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Mag Idler Gear

K

kklewin

Well Known Member
*Idler Gear

Have a Lyc 0-360-A2A (Power Sport) with 2 sets of 3 yr old PMAGs on my 6A. Rt PMag wont keep its timing. When timed and prop is pulled through, the timing (which was TDC) moves 20 deg left (as viewed from the back of the fly wheel) with every full propeller rotation. I pulled the emag to bench test and all seems well there. They mentioned maybe a missing tooth on the idler gear (cant feel anything missing, but going to dig into this more when I get back to the hangar.)

The pmag had been running fine when I pulled it and was sent in for a flashing yellow light error. It would drop 100 rpm compared to the left one but thats it. When I reinstalled it ran terrible, backfired, etc. This was when I noticed that the "green light" that normally is indicated at TDC was off 20 deg per rotation. All wiring and plugs check good (tested, cleaned, re-checked for proper wiring).

If it is an idler gear, does the engine need to be torn down to repair? Thoughts?

Thanks,

Kurt
 
That’s an odd situation for sure.. you can usually pull the accessory housing off and access all the gears from the back of the engine without pulling the engine.. has the engine experienced a prop strike? There’s an ad where they want you to look at the camshaft dowel and replace the bolt..
 
No prop strike. 600TT since new. Pmag folks just suggested this might be the issue (Im out of town and cant verify currently). They also thought maybe the board in the pmag might be the issue, but are leaning towards mag idler gear. Just wanted to ping the corporate knowledge here first.

Kurt
 
kklewin said:
No prop strike. 600TT since new. Pmag folks just suggested this might be the issue (Im out of town and cant verify currently). They also thought maybe the board in the pmag might be the issue, but are leaning towards mag idler gear. Just wanted to ping the corporate knowledge here first.

Kurt
Click to expand...

Try switching the mags from side to side and see if the problem follows the mag?
 
Its moving 20 to 30 deg past TDC. Timing point is TDC, next rotation it will be 20 deg past TDC, next rotation 40 deg past TDC, etc.
 
Wow, that’s a weird one. So each full rotation of the propeller retards the timing to 20 degrees after top dead center (ATDC) ? And theoretically, if you kept rotating the prop you would eventually get to TDC again? How do you know this….just from the indicator light?
 
I sure hope this is not the reason, but a missing tooth on one of the gears would cause these symptoms.
 
Raiz said:
I sure hope this is not the reason, but a missing tooth on one of the gears would cause these symptoms.
Click to expand...

Highly unlikely but possible and would put metal in the oil or the sump. Let’s see….the cam rotates at * half speed of the crankshaft right? The theory of a missing tooth should be pretty easy to rule in or out w/o picking up a wrench IMO. Not sure an idiot light will help get us there however.
 
It worked fine before it was sent off for repair. Why would it be a gear issue now?

Wild thought - might there be some 6 cyl parts in the PMag now?
 
Mark a tooth on the the accessory drive gears and turn the engine two turns. Does the mark come to the same spot?

Swap the mags. Does the problem stay or move with the mag?

GM
 
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