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P-MAG Sending Erratic RPM

hydraviper

I'm New Here
I'm a long time RV flyer, but a new RV owner. I just bought an -8 with dual Emagair P114L4 P-MAGs on an IO-360-C1C6. Midway through my first flight as the owner, I started getting extremely erratic RPM indications. It's CLEARLY an indication-only error, and the engine is holding actual RPM just fine. The previous owner has never experienced this before, but suggested it was likely one of the P-MAGs sending a faulty reading. Next flight I confirmed it's coming from the Right P-MAG, as when I ground it and run on just the Left P-MAG I get a normal, steady, RPM reading.

My question is, where to start the troubleshooting from here. Can the P-MAG be reset. Is it likely a wiring short? Do I need to have the P-MAG serviced/replaced? This might be a really dumb first post, but I'm just looking for anyone out there who may have experienced similar symptoms before. I'm trying to get to know my plane better, rather than just running straight to an A&P to fix every little nuisance.

Mike
 
Mike, not at all a dumb post I have duel p mags and get the same faulty reading some times but not all the time. I have not checked into it (as it just started about 10 hours ago) so I am hopeful some one who has found the problem will explained what they found. I have a io 360 and some times the rpm on my sky view will fluctuate or bounce from 2400 to 1600 and then it will be rock steady for a while and than be erratic again. Other flights it never happens and is rock steady which leads me to think it is a bad connection at the p mag, the next time I have the cowl off that is where I am going to start looking. Sorry can’t give you a better answer but you are not the only one. Btw the motor is smooth as silk when I get the fluctuation.
 
Eradic reading

Check your wire connections on the p mag. I had the rpm sending wire come loose and it would occasionally make contact and show an erratic reading on the rpm gauge.
 
I'm a long time RV flyer, but a new RV owner. I just bought an -8 with dual Emagair P114L4 P-MAGs on an IO-360-C1C6. Midway through my first flight as the owner, I started getting extremely erratic RPM indications. It's CLEARLY an indication-only error, and the engine is holding actual RPM just fine. The previous owner has never experienced this before, but suggested it was likely one of the P-MAGs sending a faulty reading. Next flight I confirmed it's coming from the Right P-MAG, as when I ground it and run on just the Left P-MAG I get a normal, steady, RPM reading.

My question is, where to start the troubleshooting from here. Can the P-MAG be reset. Is it likely a wiring short? Do I need to have the P-MAG serviced/replaced? This might be a really dumb first post, but I'm just looking for anyone out there who may have experienced similar symptoms before. I'm trying to get to know my plane better, rather than just running straight to an A&P to fix every little nuisance.

Mike

I have this pesky problem coming up on mine too. I have one brand new one with about 50hrs on it and the sister pmag that was rebuilt by emag at the same time. During my engine overhaul I was timing the mags and noticed the rpm was erratic with the engine OFF. Well I was there already and decided to trouble shoot. Left both mags on, pulled tach wire #6 on each and one mag (the brand new one) kept showing the erratic readings. I tried wiring a ground to the common source of the Dynon with a jumper. Looked at wires. Could not get it to stop. I have experimented with capacitance filters prior to my overhaul at the Dynon input with no luck. These are professional inline noise filters I bought specifically for this use. Anyways, the problem is back and I’m going to call them after my break in to try to get the tach source for the 2nd mag fixed. I like backups.
 
As a data point, I looked through all my logs, and I see about 98% of the readings show the same RPM for both PMAGs, ±2 RPM.

I have two RPM inputs in my GRT EFIS, and I have one PMAG connected to each. I think the second RPM input is usually used for twin engine aircraft.

Here are the top values:
Code:
RPM_Diff	Count
0	2798056
-2	452720
2	445247
4	9200
-4	8658
6	5867
-6	4820
8	4647
10	3670
-8	3313
12	2953
14	2451
-10	2362
16	2072
-12	1924
18	1735
-14	1603
20	1424
-16	1265
22	1240
-18	1063
24	1047
-20	967
26	860
...

Probably obvious, but it seems having bad values in a pmag is not normal, and should be investigated.
 
The wire connections (screw smashing a bare wire) at the Pmag unit are sketchy at best. I crimped a bit of brass tubing to the ends and that worked great. Also look at grounding.

I've had some weird RPM indications on Pmags in the past and in my case it was always a basic wiring issue.
 
Thanks to all for your feedback. The best piece of advice was from Carl, who told me to contact Brad at Emagair. Within minutes of me emailing him, Brad emailed me back with his phone number, at 6:00 in the evening. We talked on the phone the next day, while I was out at the plane. Pin #6 on the six-pin connector is the tachometer. He explained how a loose connection could easily cause excess RPM inputs, resulting in a high reading. He gave me detailed instructions on how to troubleshoot with a multimeter. Though it did not feel loose, I simply tried tightening the connection right there at the connector. That fixed the problem. It was a simple fix, and kind of the obvious first place to go. But it was reassuring to talk to THE subject matter expert, and confirm both my suspicions, and also that my "troubleshooting" wasn't misguided, and wasn't doing more harm than good.

I concur with Michael (Toobuilder) that the wire connections there on the PMAG leave a lot to be desired.
 
Within the last month I too had erratic tach readings with my P-Mags. Discussed all the problems and potential solutions with Brad. After removing the cowling I discovered the ground wire come from the left P-Mag harness had vibrated loose from the set screw assembly. Easy fix.
 
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