/-0.1v is huge.

Just save yourself the hassle and run another through whole.
 
RV4 P Leads

Thanks for the information, I think I will route the P-Leads solo through the firewall so the low voltage CHT/OP/OT wires will not be effected.
 
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