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Hot Mag

T

Tsabean

Active Member
After engine start, both mags show normal rpm drop during mag check. After a flight, prior to shutting down, I always check the mags to ensure they actually will turn off. The left mag operates normally but the right mag will not turn off.
Shut engine down with the mixture control. Hangar the aircraft with a “Live Prop” warning sign on the prop.
Next morning, start engine, mag check shows both mags again working normally. Apparently, when the mag gets warm, it won’t shut off.
Checked all the wiring, replaced the switch etc but no fix. My engine guy suspects something internally wrong with mag. He suggests I pull it and bring it to his shop for inspection.
Before I do that, has anyone experienced a similar issue?
O-320
Overhauled Slick mags (less than 5 hour run time)

Thanks, Tom
 
May not apply, but: many years ago in a 182 I did a mag check prior to shutdown, all okay. After shutdown, we turned the prop by hand (listening for suspected exhaust valve leak). On about the 5th turn, one cylinder (only) fired. No one killed, but close call. Subsequent investigation showed mag P lead broken right at the mag, but held in contact by cable ties - sometimes. On 5th turn it must have moved slightly, disconnecting.
 
Tsabean said:
After engine start, both mags show normal rpm drop during mag check. After a flight, prior to shutting down, I always check the mags to ensure they actually will turn off. The left mag operates normally but the right mag will not turn off.
Shut engine down with the mixture control. Hangar the aircraft with a “Live Prop” warning sign on the prop.
Next morning, start engine, mag check shows both mags again working normally. Apparently, when the mag gets warm, it won’t shut off.
Checked all the wiring, replaced the switch etc but no fix. My engine guy suspects something internally wrong with mag. He suggests I pull it and bring it to his shop for inspection.
Before I do that, has anyone experienced a similar issue?
O-320
Overhauled Slick mags (less than 5 hour run time)

Thanks, Tom
Click to expand...
If it has only 5 hours since the mag was overhauled, one would hope the shop will check it for free, obviously something wrong.
Can you run a temporary wire to the mag Plead and after it has warmed up (failed scenario) ground the mag with that wire to rule out your PLead wire?
 
I attached a temporary wire once before but can’t remember if tried it once the mag was warmed up. Removed the wire prior to first flight. I’ll try again before I pull the mag.
 
Bavafa said:
If it has only 5 hours since the mag was overhauled, one would hope the shop will check it for free, obviously something wrong.
Can you run a temporary wire to the mag Plead and after it has warmed up (failed scenario) ground the mag with that wire to rule out your PLead wire?
Click to expand...

You can run as many temporary wires as you want to your P-lead post to check it (it only takes one) - don’t fly with this setup, test only. If this fixes your mag grounding issue, then the problem is the P-lead. If not, your problem may be your mag. Make sure your P-lead connection is tight on both ends, and also make sure the shield connection on the mag is solid. This is often used as the grounding path in your mag/ignition switch in the off position. Make sure those connections on you ignitions switch(es) is also solid. If they are not solid, you don’t have a ground for your P-lead.
 
As a follow up, removed the connections from the shielded wire ends. Suspect there was an issue with the solder link. Ran a separate ground wire from the switch to the ground point on the mag. Problem solved.
 
There is another indication that you have an ungrounded mag, and you will notice it before flying. When you do the runup and turn off one mag, if the rpm does not drop at all, you are still running on two mags, one being ungrounded. That caught my attention a few years back, and it was a broken wire at the mag. We were flying Young Eagles that day. Imagine that.
 
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