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Right Slick Mag Check, RPM to Zero

Alchemist

Well Known Member
Hello,

Every since I've own this plane when I check my right mag RPM goes to zero therefore you have to listen for an audible difference in RPM drop. I'm used to it but I'd rather it do what the LEFT mag does and see a slight dip in RPM instead of it going all the way to zero.

My panel is basic VFR , old steam gauges so i'm not exactly sure what I need to do to fix this slight annoyance. RIGHT and LEFT Mag are both Slick, I will have to find a model number if you need specifics.

Any wiring diagram or suggestions would be greatly appreciated.
 

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Just guessing here. If your RPM gage is electrically/electronically driven, I would suspect that it is sensing one of the mags.

When you say "checking the Right Mag" do you mean running on the Right mag only? If so, then the RPM sensor might be on the left mag.

A little wire-tracing should tell the tale.
 
Our eight is the same only it has a little switch that, when moved will supply the tach with the signal from the mag. Essentially it is switching the tach signal source back to the mag you deselected with the mag switch. Never understood why it was built this way. We're going to be doing a panel upgrade soon and this "feature" will be history once the Dynon is installed. Is there any chance that you have a switch on your panel that you have never been able to figure out what it does? If so, try moving it when the tach goes to zero.
 
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Tach issue

Just found this. Wiring diagram for my RV-8 tach described in last post. . Hope this helps.
 

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Just guessing here. If your RPM gage is electrically/electronically driven, I would suspect that it is sensing one of the mags.

When you say "checking the Right Mag" do you mean running on the Right mag only? If so, then the RPM sensor might be on the left mag.

A little wire-tracing should tell the tale.

That is correct , turning the key to right mag rpm goes to zero. I will look and see if there is a wire going to the right mag.

Just found this. Wiring diagram for my RV-8 tach described in last post. . Hope this helps.


Thank you ! I don’t have that switch but looks like I can easily add it.
 
Our eight is the same only it has a little switch that, when moved will supply the tach with the signal from the mag. Essentially it is switching the tach signal source back to the mag you deselected with the mag switch. Never understood why it was built this way. We're going to be doing a panel upgrade soon and this "feature" will be history once the Dynon is installed. Is there any chance that you have a switch on your panel that you have never been able to figure out what it does? If so, try moving it when the tach goes to zero.

My 6A is also like this - have to flip the switch between mags whenever I check them on the GRT.

I've gotten used to it, I'm debating switching to straight push-button start and mag switches for the new panel....
 
Speaking only of magneto ignition: If your EFIS (or electric tach) senses RPM via a P-lead connection, you need to wire both mags. If the instrument can't have both p-lead inputs at the same time a switch would be needed. If you use a vent hole pickup you only need one. If the mag is turning, the vent hole pickup will see the rotating magnet, whether the mag p-lead is grounded or not. I've had both (Dynon and then G3X) and that is how mine was wired.
 
I have the same issue. The problem arises - for me - because there is only one mag RPM input into the EFIS. So I have to listen to the engine when the left mag only is on.

I would need to rig the switch that 74-07 showed, or wait until my panel upgrade.

OR

Get one of those hand help RPM measuring devices you can use from the cockpit


OR

In standard mags there is, I believe, a port available into which you can place a Hall Effect sensor. I am told this will report RPMs. So it doesn't matter which mag you have grounded, you will be reading RPM's. This is what you feed to your display etc.

However I do not know:

1) Which hall effect sensor to use

2) What signal conditioning I would need to provide in order to translate the output of the sensor to the input values required by the EFIS.
 
You should be checking EGT rise

It's not all about the rpm drop, it's about checking that all cylinders have a rise in egt during the check to ensure you have ignition in all cylinders.
 
Back in the day

I went to the locally owned ( by my grandfather ) auto parts store. I needed a tach drive cable for my Rv4 under construction. I gave the older guy behind the counter ( where the giant set of books containing millions of part numbers from dozens of suppliers resided ) a drawing I made with the length of cable i needed and asked if they had anything close. They did not. He said but im sure we can fix you up one. He walked back down a row of shelves until he was out of sight and came back with a spool of tachometer cable and the housing and a other parts. He said “ follow” me, and we went into the back shop and he proceeded to cut the cable and housing to length and used a small metal die and form block clamped in a vice and attached the drive end to the cable. He let me do the second end. I now had the exact tach drive cable I needed and it was only a few bucks.

Still working, and the tach never quits when I do a mag check :)

Cm
 
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Thanks for the inputs guys , it looks like I don’t have a wire from the right mag going to a switch or rpm gauge. What it looks like I need to do is run a wire from the right mag to a switch that was posted above so I can toggle between left /right mag check. I don’t have any fancy EFIS , my panel is old so this looks like a quick fix until I can afford a fancy panel upgrade.

Thanks for the help guys !
 

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UMA Hall effect sebsor

Seems you have all the answers you were looking for.
Here is the Hall effect sensor from Spruce/UMA in case you don't want to fool with a switch while you are checking RPMS.
https://www.aircraftspruce.com/catalog/inpages/umamagtachsend.php
I had one of these in my RV-8 built 20 years ago with a round gauge panel.
worked very well and attaches right to the tach drive port on top of the crank case. You then route the wires to your tach and you will have a reliable RPM input source independent of magneto operations.
 
Seems you have all the answers you were looking for.
Here is the Hall effect sensor from Spruce/UMA in case you don't want to fool with a switch while you are checking RPMS.
https://www.aircraftspruce.com/catalog/inpages/umamagtachsend.php
I had one of these in my RV-8 built 20 years ago with a round gauge panel.
worked very well and attaches right to the tach drive port on top of the crank case. You then route the wires to your tach and you will have a reliable RPM input source independent of magneto operations.

That does sounds like a better solution, what rpm gauge would I need ?
 
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