SonexBuilders.net

If any of you guy's have a Jab engine and a MGL Rdac unit how did you connect the rev counter??. I have just run my engine for the first time but no matter what I do I can not get any reading using the Jab sensor picking up off the tabs on the flywheel.

Thanks

Hi Phil
I cant help with the Rdac but I had the same issue with the tacho on the MGL single. Fellow Sonex builder, friend and electronics tech Adrian Clout came round with his cro and a signal generator he worked out the output voltage from the hall effect sender wasnt high enough to register. He made me a simple amplifier which has been working great for 6 years now.

From memory I think the sender output was 3-4 volts and the MGL needed about 5volts

Steve
Sonex 892
VH-ZSX 195 hrs

If low voltage is the problem, the optocoupler solution I posted here some time ago would solve your problem:
viewtopic.php?f=6&t=2294&p=17456&hilit=optocoupler#p17456

Just a matter of choosing the right resistors, I used a 2K2 on the input side because the pulses on the alternator side can get up to 60V, if you're getting pulses of 3V or so you probably won't even need a resistor on that side (need to look at the specs of the optocoupler), you will get a perfectly square 12V pulse on the output side.

You can do away with the diode as well since you won't have AC current.

Hi Phil, I have the same set-up - Jab engine and MGL RDAC. My tacho works very reliably. My memory of it that I connected it as per the MGL installation manual, but I'm in Queensland right now, heading home tomorrow, and my documentation is unreliable. I think I had to pull out the link that connects the ballast resistor and also use a series resistor. I'll check it Friday but hopefully you'll have an answer before then.

Peter

Update on my rev counter issue. I found some instructions put out by Sonex where they say use a tapping off one of the Alternator wires as a signal so I tried that and I got it to work sort of. Anything above 1500 rpm seems ok, below that it is very erratic. I have asked Matt at MGL for suggestions. One of the guys at the airfield had a similar problem with a Jab 6 and a Dynon so he fitted a different type of Hall effects sensor that gives out a stronger voltage (same as Steve mentioned).

The RPM fluctuations seem to be a common problem with MGL. I've had them for quite some time. My wire was originally picked up off of one of the alternator wires. It would read the RPM but started fluctuating wildly when they got above 1600. So I switched the pickup wire from one wire to the other on the alternator and installed a ferrite bead thinking it was getting interference. That didn't work either. I moved the ground wire for the RDAC hoping that was the problem. Didn't help. I adjusted the dip switch and the sensitivity. Couldn't fix the issue. I even tried using RPM #2 input on the RDAC (usually for tail rotor speed or a second engine) and that didn't work. Eventually I emailed MGL and they said I should try to pick up the RPM off the Mag. I did that and it worked great...until the engine warmed up a bit. After the engine would warm up, the Mag I picked up the RPM from would intermittently cut out. It was picking up a ground from the RDAC and causing the Mag to go out. I traced the problem using a meter and found out that flipping the dip switch eliminated the grounding issue and the Mag ran fine. After I flipped the switch, however, the RPM wasn't as accurate. The RPM would read incorrectly (higher) as you applied more power. My cruise RPM was showing about 100+ more than it should have and minor fluctuations were back. It was usable until Friday. I went for a flight in the afternoon and, when I landed, the RPM was bouncing between 900 and 1500 and the application of power made it go crazy again.

I'm not an electrical guy. The only thing I can think of at this point is to install a resistor in the wire and flip the dip switch back to where it was when it was grounding the Mag but working perfectly. Still not an ideal fix because you can't see the mag drop when checking the Mags because the RPM goes to zero.

Decided to start my condition inspection a couple weeks early so I'll post any fixes I might find back here.

The MGL instructions for the RDAC suggest using the same connection as for a Bendix magneto - connect from the low tension side of one of the coils to the RDAC tacho input via a 10K resistor and remove the ballast resistor link. I actually used a 12K resistor, presumably because that was what I had available at the time. It works perfectly, completely reliable. I have checked it against an optical tacho and it is accurate to the resolution of the optical tacho.

Peter
Sonex 894

peter anson wrote:The MGL instructions for the RDAC suggest using the same connection as for a Bendix magneto - connect from the low tension side of one of the coils to the RDAC tacho input via a 10K resistor and remove the ballast resistor link. I actually used a 12K resistor, presumably because that was what I had available at the time. It works perfectly, completely reliable. I have checked it against an optical tacho and it is accurate to the resolution of the optical tacho.

Peter
Sonex 894

So essentially you just installed an in-line 12k resistor on the wire and are having success? Don't remember reading about that in the manual but I probably blew over the Bendix mag part.

vwglenn wrote:

peter anson wrote:The MGL instructions for the RDAC suggest using the same connection as for a Bendix magneto - connect from the low tension side of one of the coils to the RDAC tacho input via a 10K resistor and remove the ballast resistor link. I actually used a 12K resistor, presumably because that was what I had available at the time. It works perfectly, completely reliable. I have checked it against an optical tacho and it is accurate to the resolution of the optical tacho.

Peter
Sonex 894

So essentially you just installed an in-line 12k resistor on the wire and are having success? Don't remember reading about that in the manual but I probably blew over the Bendix mag part.

I just reread the RDACXF manual on this and it seems pretty simple. I've never gotten a good reading from the alternator across the whole range.
Not familiar with the "low tension side" term, but it seems you just connect were the p-lead is. Will be experimenting myself with this.

The link to the manual if you don't have it is http://www.mglavionics.com/RDACXins.pdf. The relevant detail is on page 27. Resistor should be 10K. By "low tension" side of the coil I just mean the connection for the kill switch. The tacho works even with the kill switch on. I guess that there is some resistance in the kill switch circuit so there is still a big enough spike getting through to drive the tacho.
The other part of the set-up is in the Stratomaster calibration where you set the number of pulses per 10 revs, which looks like it should be set at 20 for the 4 cylinder engine or 30 for the 6 cylinder engine. If your engine goes to 6000 rpm I'll admit to poor memory on this one.

Peter
Sonex 894