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How to test GRT EIS Tach input?

KazooRV-9A

Well Known Member
Patron
I'd like to test the Tachometer input to my GRT Sport (with EIS) EFIS unit before running the aircraft engine.
The EIS Tach lead connects to the "P" lead of the Magneto through a resistor to the EIS tach input.
I was wondering if anyone can suggest a way to test the Tachometer readout of the EFIS/EIS without running the engine?
Tnx, AC
 
If you are using traditional mags and have any mx savvy practical jokers around you- heads up on the shock trick.
 
Andy,
I don't know much about mags, but could you pull the mag out and spin it with a drill motor?

Jim Butcher
 
The level of effort required to pre-test this interface is not at all in alignment with the level of benefit associated with doing this testing.

If one is really concerned about running the engine and having valid engine RPM indications, purchase an optical tachometer and have it on hand as a double-check of the on-screen RPM readout. If the on-screen numbers are wrong use the optical tach as your primary until you find the source of error on the on-screen numbers.

https://www.aircraftspruce.com/catalog/topages/trutach.php?clickkey=39018

There's a very good chance that somebody in your area already has one of these and would loan it to you.
 
My reason to test the Tach input (and ALL other EIS and EFIS inputs) is to make sure that they are functional electronically before I rivet the front upper cowl on. The relative ease of correcting any wiring issues before closing the front, is orders of magnitude simpler than afterwards.
I'm not worried about fully verifying the accuracy of the tach or other inputs, only that they are indicating and look generally correct in magnitude.
That said, is there any reason that I can't for instance, breadboard up an automotive ignition circuit (car distributor) driven by a drill, using the RV's battery, and connecting the GRT EIS Tach wire to the distributor's points? I believe that this would provide a voltage square wave input, similar to the signature of a Magneto "P" lead, to the EIS that will drive an RPM readout?
I've been trying to call GRT support but have been unsuccessful here during the Holiday's, maybe I'll be able to reach them soon to ask about such testing...

IMG_2247_iHPEnoAuwWQHhjG9ShkKKN.JPEG
 
The only issue I have ever had with a GRT EIS tach that required time "upside down the wrong way in an airplane" is wire continuity. Every other issue was solved with work at the ignition source or with programming at the EIS display. Personally, I would ring out your tach wires end to end and be done with it.
 
That said, is there any reason that I can't for instance, breadboard up an automotive ignition circuit (car distributor) driven by a drill, using the RV's battery, and connecting the GRT EIS Tach wire to the distributor's points? I believe that this would provide a voltage square wave input, similar to the signature of a Magneto "P" lead, to the EIS that will drive an RPM readout?
I've been trying to call GRT support but have been unsuccessful here during the Holiday's, maybe I'll be able to reach them soon to ask about such testing...

IMG_2247_iHPEnoAuwWQHhjG9ShkKKN.JPEG

The P lead from a mag provides anything but a square wave though most EMS tach inputs can capture the mag signal as well as a traditional square wave. I have no idea what kind of voltage is seen on a P lead, but if the port can also handle a tach output, they are typically 12V. Be sure to get the polarity right and better use some resistors to keep the current down, else risk blowing up the components. I know that some EFIS systems require the installer to put the resistors in the circuit, so may not have a lot of protection built in. This is definitely the case for GRT. Connecting a battery directly to the tach input port would scare me to no end and you couldn't pay me enough to try that, but this is YOUR plane. The challenge in getting a good signal from a mag in an EIS installation is usually not in the wiring but in getting the right sized resistor installed, so some trial and error may be in your future anyways.

I am sure you have read it here before, but maintenance is a real thing and you don't ever want to paint yourself into a corner and not have access to things once the aircraft is assembled. Having to pull off a riveted panel to fix something is bad design and I would consider addressing that now. I can almost promise you that at some time you are going to need to get to things.
 
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Low-level signal ideas

Without knowing what kind of signal the EIS tach input requires, here are some ideas for generating known frequencies -

1. output of an electric guitar
2. output of a 9V wall adapter
3. earphone output of cellphone/PC running a tone generator app

Each of these has the potential to damage the EIS input - talk to GRT first.

Bo
 
Is this a remote mount EIS, NOT available from panel face? IMHO. the EIS from GRT is dam near bullet proof. Wire it up and look at cht egt, oil temp, pressure….zero risk on these.
 
Oh, and GRT support is stellar via emai. Alway less than 24 hours….most answers come from 6pm to 6am via email.
 
I have tested most of the other EIS inputs. The EGT, OAT, oil temp and CHT thermocouples were easy, just aim a heat gun at them and watch for temp rise.
Volts no problem, other Aux inputs such as oil pressure, fuel pressure, fuel tank level etc, are not too difficult. (See video of how we tested alternator, it was fun). Definitely plan to contact GRT before attaching the Tach lead to something other than the engine Mag.
The Mag "P" lead is attached to the primary windings of the coil, exactly like an automotive coil. The contacts in a Mag are essentially like the points in a distributor. Other than the effects of the capacitance in the primary circuit, the signal generated has to be like a square wave.
There was a 39Kohm resister included with the EIS, that installs in the Tach input wire. I have a signal generator, but no function generator...

I wish GRT would return my voicemail or that I could actually get something other than a recording during their support hours,,,

XYSzDIUemc4


IMG_2276.jpeg
2101p_pe_savvy2_16x9.jpg
 
I have tested most of the other EIS inputs. The EGT, OAT, oil temp and CHT thermocouples were easy, just aim a heat gun at them and watch for temp rise.
Volts no problem, other Aux inputs such as oil pressure, fuel pressure, fuel tank level etc, are not too difficult. (See video of how we tested alternator, it was fun). Definitely plan to contact GRT before attaching the Tach lead to something other than the engine Mag.
The Mag "P" lead is attached to the primary windings of the coil, exactly like an automotive coil. The contacts in a Mag are essentially like the points in a distributor. Other than the effects of the capacitance in the primary circuit, the signal generated has to be like a square wave.
There was a 39Kohm resister included with the EIS, that installs in the Tach input wire. I have a signal generator, but no function generator...

I wish GRT would return my voicemail or that I could actually get something other than a recording during their support hours,,,

XYSzDIUemc4


IMG_2276.jpeg
2101p_pe_savvy2_16x9.jpg

It is not like an auto spark system. It uses a magnetic generated power source and has a slow ramp followed by a sharp drop off. Looks more like a VR sensor output and most EFII systems use very different software for capturing a VR output vs a hall effect, which does look like a square wave (whole bunch more tuning required in the former). It also AC and is reversing polarity on each event and square wave is consisten polarity. Not saying it won't work only that it is not like a square wave 12V tach signal, which has sharp transitions from 0V to +12V.
 
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GRT Avionics returned my voicemail at their return from their Holiday time off, they were very helpful!!
As I had thought, the way to test the Tach inputs was to simply pulse +12V to the Tach leads. There are two leads for the EIS I have, one for each Magneto. The EIS Primarily works off of the Tach 1 input, but if there's no signal from Tach 1 (as when doing a pre-flight Mag check), the EIS reads from the Tach 2 input.
I used an old Datsun 240Z points distributor that I have (I used to race Datsun's in SCCA Regional road racing) and connected the RV Battery B+ to the distributor lug, and the Tach input lead(s) to the distributor body being very careful not to let the body touch anything else on the plane. Then I attached a battery powered drill to spin it. The EIS Tach input was then configured for the 4 cyl/4 stroke Lycoming and for Low sensing. Everything worked perfectly. See the attached video.
https://youtu.be/NQc9J1dz_P4
 
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