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P mag drop

Dennis Harm

Active Member
Hi everyone, just into my third flight on my 14A. I have the IO-390 thunderbolt with dual p mags. During run up and mag check I’m not getting any rpm drop when I move my toggle switch to test. Is this normal for p mags , just doesn’t seem right but there new to me. Wondering what everyone else is seeing with this setup, thanks,Dennis
 
Hi everyone, just into my third flight on my 14A. I have the IO-390 thunderbolt with dual p mags. During run up and mag check I’m not getting any rpm drop when I move my toggle switch to test. Is this normal for p mags , just doesn’t seem right but there new to me. Wondering what everyone else is seeing with this setup, thanks,Dennis

Assuming you are talking about testing the internal power of each Pmag I do believe you are correct.

The test switch is just taking ship power off and the Pmag is running on it's own power.
 
Watch your EGTs on the mag check. Turning one off you will see a slight rise in EGTs.

There is usually an RPM drop as well, but it tends to be much smaller than with standard mags.

To verify, did you time for 20 degrees BTDC for your IO-390? This is typically done by timing the pMags with the crank at 5 degrees ATDC.

Carl
 
I would suggest you study the system diagram/schematic to understand what it is you are doing with the test switch, then read the PMAG manual so you have a thorough understanding of it's operation.

What you describe is most likely normal as you are 'probably' just removing ships power from the PMAG and checking it's internal generator.

You should know 'exactly' what every switch function is, what it does, and what you should expect when operated, otherwise you're just a passenger in your own airplane.
 
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Hi everyone, just into my third flight on my 14A. I have the IO-390 thunderbolt with dual p mags. During run up and mag check I’m not getting any rpm drop when I move my toggle switch to test. Is this normal for p mags , just doesn’t seem right but there new to me. Wondering what everyone else is seeing with this setup, thanks,Dennis

Hi Dennis, you should get a mag drop. If you are not getting any drop, then something is wrong. As suggested by Walt, it would be good to do a bit more checking - you should be able to choose 3 different states for each PMAG, plus a maintenance state:

  1. Mag on with 12v external power (normal operating mode)
  2. Mag on with internal power (in case you lose electrical system)
  3. Mag off (on the ground with engine not running)

Maintenance mode is 12v external power with the mag in the "off" position. Off for the pmag, like regular mags, means that the "p-lead" is grounded.

Brad is very accessible - might be worth a call to him if after reading the docs it's not clear - it's really important to get this right and to feel comfortable with how the pmags work, and how to time them properly for your engine.
 
In addition to Walt's suggestion, I recommend double checking the timing and noting that 390 engines have a 20 BDC timing and you need to account for that with the PMAG as it is by default set to fire at 25 BDC. If you have not account it for that, your timing is incorrect and could account for no drop in RPM during mag check.
 
Hi everyone, just into my third flight on my 14A. I have the IO-390 thunderbolt with dual p mags. During run up and mag check I’m not getting any rpm drop when I move my toggle switch to test. Is this normal for p mags , just doesn’t seem right but there new to me. Wondering what everyone else is seeing with this setup, thanks,Dennis

Dennis, I have the exact same setup in my -14 and to answer your question, I show about a 75rpm drop on each Pmag when doing a typical mag check. As others have said, when checking in Pmag "test" mode to remove the ship's 12V power, you should have a zero drop.
 
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