IGNITION SYSTEM - TM-10-3930-623-120038

TM 10-3930-623-12

d. Tighten locknut to retain governor setting. If

4-20. The primary circuit is opened and closed by the

engine speed surges at governed speed, loosen locknut

contact points and the breaker cam. The cam is rotated

on surge adjust screw, and turn screw in until surge

by the distributor shaft. The shaft is driven by gearing on

ceases.

the engine camshaft, The distributor shaft and breaker

cam are rotated at one-half engine speed and the

breaker cam has the same number of lobes as there are

4-16. IGNITION SYSTEM.

cylinders in the engine. As each breaker cam lobe

4-17. The ignition system (figure 4-2) consists of the

passes under the breaker-lever rubbing block, the points

ignition coil, condenser, ignition distributor, ignition

are opened so that a high voltage surge is induced in the

switch, low and high tension wiring, spark plugs, and a

ignition coil. Thus, with every breaker cam revolution,

source of electrical energy (battery or alternator). The

one spark will be produced for each engine cylinder.

ignition system has the function of producing high

Since each cylinder fires every other revolution in a four-

voltage surges and directing them to the spark plugs in

stroke cycle engine, each cylinder will fire once for every

the engine cylinders. The sparks must be timed to

two crankshaft revolutions or every single distributor

appear at the plugs at the correct instant near the end of

breaker cam revolution.

the compression stroke with relation to piston position.

The spark ignites the fuel-air mixture under compression

so that the power stroke follows in the engine.

4-18. FUNCTION OF DISTRIBUTOR. The distributor

has three functions. First, it opens and closes the low

tension circuit between the source of electrical energy

and the ignition coil so that the primary winding is

supplied with intermittent, timed, surges of current. Each

surge of current builds up a magnetic field in the coil.

The distributor then opens its circuit so that the magnetic

field will collapse and cause the coil to induce a high

voltage surge. Second, the distributor has to time these

surges with regard to the engine requirements. Third,

the distributor directs the high voltage surge through the

distributor rotor, cap and high tension wiring to the spark

plug of the cylinder which is ready to fire.

4-19. There are two circuits through the ignition

distributor. One of these is the primary circuit which

includes the distributor contact points and condenser.

The other is the secondary or high tension circuit which

includes the distributor cap and rotor.

4-2.1