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-   -   Pmag Issue - Diagnosis Help (https://vansairforce.net/community/showthread.php?t=182881)

Kyle Boatright 05-26-2020 07:49 PM

Pmag Issue - Diagnosis Help
 
I'm finishing up my condition inspection and ran into an issue with the pMag. It was working fine before the inspection, so there's a good chance that the problem is/was related to the maintenance/inspection.

In any case, the symptoms are roughness and lumpy idle on the pMag below about 1300 rpm. That occurs with the pMag alone or the pMag+Mag. At 1300 RPM and below, the engine runs better on the mag alone than on both ignitions.

Above 1300 rpm, the engine runs fine.

These symptoms began a week or so ago when I awakened the engine after the condition inspection. As part of the inspection, I had pulled the pMag to check for shaft play and normal rotation. No problems were identified. To remove the pMag for the inspection, I disconnected the 6 pin connector that attaches to the pMag, and subsequently reconnected it. I've checked/tightened it since then, with no improvement.

In talking with Brad at Emagair, he had me troubleshoot the system by looking for problems in the power and grounding wires. No problems were identified. Tomorrow, I'm going to run a direct circuit from an independent battery to power the pMag to rule out (or in) a power/ground problem.

Any other suggestions?

Carl Froehlich 05-26-2020 08:35 PM

Tell us more:
- Clean/replace plugs?
- Reset timing on the pMag and/or Mag?
- Check plug wire resistance and connections on both ends?

Carl

Kyle Boatright 05-26-2020 08:46 PM

Quote:

Originally Posted by Carl Froehlich (Post 1433649)
Tell us more:
- Clean/replace plugs?
- Reset timing on the pMag and/or Mag?
- Check plug wire resistance and connections on both ends?

Carl

New plugs on the pMag. Cleaned/gapped the mag plugs. All 8 plugs appear normal and the issue does not appear to be on the mag side.

The mag timing was still correct (25*btdc), so I did not change it.

Yes, I reset the pMag's timing (back to 0* btdc). It is necessary to reset the timing when you remove the unit to check the shaft play (or lack thereof). I've reset the timing 3 different times. It is possible I'm doing it wrong, but if I am, I'm doing it with the manual in my hand, and after getting it right 8 years in a row. ;-)

Initial problem was with an 8 year old set of wires. One wire didn't pass the resistance check. A new wire set didn't cure the problem.

Still, the issue didn't occur until I pulled put everything back together after maintenance. I'm wondering if the 6 wire connector didn't hold up to another remove/reseat cycle and why things smooth out at ~1300 rpm.

Ron RV8 05-26-2020 08:54 PM

Bad plug on the Pmag? Check resistance... Switch back to old plugs to check...

GalinHdz 05-26-2020 09:05 PM

Did you try setting the Pmag to the same value (not TDC) it was before the problem started? :confused:

Kyle Boatright 05-26-2020 09:22 PM

Quote:

Originally Posted by GalinHdz (Post 1433659)
Did you try setting the Pmag to the same value (not TDC) it was before the problem started? :confused:

TDC is where it is supposed to be set. That is where it should have been before, and I (re) set it to that value 3 different times. A re-read of the manual confirms that I did it correctly unless my confirmation bias is out of whack.

If I can put my hands on the old plugs (Ron's suggestion) I'll try that tomorrow to see if a bad plug is the culprit.

Skysailor 05-27-2020 05:53 AM

Jumper tight?
 
Take a look at the wires in the connector on the Pmag. I have the jumper installed (as do most). The only problem I had with my Pmag was due to one end of the jumper not being tight in the connector. This caused the timing to change as the loose jumper made and un-made contact. Inspect the pins in the connector to verify they are clean when you check this. Certainly a quick simple check.

Any chance one of the new plugs is bad? If you still have the old ones or another set of new ones it would be another possibility. I agree with the post above to look first at anything touched since it was fine before.

pa38112 05-27-2020 06:53 AM

I have noticed that p-mags do not always install correctly. The seem to have a loose fit in the mag bore. I once installed one so that it seemed fine, but the gears were not actually meshing. I never ran it like this to see the symptoms, but I would remove and then re-install the p-mag so that you are sure it is seated properly with no force. When I did it everything looked fine, but it required tightening the studs to get it seated.
I would also check for vacuum leaks.

Scott Hersha 05-27-2020 07:22 AM

I had two issues with mine that created a similar situation to what you describe. One issue was with the connector. One of the wires, I think it was the ground wire was not tight in that green connector. I couldn?t pull it out, but I could tighten the screw holding it about a full turn. If you tin the ends of the wires, they might be able to withstand the crush they experience in that connector. I also had a connector that wasn?t seated fully in the Pmag. Again, it looked OK, but when I wiggled it, it seated in further and I was able to tighten the hold down screws a little. I didn?t actually run it in that condition, as I discovered it during re-installlation. I?m not a fan of that kind of connector on an aircraft engine.

The other issue I had was a suction leak in the manifold pressure line - again it was installation error. My initial install included a section of nylon pitot/static type line after the #3 aircraft hose, with a couple of those safe-air fittings. Those fittings work great for your pitot/static lines, but not so great in a vibrating engine environment under suction. It created some erratic engine behavior. I changed it out to standard aircraft engine hose (TS Flightlines), + aluminum hard line, then transitioning to the silicone lines supplied with the Pmag kit. Much better and it fixed the erratic behavior.

GalinHdz 05-27-2020 07:34 AM

Quote:

Originally Posted by Kyle Boatright (Post 1433663)
TDC is where it is supposed to be set. That is where it should have been before, and I (re) set it to that value 3 different times. A re-read of the manual confirms that I did it correctly unless my confirmation bias is out of whack.

Yes I know the manual says TDC but if it worked then and doesn't work now the equipment is trying to tell you something and I would listen.

FWIW my pMag install works best when set at 1* AFTER TDC.

:cool:


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