Mag Check With E-mag?

I'm on the verge of ordering an EMS system for my RV-7 project and I was wondering how the units detect RPM drops during Mag checks. Do the systems (VM-1000c, AFS 2500, and/or Dynon EMS-120) show the left-right mag drop as would a mechanical tach, or is "special" wiring needed? I take itt the systems use an RPM generator on one of the mags so how do they "read" RPM drop during mag checks?

Dave
C-GCPD..ready to build the panel

Good question I think there are some threads out there

I'll tell you what I did. I have two electronic ignitions and a GRT EIS 4000 (I recommend for bang for the buck).

To get around the loosing a Tach signal during Mag check (or EI box and spark plug check), I went with a Tach sender on the engines mechanical drive. Its a hall effect unit and sends out a 5v pulse once per rotation (two pulse per crankshaft rotation). It does not matter what the mags or EI's are doing, I always have Tach as long as the engine is rotating and there's power to the sender and EIS. It works and is simple, however cost of a Tach drive is the negative (I recall only about $30-$40).

What I recall about the way other folks did it is they pick directly off only one of the Mags or EI box's as the case may be. They just deal with the loss of tach signal with a "check". Especially with EI, the RPM drop is Nil when you shut the MAG off (if running one Mag one EI). Apparently they go by ear. Not ideal but what the heck.

The other way is wire up switching to flip the Mag or EI Tach signal between left or right. You could sync the switching up with what mag you have selected (or de-selected). I see some relays involved. I have not thought that through but seems doable, however I do not like the complication. Also when it comes to ignition I don't want to go crazy with experimental switching (if there is any chance it could affect the spark makers).

In the end I did not like loosing the Tach signal with single ignition Ops and did not like the complication of switching the Tach signal; therefore I ended up with the mechanical/electro solution (tach sender). I used a UMA sender. I don't care for the one Van's sells, which looks like something off a Bicycle.
G

PS: I got this off Lightspeeds FAQ, and apparently the new Plasma III (A) allows both tach leads to be tied together in parallel:

"13.) Does the Plasma CDI work with my electronic tachometer?

All Plasma CD Ignitions have a pulse output (on the input connector) for an electronic tachometer. The pulse characteristics are shown in the manual and most tachometers will respond to that signal. If you have two Plasma systems and you want to have a tach reading from either Plasma CDI, you need to run the signals through a selector switch.

If you are using Plasma III versions "A" (available after 12/05), you can connect the tach pulses from two systems together to one tach terminal.

Plasma III and Plasma II Plus CD ignitions also have a built in tach which provides a voltage output for a millivolt meter on the output connector. See the wiring diagrams for details."

4 methods

1. Install a switch to swop over which mag drives the tach function
2. Use a tacho sender on the tach drive output from the engine
3. Use an E-mag or P-mag that keeps sending the tach output when the sparks are turned off.
4. Drive from one ignition only and use your ears to compare the mag drop.

Pete

penguin said:

4 methods

1. Install a switch to swop over which mag drives the tach function
2. Use a tacho sender on the tach drive output from the engine
3. Use an E-mag or P-mag that keeps sending the tach output when the sparks are turned off.
4. Drive from one ignition only and use your ears to compare the mag drop.

Pete

Click to expand...

Copy cat George ( you left off dual LS plasma III's)

Sorry George, didn't have time to read everything that you wrote

I was worried about the same thing - only reading the RPM on one mag - until I really got thinking hard about it. Over all my years of flying, I have never had a mag check failure that wasn't 100% obvious by listening to the engine. The old "difference of 75 rpm" rule is good, but if you have a fouled plug, you're going to see a whole lot more drop than that, and the missing will be really apparent. I suppose that a subtle mis-timing between mags might be a little less noticable, but since you're going to be flying your own plane, and know what and when the maintenance has been done, you'll probably find that those kinds of problems won't crop up.

You can certainly do the switch thing if you're worried about it, but it's really low on my worry list, based on experience.


Paul

P-leads tied together?

penguin said:

Sorry George, didn't have time to read everything that you wrote

Click to expand...

I understand G

PS I just came across a post about tying two P-leads (dual mags) together with a capacitor? Don't know how that works or if its a good/bad idea. George

Good point, especially EI/mag combo

Ironflight said:

I was worried about the same thing - only reading the RPM on one mag - until I really got thinking hard about it. Over all my years of flying, I have never had a mag check failure that wasn't 100% obvious by listening to the engine. The old "difference of 75 rpm" rule is good, but if you have a fouled plug, you're going to see a whole lot more drop than that, and the missing will be really apparent. I suppose that a subtle mis-timing between mags might be a little less noticeable, but since you're going to be flying your own plane, and know what and when the maintenance has been done, you'll probably find that those kinds of problems won't crop up.

You can certainly do the switch thing if you're worried about it, but it's really low on my worry list, based on experience. Paul

Click to expand...

I think the most obvious (or NO rpm change) is when you have one Electronic ignition and one regular MAG. Turning the mag off results in no RPM change. So even with out a tach reading, if you go to EI only (Mag off) and you hear any change, than that would be unusual.

Also as you point out, dual traditional Mags is more subtial and having RPM on both Mags is more important to evaluate the quality of the spark for each Mag. I guess you than need to do the switch solution or the solution I mention above.

Some claim that you can tie the two p-leads of traditional mags together and isolate them with a capacitor? Don't know how that works, but it would be nice if it worked. Is there a danger of taking one Mag down with the P-leads tied? I would not do it unless I understood it better. May be some one can explain this idea?

George

Original question

Don't make this so difficult, The VM1000 or 1000c both come with a RPM sensor. The sensor style is determined by the type of mags you are using (slick or bendix). The sensor is installed in ONE magneto. The sensor counts the RPM of the internal working parts of the engine and displays the RPM on the indicator on the panel. A mag check is very simple. As RPM decreases the internal parts RPM decrease and the drop of RPM is shown on the Indicator. No switches, fancy wiring, etc. are necessary. You simply remove the vent plug on the magneto, install the included mag sensor, plug in the two included connectors, and you are finished. The computer does the rest. The only way the system could fail is if... 1- you would lose your battery voltage, 2-the internal gears in the engine broke (causing the mag to stop turning), or 3-a system component failure. I have one mag and one EI on my RV-6A and during runup it works fine. I have 2 mags on my RV-10 with excellent results. I think what is confusing is the fact that the sensor senses the RPM of a gear turning in front of it. When you switch from one mag to the other the mags still turn. The sensor does not care where you have the switch positioned. As long as the gear is rotating in front of it, it will show you the RPM.I hope I have better described this for you. If not I would be happy to try again. GOOD LUCK!!!


John
RV-6A 800 hrs
RV-10 100hrs

The Dynon Engine Monitors have two mag inputs, so no switch or other arrangements are needed. The unit will automatically switch between the two as you do the mag check.

Regards,
Mel Jordan
RV-6A Tucson

So more options (dual EI)

stewartj said:

The sensor is installed in ONE magneto. The sensor counts the RPM of the internal working parts of the engine and displays the RPM on the indicator on the panel. I have better described this for you. If not I would be happy to try again. GOOD LUCK!!!

John RV-6A 800 hrs RV-10 100hrs

Click to expand...

Good for you John, good info on the VM-1000c. I am not 100% what you mean about "internal working parts", but it sounds like you need at least one traditional magneto. Many folk are running dual EI. I guess I am a little turned off by the idea of modifying the Magneto.

Good info lets recap the ways to get around a switch for selecting a Tach input with electronic engine monitors:

1) use a tack drive independent of ignition. Type of engine monitor does not matter.

2) Have at least one E/P-mag, which always sends tach signal even while ignition spark is off. Type of engine monitor does not matter.

3) Dynon has a way to handle two separate inputs, mix match. It does not matter what ignition you have. I don't know if it can mix two tach signals from different types of ignitions like a Mag and EI (?).

4)Dual Lightspeed III (models since 12/2005) sends a coordinate signal from the two units. Type of engine monitor does not matter.

5) VM-1000c uses a modification to regular magneto for signal independent of P-lead. Requires at least one regular mag.

I have a GRT EIS 4000 so it takes one input. I have not decided on an ignition but my first choice is dual Lightspeed II's. That is why I went with the mechanical tach sender. Simple no switches and does not rely on the ignition at all.

It seems like it would be easy to make a little electronic module which can take two tach inputs and mix or switch it to one output signal. Any electronic experts out there who can make a little board to do the job for those who don't have a Dynon, E/P-mags or a VM-1000c with magneto ignition?

George.