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Kato's 8

Well Known Member
Could someone give me a cliff note run down on how to get rpm readings?
I have brand new uninstalled Lycoming from Vans with standard slick magnetos
I do not have Tachometer or tach cable
I do have Dynon EMS 120 for Tach readings with wire leads run forward.
I have ACS ignition switch

How does this work with out tach cable? Ive seen other explanations and got confused. The easiest approach is what Im looking for

Do I hook the EMS directly to the P-leads with resistors? If so, how does thateven work?

Do I hook up a sensor to the tach cable fitting?

If I need a sensor, which one?

Any advice greatly appreciated!
 
Maybe this will help. I do not know about dynon. I have mgl avionics, here is how mine works. There is a sensor, it screws into the mag. There are two removable plugs on the side of mag. It uses the one closest to the engine. The sensor is actually what we (electricians) call a prox or proximity switch. As the mag turns a part inside passes close to this prox, it senses this and relays this "pulse" to the ems. The ems has to be set up for the number of pulses for ONE engine revolution. This interprets to rpm.

Bird
 
The Dynon connects to the P-leads thru resistors. In the two RVs that I installed D-120 EMS units, I just connected short jumper wires to the ignition switch terminals where the P-Leads connect, and ran the jumpers over to a ring terminal type of terminal strip (from Radio Shack) mounted behind the panel near the switch. I installed the resistors across terminals of the terminal strip (so I could easily change the values) and then ran the D-120's tach input leads from its harness over to the terminal strip.

I did end up having to use resistor values well in excess of 100K ohms (130-150k, can't remember exactly) on both these RVs (a -10 with IO-540 and a -8 with IO-360, both equipped with new Slick mags only) to get the Dynon tach readings stable.
 
Tim, thanks. Your explanation helps me understand how that works and it makes sense. I suppose ill need to find out from Dynon if that method is supported or not and if so, determine the proper sensor.

Neal, your method I've read about too and I fact, I have resistors supplied to me from Dynon kit. That in itself makes me think that is how I'm supposed to do it. I don't understand how the rpm is measured with out a sensor of some kind however.. Anyway, I already ran the Dynon wires for rpm thru the firewall. Dang! it sounds like I'd be needing to pull those back to back of panel and then over to ignition switch? The bus idea sounds good for easy trial and error with resistor values. do you have any photos that shows a little more clarity?

Thanks for such a quick response guys!!
 
I forgot one thing: on both methods, does rpm readings exist even when one mag is shut off during run up? I heard of problems in that area
Thanks again
 
The Dynon tach reads "pulses" from the p-lead. Not sure about the D-120 but the Dynon FD 180 has two tach inputs (a primary and secondary) so you do not loose RPM with one tach off.
 
The Dynon Tach setup is well explained in their manuals online e.g. D-120 Link

My understanding is that the resistor is for "safety", not (just) to make the Dynon work. If you connected the P-Leads directly to the Dynon, and your Dynon decided to have some fun and ground both leads, you suddenly have no engine :eek: By passing the P-Leads through the resistors, then even if the Dynon grounds the inputs, the Mags continue to run. If I am correct (no idea) that might affect where you put the resistors i.e. put them as close to the Mag switches as possible to avoid similar inadvertent grounding issues.

If you have run some extra wiring, suggest you leave it there. If / when you upgrade to P-Mags you need separate "P-Leads" and Tach wires, as well as Power for the P-Mag...
 
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