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Checking the Aircraft Magneto  filter or magneto capacitor

First a little background: 

A magneto capacitor has three functions that should be tested:

1. Capacitance
2. Shunt AC voltages to ground
3. Insulate DC voltages from ground

A modern capacitor checker will check for all three and at the voltages experienced in the magneto. As the best capacitor checker only simulates a magneto circuit the best checker is the magneto itself.  A capacitor that has no capacitance causes the magneto points to arc. This results in point overheating which damages the point surface and causes the plastic cam follower to melt. So if you have bad looking points or a melted cam follower replace the capacitor.

How do you know the cam follower is melted rather than worn? A worn cam follower will have sharp edges; a melted follower will have blunt edges and often have melted plastic balled up on the backside of the wear surface. An 8X magnifying lens works best at distinguishing worn from melted.

Magneto capacitors often fail because the fragile electrical connection from the pigtail lead or the stud breaks inside the capacitor. This causes intermittent capacitance as the connection makes and breaks contact. This causes the capacitor to possibly test good but be intermittently bad. Point condition is the best detector of this problem.

Capacitors that do not insulate DC from ground effectively prevent the magneto from firing.

AB-349276 Magneto capacitor

Field Method of testing capacitors (warning: some bad capacitors will pass this test so if the capacitor fails the test then you know its bad. If it passes the test then well maybe it's bad and maybe its good).

 Both the aircraft magneto filter and magneto condenser are capacitors. Remove them from the circuit and discharge them by grounding the lead-in wire against the capacitor housing. Using an ohm meter check capacitor resistance between the housing and the lead-in wire. As the ohm meter charges the capacitor with the low voltage from the meter's batteries, the resistance should quickly increase until it becomes infinite. Capacitors should have an open circuit to ground for DC voltages. Anything less than infinite resistance shows that the capacitor is leaking. A leaking magneto filter  lowers primary voltage resulting in a weak magneto. Now that the meter has charged the capacitor you can switch your meter to the volt setting (2 volts or less) and see that the capacitor has retained the charge that your meter has fed it. If the capacitor hasn't retained the charge (reads zero volts) then the capacitor doesn't have any capacitance. The VOM meter is using substantially lower voltage than the circuit experiences in operation. Higher voltages may cause leakage whereas low voltages may not. For this reason this test will identify an obviously bad capacitor but will not absolutely prove the capacitor is good.

MF-3 Aircraft Magneto Filter.

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Disclaimer: sacskyranch.com contains abundant information relating to aircraft maintenance. The information provided  is not intended to supercede or supplement the F.A.A. approved  maintenance and/or operator’s manuals. Those F.A.A. approved manuals must be utilized when performing maintenance and/or operating aircraft.