Skyview EMS and IO-540 P-Lead Resistors

[lolachampcar]

This appears to be an old subject but I am finding a dozen or so posts that show no resolution.

I have an IO-540 with Slick Mags and the "standard" Dynon resistors installed on the P-Lead pick ups.  Pulses/revolution is set to 1.5 and Dynon agrees with an optical tach a 1000 rpm.

As per other's experience, my RPM reading begins to jump at around 2300 rpm or so which indicates the need to reduce the signal at the Skyview EMS (increase the pick off series resistance).  I need to pull my center fuel tank to do this which is a bit painful.  I thought it best to check on the forum before embarking on this path.

What is the current status of IO-540 Slick mag P-Lead pick off with the Skyview EMS?  Are all (or nearly all) installations working well at both high and low rpm with the correct resistor size?  If they are, I would appreciate anyone with this configuration posting their resistor values and support providing their best recommendation as it is a time consuming process for me to pull the fuel tank each time to change the resistors.

Thank You

 

[jakej]

lolachampcar

FWIW - I've done more than 20 Dynon installations with Lycoming engines & never had to change the resistor value from the supplied standard 30KoHm. My guess the issues are to do with the magnetos, a bench test of them with a 'scope' at cruise rpm may help identify the fault.
Interesting to note too that I always add resistors, to dampen the rpm fluctuations, to the Jabiru setup despite Dynon saying they are not needed

HTH

Jake J

 

[lolachampcar]

Thank you for the feedback. I tried 100K, flew the plane for the first time and rpm went errtic on roll out (skipping high to the point of being useless).

For my application, I find the P Lead pick off functionality fails. I have an electric tach in another certified aircraft (from 1994 with Slick Mags and an IO-540) which uses the P Leads without external resisors and without fail. Speaking as an engineer, we either acknowledge a problem and correct it or pretend it is not a problem. I am guilty of this myself. Doing a search on P Lead and Dynon then counting the number of hits I get that include Slick and IO-540 tells the story.

The Skyview system continues to be great value and I would not have another system in a non-certified aircraft.

 

[dynonsupport]

The resistors are outside the box for safety. The whole idea is that you hook them near the mag, and then if the wire shorts out anywhere, the engine still runs. We could have put them inside, but then the wire would have been an issue. In fact, there ARE resistors inside, but we still want some outside.

The reason we need to suggest different resistors is that we need to support so many engines and ignitions. We support a C85 with normal mags, a IO-540 with plasma ignition, Rotax with CID, Jabiru with CID, etc. Sure, if we made SkyView for just one engine we could tune it better.

Remember, you can add resistance anywhere along the line. If you already have 30K at the mag, and it's hard to add a resistor there, add a 100K anywhere else along the line. Same effect, same protection since the 30K is there near the mag.

My personal opinion from experience is I'd go 150K with those mags. It's also important to do a mag check at the problem RPM and make sure that it's not just one mag acting up, because it can be a sign of the mag failing.

 

[lolachampcar]

Again, thank you for the feedback.  Regretfully, I've spent too much time tweaking notch filters trying to get these types of things working only to get it right in one set of circumstances only to have it "soft" fail with changes in temperature and the like.  I suspect a cap to ground to shunt the high frequency content as engine RPM increases might do the trick but there are just too many other items on  my ding list to head down that path.  Given that I have to pull a fuel tank for every try, I would prefer to go with a more concrete solution.

I've ordered both a hall sensor for the vent port of the mag and what I assume is a variable reluctance sensor based (described as providing an AC waveform) electronic tach transducer that attaches directly to the tach drive on the accessory case.  I'll have a better idea of exactly what it is when I can spin it up on a scope.  I've not re-read the Dynon documentation to see if the "low voltage" inputs can tolerate a zero crossing VR sensor input.  Will the Skyview EMS accept a VR input?  Also, not sure if the MGL hall sensor has a built in pull up or if it is just open collector.  If OC, can the low voltage inputs to the EMS be configured with a pull up or must this be done externally?

Thanks much for the help.

 

[dynonsupport]

We're not trying to build a filter with the resistor. We're just trying to reduce amplitudes with a straight resistor divider.

A P-lead signal has a big spike for the main pulse, then a trail of rings after that. As RPM increases, the amplitude of all of this increases. Eventually, the rings get big enough that they count for real signals and get counted.

Upping the resistance reduces all signals and reduces the amplitude of the ring below our threshold.

Remember, the resistor can be anywhere along the line. So you can add it at the magneto, anywhere along it, or at the EMS. So I doubt you need to remove the fuel tank to get to the wire.

If you really want to go with something else, your best choice is the RPM pickup from Vans. They sell this under PN VTACHGEN. This has been used many times successfully.

We've never messed with a VR sensor, so I don't know. We detect voltage above a threshold, not zero crossings.

An open collector input will need an external pull-up. You cannot configure a pull-up on the RPM inputs and there is not one internally.

 

[DBRV10]

I have been down this road with a perfectly serviceable (according to the mag shop) slick that was giving very high erratic readings at 2700 RPM, flickering to 3800 etc, and no amount of resistance would fix it.

Just on one mag, the right one in this case.

We found there were other issues. Possibly caused by some defect in the mag. So we replaced the mag with a new one. Problems all vanished.

Talking to George Braly today at lunch, and we both agreed that I will have to get a scope on this mag one day on a test bench.

I have isolated the problem, but never really gotten to the bottom of it :-?

 

[vlittle]

I have a new install, with SV 6.0 software;
Standard magneto on left, Electroair on right, IO-540 D4A5.

Tach indicates properly with Electroair, but drops to zero when Electroair is turned off. Magneto is still operating fine. Checked the data log, it shows identical rpm on both L and R inputs at all times, so it looks like the EMS is picking up the P-lead pulses.

I did mis-program the magneto input to 3 pulses/rev... I have since reset this to 1.5 but can't run the engine until my new prop governor arrives.

I've also since added a trimpot(100K) in series with the 30K ohm resistor so that I have in-cockpit control.


Something is not right: If I misprogrammed the input to 3 pulses/rev, why would it inidcate the correct rpm in the data log? Why would it go to zero when the electronic ignition is turned off.

What am I missing. The wiring is correct!

 

[Dynon]

So the datalog may not be the best indicator of what's going on on that pin, as there are some special behaviors that the rpm inputs use which may influence the data log. Start by confirming that the pin is actually configured in the EMS pin mapping setup.

 

[Dynon]

Also, if you export your settings to a USB stick and then look at the "rpm_policy" line, what is it set to?

 

[vlittle]

So the datalog may not be the best indicator of what's going on on that pin, as there are some special behaviors that the rpm inputs use which may influence the data log. Start by confirming that the pin is actually configured in the EMS pin mapping setup.

Sometimes it's a configuration problem! I found that the pin was not configured, so hopefully that's the fix.

Thanks,
Vern