The low oil pressure switch, normally open, closes on increasing oil pressure to 5 to 7 psi.
When the switch contacts close, the low oil-pressure circuit is deenergized. At rated engine speed, a
drop in oil pressure below 5-7 psi opens the low oil-pressure switch contacts, and, through electrical
Circuitry for ignition and engine electrical accessories is included in the electrical system.
The system includes the ignition exciter, spark plug, speed switch, tachometer generator, thermocouple,
and hour meter. The engine harness-assembly connectors mate with receptacles on the junction box, oil
pressure switch, ignition exciter, speed switch, and tachometer-generator.
Descriptions of the
components in the electrical system are given in the following subparagraphs.
Ignition exciter. The input voltage to the ignition exciter is 10 to 29v dc. Input voltage
is converted to an intermittent high-energy current which is directed to the spark plug for ignition.
Minimum spark rate is two per second at 14 volts.
Spark plug. The shunted surface-gap type spark plug is threaded into a boss in the end
of the combustor. The plug furnishes the spark necessary for initial ignition of fuel during the starting
cycle. Ignition is terminated by the No. 1 switch in the speed switch when the engine attains 90 percent
Speed switch. Mounted on the rear accessory drive pad in tandem with the tachometer
is the speed switch. Inside the switch housing, two flyweights are mounted to the speed switch drive
shaft. The flyweights move an actuating plate held in position by two springs of unequal strength. The
speed switch is two switches in one housing. The No. 1 switch is set at 90 percent rated speed, and the
No. 2 switch is set at 110 percent rated speed. As rpm is increased, the force of the lighter and then the
heavier spring is overcome, allowing the actuating plate to actuate the No. 1 switch (90 percent speed).
When the No. 1 switch is actuated, the start fuel and ignition systems are shut off. When the No. 2
switch is actuated at 110 percent speed, the main fuel solenoid valve is closed, and this shuts off fuel to