since 1940

Testing the aircraft magneto coil

10-357164-1 Bendix magneto coil

hard starting
rough engine
fouled spark plugs
preignition, low power, and
unusual vibrations.
 

Worn or fouled spark plugs can dramatically raise the voltage required to fire the spark plug. Firing resistance is increased by raising the coil's temperature, increasing spark plug gap, increasing combustion pressure, dirty contacts at the spark plug, lead, and distributor. The higher the resistance to the spark plug the more voltage the coil's secondary windings produce. This high voltage in the magneto coil's secondary windings may cause the coil to fail. When this happens the engine becomes progressively harder to start and the engine may run rough.

The coil produces enough voltage to arc the gap. The intended gap is at the spark plug, but the high voltage arcs at the point of least resistance. The Slick magneto coil, for example, can produce up to 20,000 volts at up to 250 degrees F. Coils produce the high voltage in the 15,000 turns of #44 wire (smaller than a human hair) in the secondary windings. Normal spark plug firing voltage occurs at 2,000 to 4,000 volts.

When the magneto coil has to produce higher than normal voltage to fire the spark plug, high coil insulation stress occurs within the coil when the secondary field is at maximum saturation and the contact points haven't opened. If the resistance is too high the cellophane insulation between the secondary winding layers breaks down. When the cellophane insulation breaks down, the arc occurs within the coil instead of at the spark plug. Arching in the coil burns the insulation creating lower resistance within the coil windings. Lowered resistance allows more arching, which raises the coil's internal temperature further damaging the insulation. The coil's internal temperature rise eventually cracks the coil's epoxy covering.

High voltage from the secondary may then jump out of the crack and arc to the magneto housing. A cracked magneto coil means that the secondary windings are arching and is cause for rejection. Before the coil cracks there is a period in which the secondary is shorted. Secondary shortage lowers the voltage delivered to the spark plug causing a ragged and intermittent spark. This causes a potpourri of symptoms.

How can we detect a short in the secondary windings? The most common field test of the coil is an ohm meter from the secondary lead to the coil core. Unfortunately, the ohm meter won't detect many secondary shorts. Secondary coil short lowers resistance in the secondary. Shorts that occur near the end of the 15,000 winding change the total resistance by an undetectable amount. The ohm meter doesn't detect shorts that only occur under the stress of high voltage. Insulation between the secondary windings may be enough to insulate the thousandth of a volt that the ohm meter puts in the coil to measure resistance, but at 12,000 volts the insulation breaks down and produces a short.

Marginal coils may work at room temperature but during engine operation the coil can get as hot as 250 degrees F. Increased coil temperature increases the resistance in the secondary windings and may cause internal arching where the insulation has weakened. The engine runs fine for a half hour or so until the coil temperature rises. Sometimes the pilot complains about engine roughness at altitude. It's not altitude related but time related! When the mechanic checks the aircraft the coil is cold and the magneto fires normally. If the magneto tests fine on the bench test it also bench tests ok.

If you don't have the equipment to measure coil output voltage then you need to heat soak the magneto or coil before testing. We recommend 120 degrees F. as a safe temperature. The most accurate coil test is to measure coil output voltage on an oscilloscope or by firing the coil across an air gap. For example, a 6 millimeter air gap requires 8,000 volts from the coil. Coil's with secondary winding shorts fire intermittently across the gap. At a gap of 8 millimeter requires 12,000 volts. Any higher of a gap and you risk internal arching the coil's secondary windings, which shortens the coil's life.

Spinning a magneto in your hand may take 100 hours off the magneto. The voltage has to go somewhere. Without a spark plug to arc it may arc inside the coil creating a weak spot in the secondary insulation.

Slick magneto coil with small crack in side. When first found the crack had black soot around the crack from burning inside the coil. Because the crack is on the side of the coil it is very hard to see without removing the coil from the magneto. 

Old style Bendix coil. Failure caused by too many years of operation, neglect, and no preventative maintenance.

Cross section of Slick magneto coil

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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.