Thanks to all who offered advice about my ignition problem from 3 weeks ago.
The popping noise in the headphones and the rough mag check were the result
of a broken wire on one of the ignition modules, not a high-voltage spark
plug wire. This is the same problem that Bob Jacobsen reported. When the
wire got hot from heat-soaking after engine shutdown, the insulation
softened and expanded, causing the broken wire inside to separate (no
problem when cool). We verified the problem by checking each plug wire with
an inductively-coupled timing light, while it was running roughly on one
ignition. Wiggling the bad wire to the troublesome ignition module caused
an immediate response. This wire could be stretched right at the connector
when hot, since the wire inside was broken.
In troubleshooting the problem, we also discovered that each ignition coil
drives a pair of spark plugs located opposite each other on the engine
(e.g., lower port front and lower starboard front, etc.). The normal
resistance between the plug connectors is about 17,000 Ohms, which is split
about evenly between the resistor in each plug connector and the resistance
through the coil from one wire to the other. Two of the wire pairs showed
open circuits, so we unscrewed the plug connectors, cut off about 6 or 8
mm., coated them with dielectric grease, and reassembled them. This
corrected the resistance readings, but didn't affect operation noticeably.
Still, it is an easy way to check the integrity of the spark plug
connections.
The broken wire made a very sharp 90-degree bend as it left the connector,
which is where it broke. (Another wire is routed similarly, and I won't be
surprised to see it break in the future as well.) We replaced the contact
in the connector, crimping a short wire into it that we routed in a loop for
a 270-degree bend, with the loop supported by cable ties. Even with the
connectors secured as well as possible with plenty of cable ties, they still
receive a rather vicious snap during the starting jolt of the 912S engine.
At least now I have plenty of spare contacts for the connectors.
Dave DeFord
N135TD (flying, 400 hours)
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