Failed Mag Check

[Brandontwalker]

I have a new issue and could use some insight. I was departing on the way home from a long cross country and the left mag did not check on the run up. In fact, the engine just died when switching to left only. Taxied back to the FBO and the engine would not start again. A local A/P came over and diagnosed the issue as a failed ignition switch. He disconnected the p-lead to the left mag and all was well. Engine started fine again and the return trip was made. I had the ignition switch overhauled, reconnected the p-lead and now the same problem again. Ideas?  Is it possible that the p-lead is shorted and grounded to the airframe?

[kmyfm20s]

I had what I thought was the same problem. I was on my way to Texas from California when I stopped in Arizona to visit a friend. On run up before take off for one mag would run ruff and misfire. At first I thought it was a fouled plug but it wouldn't clear. I went to the mechanic on the field and he convinced me it was the ignition switch. He said the switch was fine on both mags but was causing problems when only the individual mag was selected. I flew straight to San Antionio and it ran great. One thing I did notice was the EGT's where higher than normal. I was flying at a lot higher altitude than I normally did so I thought it was because of that. Of course thats backward since you have less O2 higher so your EGT's should be Lower. On final approach to KSAT when I started reduce power it really ran ruff and was misfiring bad. After landing it almost quit on the runway and it was busy that day, yikes! I pulled into Landmark and as everyone was getting off their jets and I roll in backfiring and sounding like a jelopy, kinda embarrassing. Needless to say it was not the ignition switch it was actually a failed contacts(I think)in one of the mags. A relatively inexpensive fix and ran great after that.

[cliffy]

You might take a look at the P lead wiring right at the mag connection. I've had a couple over the years that the inner insulation was old and brittle with the core wire exposed allowing the shielding to contact the core and ground out the mag. Kind of an intermittent problem. Move the wire, problem goes away for a while then comes back. New P lead wiring -problem solved.

[Mcstealth]

On November 21, 2015 at 7:18:08 PM, Brandontwalker said:

I have a new issue and could use some insight. I was departing on the way home from a long cross country and the left mag did not check on the run up. In fact, the engine just died when switching to left only. Taxied back to the FBO and the engine would not start again. A local A/P came over and diagnosed the issue as a failed ignition switch. He disconnected the p-lead to the left mag and all was well. Engine started fine again and the return trip was made. I had the ignition switch overhauled, reconnected the p-lead and now the same problem again. Ideas?  Is it possible that the p-lead is shorted and grounded to the airframe?

I second the thought of looking for a loose wire or cracking solder. So many times intermittent issues boil down to a loose connection. 

Any updates?

 

[Brandontwalker]

Sorry for not posting an update. It was the p-lead to the left mag. The wire was badly worn and the outer shielding/ground was contacting the core. Easy fix though.

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[rbridges]

just for my education, do you simply replace the P lead?

[Brandontwalker]

My A/P desoldered the small washer on the end, cut off the frayed part of the wire and then reassembled the lead.

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[Guest]

8 hours ago, Brandontwalker said:

Sorry for not posting an update. It was the p-lead to the left mag. The wire was badly worn and the outer shielding/ground was contacting the core. Easy fix though.

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That's pretty common with old worn out wiring.  It's also a fast way to become a glider pilot.

Clarence

[jetdriven]

It's also a cause of alternator failure. The field wire shorts to ground in the same manner