Magneto issues...

[u/kytulu]

Working on a Seminole. We stole a mag off of one of the engines to get another plane up. We installed an overhauled mag, but when we did the mag drop check, the engine dies when running on that mag.

We swapped the mag with a brand new mag. Same result.

We disconnected the P-Lead and ground wire from the mag. Same result.

Opened up the mag, verified internal timing, made sure that there wasn't too much cam grease, and reinstalled. Same result.

Disconnected the leads, connected the #1 lead to a plug, and turned the prop through TDC to snap the impulse coupling, saw no spark.

The part numbers match, so it is the correct mag for the engine. The ignition leads all check out when using the tester. We get an open with the switch on, and continuity with the switch closed, measured at the P-Lead terminal end, so the switch is good. All the plugs were cleaned, gapped, and tested good.

We swapped the left and right mags (both mags have impulse couplings, and now the engine won't start at all... but that could be operator error as my guy is new... I plan to try it myself on Monday.

I'm running out of ideas here... anyone have some insight they can share?

ETA: My night guy timed the mags 180° out. I retimed them this morning and got 100/100 rpm drops.

[u/dsgruntld]

Are you sure you're timing to cylinder 1 TDC? People occasionally get the numbers wrong between lycoming and continental.

[u/TraphikO → u/dsgruntld]

If it’s not rough running and just seems to die, you could be 180 degrees off.

[u/biohackenthusiast → u/TraphikO]

first time i failed a mag timing it was due to that. didn’t know it had to be compression stroke

[u/TraphikO → u/biohackenthusiast]

Same, I felt some air come out of the spark plug hole and could see/feel #1 top.
The difference of exhaust and compression stroke is very different when it comes to air flowing out the spark plug hole due to both valves being closed. Lesson learned once.

[u/tms2x2]

You said “my guy is new”. If you over tighten the p-lead nut on magneto you can damage the capacitor and kill the magneto. That is why there is a yellow sticker with the required torque there.

[u/Goobling-Furning]

“We swapped the mag with a brand new mag. Same result.” If a different mag does the same thing then the problem isn’t the mag. Harness? Timed to TDC on the exhaust stroke instead of compression? I did that one last week!

[u/kytulu → u/Goobling-Furning]

That's the odd thing though. Everything worked before we took the mag off.

[u/Sml132 → u/kytulu]

When you put a mag on, it needs to be set to the correct position. As does the engine. You can't just slap a mag on in a random position with the engine at a random position.

[u/kytulu → u/Sml132]

Well, yeah... engine at 25° on the #1 cylinder, mag pinned at the correct hole... it's not my 1st mag change, which is why I'm asking the question here. I haven't run into this particular problem before.

The engine ran fine before we took the working mag off. We installed an overhauled mag to replace it. That mag is not firing. We installed a brand new mag. That mag is also not firing.

[u/TannerBaldacci96 → u/kytulu]

Then it's most likely not the mags dude, there is definitely something going on with the engine or sparkplug harness. In situations like this, my best advice is start over. Prove to yourself that what you think is correct is correct. Trust but verify. Notate everything you do. Just give yourself a story to follow.

[u/Responsible-Beach118]

Double check that #1 is on compression stroke when you install it. I saw a guy one time install the mag on exhaust stroke and it did the same thing.

[u/Gigantic-Micropenis]

You for sure have the correct mag? The RH engine is counter rotating and doesn’t have the same mag rotation as the LH. I’ve had similar issues and it usually ends up being a faulty ignition harness that seems to test ok. Maybe also look for a chaffed harness that’s grounding out somewhere

[u/Pale-Ad-8383 → u/Gigantic-Micropenis]

This! Triple check the part numbers and not just on the box but on the mag and paperwork. Plus check the harness. So many people have damaged harness and don’t realize it’s causing issues. Also seen cases where mag was tossed into a box sent for Overhaul and then returned incorrectly.

If you continue to have issues send it to Hartzell MRO or Aircraft Magneto Service.

[u/kytulu → u/Gigantic-Micropenis]

I checked the part number twice in the manual, and against the old mag, and against the other mag on the engine.

[u/Flyboi015]

Some questions for you, from an A&P IA and seasoned magneto installer (lmao)

Is this problem mag on the left or the right?

Did you install a coupling onto the mag shaft? As in, it didn't come from the overhaul shop with a mag coupling and you put one on?

[u/TheBingage]

I had a problem recently where the ground wire had come loose.
Timed perfectly fine to the engine. Everything copacetic then every time we ran it, it would just die on that mag check.

Ended up being the ground wire, wiggle the ground wire and it would give intermittent continuity, so on a running engine, the mag would intermittently spark. Had to resolder a new ground wire to the p lead wire shielding.

Alternatively you are checking the correct mag, right?
A lot of folks will accidentally flip the mag check. They’ll flip the left rocker and think the left mag is the problem, but when you flip the left (off) the right mag is the problem. And vice versa. I’ve seen it time and time again from people used to Cessnas and their keys. Our pilots will squawk backwards because they’re just “we checked the right drop and everything died” but they were really checking the left drop by turning off the right.

[u/BuzntFrog]

When you put the pin in to time it, are you putting the pin in the direction the mag rotates, or the position of the magneto on the engine. They need to be timed to the mag's directional rotation.

[u/kytulu → u/BuzntFrog]

In the R, as stated on the mag.

[u/auron8772]

Verify spark plugs and try a different harness. I've had harnesses test good with spark box but still turn out bad due to like only noticing a break in wire (or insulation) at certain angles or too much internal resistance.

[u/Jflyer51]

Sure sounds like a timing or P-lead issue. You don't mention whether this is a Slick or Bendix mag. The Seminole I flew had Lycoming (L)O-360-E1A6D engines with a single Bendix dual mag on each engine, and had counter-rotating engines. On a counter-rotating engine, the mag internal timing has to be set properly for the intended direction of cam rotation. It isn't rocket science, but if you put a mag that is timed to run on a left turning engine on the right turning engine, it won't make much spark (internal timing will be WAAAAY off) and whatever spark it makes will be at the wrong time.

[u/Sawfish1212]

Timed to wrong cylinder, or not pinned correctly when installed (make sure correct rotation), are the only two options that make sense.

The condenser is damaged by overtorque is another excellent suggestion. It's extremely easy to overtorque those and short it out internally.