RPM Sensor 30K Resistor Location

[baldcaldwell]

The book says, "...connect each resistor as close as possible to the spot where you tap into the P-Lead."

I installed my resistor on the end of the wire that attaches to the P-Lead terminal on the Slick Magnetos. Did I do that correctly?

Thanks!

Also, while I'm here.... My engine is the Lycoming IO-540, do I understand that the tach pulse setting on the HDX should be set to 1.5 for a six cylinder?

 

[Albee]

Interesting choice to connect near the mag, when the manual shows connecting at the switch. Should work fine, and the only (minor) downside I can imagine is now you have a second wire leading from each mag through the engine compartment, firewall to your panel. Odds of that getting pinched & grounded are pretty low. I suppose since that wire is beyond the resistor it's possible grounding it wouldn't kill the mag too, so maybe it can't matter at all.
On the other, I just set my engine type (Lycoming O-360) and the defaults worked. One of my mags (the older one) needed a lower resistance value to read correctly.

 

[baldcaldwell]

I wouldn't say it was my choice to wire near the mag... My question was about the wording "...the spot where you tap into the P-Lead." That seems open for interpretation. I must have a different installation manual. Mine has no picture connecting to the switch.

 

[PYoder]

Maybe I misinterpreted this ..... I put the resistor ‘in-line’ at the mag switch - on the wire going to the EMS - right?

 

[baldcaldwell]

Not sure. That's what I'm trying to ascertain.

 

[kellym]

Not sure. That's what I'm trying to ascertain.

The optimal location is for the resistor(s) to be as close to the ignition switch as possible, where the P leads connect to the switch, and yes, you need a factor of 1.5 for six cylinder engines. That is how I have it in my RV-10 with IO-540 and works fine.

 

[baldcaldwell]

Excellent! Thank you!!!

 

[preid]

you'll need more than a 30K resistor on a io540, more like three times that.
How will you know? at full throttle it will spike well above 2700, save the time get a higher resistor from the start.
As already mentioned, near the switch on panel not in the engine p-lead directly.
Pascal

 

[kellym]

you'll need more than a 30K resistor on a io540, more like three times that.
How will you know? at full throttle it will spike well above 2700, save the time get a higher resistor from the start.
As already mentioned, near the switch on panel not in the engine p-lead directly.
Pascal

Not necessarily, probably depends on which mags you have and wiring specifics. IIRC I used 22K resistors. I have Bendix S1200 mags.

 

[baldcaldwell]

Not necessarily, probably depends on which mags you have and wiring specifics. IIRC I used 22K resistors. I have Bendix S1200 mags.

Interesting. I have Champion Slick Mags. I've run the engine up to 2,300 RPM (not flying yet) and no fluctuations so far. So I wonder....

 

[preid]

you won’t at 2300 but at full throttle 2700 you probably will.
go With it and see what happens and if it starts to go crazy move up resistors knowing why, if you even need to.
best of success!

 

[Dynon]

So just to explain the resistor location and the importance of it. It serves two functions: First, it attenuates the voltages so that they're usable by the EMS. As you've read, sometimes that needs tuning. The placement, though, protects you against grounding out the mag. If you ground the p-lead, the mag is hot. The resistor prevents that grounding for the wire that is upstream of the pick-off point. IE, if you somehow grounded the wired (it gets frayed, etc) that goes from the p-lead to the EMS, but it's upstream of the resistor, it won't ground the mag and make it hot. So you want to put the resistor right at the p-lead.

 

[baldcaldwell]

So just to explain the resistor location and the importance of it. It serves two functions: First, it attenuates the voltages so that they're usable by the EMS. As you've read, sometimes that needs tuning. The placement, though, protects you against grounding out the mag. If you ground the p-lead, the mag is hot. The resistor prevents that grounding for the wire that is upstream of the pick-off point. IE, if you somehow grounded the wired (it gets frayed, etc) that goes from the p-lead to the EMS, but it's upstream of the resistor, it won't ground the mag and make it hot. So you want to put the resistor right at the p-lead.

Excellent! Thanks for the clarification.