Low RPM on D-180 w LSE II+ and P-lead

[himslv]

Aerosport 180 (Lycoming clone)
Lightspeed Plasma II+ Electronic Ignition digital rpm out
Slick Mag - P-lead tap through 30k resistor
Dynon D-180
ACS ignition switch

Problem:
Low RPM readings verified using Tru-Tach tachometer.

Details:
I have the digital pulse information from my Lightspeed plasma II+ electronic ignition feeding one input to my Dynon D-180 and a magneto P-lead tap through 30 k resistor (per Dynon manual) to the other RPM input of my Dynon D-180.

Problem is that with the P-lead Pulse per revolution set to 1, and the Plasm II+ Pulse / rev set to 2, my Dynon reads 2 to 3 hundred rpm low at 900 rpm (verified by Tru-Tach) and 4 to 6 hundred low at the high end (2100 to 2700 rpm).

I removed the Dynon and using a square wave generator providing 12 volt pk-pk referenced to ground tested both inputs. It is right on. I also dropped the voltage down to as little as 8 Volts Pk-Pk and the Dynon remained accurate.

Suspecting that the Dynon averages both inputs, I tested each input by removing the other. Both the Plasma II+ and the P-lead were reading low rpm's compared to a tru-tac (which can be calibrated by aiming it at flourescent lighting...pretty slick).

By Resetting the pulse rate of the Dynon to 1.5 for the Plasma II+, I get readings that are consistently 60 rpm low.  The P-lead with the recommended Dynon resistance of 30K reads 2 to 300 low.

My temporary fix at the moment is to remove the P-lead input to the Dynon from the Magneto (Slick brand via aerosport) and keep the Plasma II+ input set to 1.5 pulses / rev even though the Lightspeed manual says it should be 2 pulses.

My friend with the same identical arrangement doesn't have this problem.

Have searched this forum and have sent an email to Lightspeed requesting true nature of its digital output for my Lightspeed unit.

Would also like to know what the D-180 actually does with dual tack inputs. in other words, what is the display showing me? Averages? One or the other? One then another?

Will increasing the P-Lead resistance value (mine is not a 6 cylinder) increase the rpm?

I could haul an oscilloscope to the airport to actually see the signal inputs to the D-180, but I am hopeful this post will spare me that.

Thanks,
Vince_Himsl
RV8 N8432 Flying! (10 hours)

 

[dynonsupport]

The D180 doesn't average inputs. It uses either one or the other. Which one it uses is actually kind of random. If it sees pulses on one, it uses that one. If the one it's using goes to zero (like when you do a mag check) it switches to the other one. If that one is active, it uses it, if not, it flops back and forth until one is active.

Beyond that, the D180 only looks for pulses, so changing a resistor doesn't change the actual RPM value, unless it gets so big that we don't see some of the pulses, or so small that we see noise. In either of these cases, you usually get a lot of noise on the RPM value.

Not sure what is up with your setup given all of this. We generally don't just see fixed low values. The only thing I can think of would be that the clock chip in your unit is running slow. This is the same device that runs the actual clock in the EMS- can you leave the EMS on for 10 or so minutes and see if the clock seems accurate? Make sure you don't have a GPS or EFIS sending it time since this will reset it constantly and you won't be able to see any errors.

 

[himslv]

Thanks for the quick response!
To verify the Dynon clock, I ran it for one hour and measured it against my cell phone. At the end of an hour both read the same.

This didn't surprise me as when I was bench testing my D-180 with a square wave generatro, the Dynon read correctly.

I received word from Klaus at Lightspeed that my unit's tach output is identical to my friends.  

I have ordered a tachometer pulse generator from Van's aircraft and will use that instead of either the Plasma or P-lead inputs. This way I can foxus on rpm drop indications at runup rather than which input do I believe.

I will report on the accuracy of the pulse generator rpm readings when it is installed. For if it gives low RPM values then I will have to conclude there is a problem with my Dynon in spite of my tests saying it works correctly.

 

[PhantomPholly]

Some of the LightSpeed units do NOT output a 10v pulse. Klaus will fix it for you if send it in.

 

[himslv]

I have disconnected the Lightspeed, the P-lead, and installed Van's tach generator transducer. It has a tag that says it puts out 8 pulses / revolution. To get anything close to accurate at low end, I have to set my Dynon set to 3.5 pulses.

Still, as with the other inputs, I can't get the Dynon to read above roughly 2100 RPM at full power. I am going nuts trying to figure out why I can't get reliable tach readings on my Dynon. Hooking up square wave generator to Dynon tach input with unit removed from plane shows the Dynon to read correctly. This leads me to believe that there is not much signal conditioning on the tach input if only a perfect square wave will read correctly.

I will call Dynon support tomorrow as I really have no clue other than fact that three different tachometer inputs are read identically incorrect by the Dynon.

 

[PhantomPholly]

Vince,

It is EXTREMELY unlikely that the Dynon is mis-representing it's readings.  It simply uses a digital counter, which in turn relies upon pulses of at least 10v.

You mentioned problems with the LightSpeed unit, but never said whether you confirmed with Klaus that your unit had the mod capable of outputting 10v or not.  Since that is free and doesn't require installing new equipment, I would recommend going that route first.

For myself, I have a LightSpeed III that does NOT output 10v.  I have been satisfied simply having a single rpm source from the P-lead.  Also, at the time I made that decision Dynon was having some difficulties when multiple rpm sources output a different number of pulses per revolution - thus, at that time it would not have worked anyway.  I think they have solved that latter problem.

The only thing you really need 2 rpm inputs for is when you do your runup to determine mag drop.  If you've got a good sense of pitch, you can tell whether the engine slows more than an acceptable amount even though the rpm meter drops to zero.  If you lose rpms in flight, you really should be worried more about landing than your precise rpms.

Just my 2 cents worth...

 

[dynonsupport]

Vince,
We do have a lot of filtering on the input. We couldn't take a standard P-lead if we expected any kind of nice square wave. P-leads are nasty, 600V pulses that ring all over the place.

Given that the problem is persisting with different sender types, I think we should look at the things that don't change. I'd guess ground in this case. If all the senders read weird, this very well could be because there is something weird about how the EMS is relating ground to the sender.

If you say a frequency generator works, then look at how it's grounded and see if the senders you are trying to use also have an equivalent ground path.