1942 - 1947 Chevrolet Shop Manual



	12-24

		side of the condenser, which is grounded, equalizing the two sides of the condenser. This is fundamentally the same as when the counter air pressure in the surge chamber of the water system, overcame the surge pressure on the line. The surge chamber discharged and forced the water back in the line equalizing the pressure. IGNITION DISTRIBUTOR Since the spark must occur at a given instant in the engine crankshaft rotation, the switch must be operated by the engine itself. This switch is called the mechanical breaker, timer or interrupter. The breaker arm is pivoted at one end and carries at its free end a contact which is held against a similar contact by a spring acting on the breaker arm. The stationary contact is mounted on the breaker plate to permit adjustment of the points. When the contact points are together and the ignition switch is closed, current will flow through the breaker arm contacts and primary winding of the coil. The cam is positively driven by the engine. As it rotates, the lobes of the cam strike the rubbing block on the breaker arm, causing the arm to swing outward, thus separating the contacts or points. This interrupts the current which was flowing through the primary winding of the coil, and produces a spark as previously described. As the cam continues to turn, the lobe passes the rubbing block and the spring pulls the breaker arm back until the contacts come together again. The current then flows through the primary winding of the coil and the operation is repeated as before. The cam is not only positively driven from the engine, but it is also "timed" to the engine so that it interrupts the circuit through the coil at the instant the spark is required. The mechanical breaker is made in this form in order to open the primary circuit quickly and to operate at the highest speed. AUTOMATIC SPARK CONTROL The spark control is of the automatic type, that is, the spark is automatically advanced or retarded by two methods. First, the vacuum spark control gives quick advance to the spark, to give power at low engine speeds and when the throttle is opened quickly the spark is momentarily retarded, preventing laboring of the engine and pinging, until the load is relieved, at which time the spark is automatically advanced. At higher engine speeds, this vacuum spark control cuts out, retarding the spark for better economy and engine performance at these higher speeds.
	Fig. 52-High Pressure Water Line In Fig. 52 we have a high pressure water line with a valve located in the line. Just ahead of the valve is located an air chamber called a surge chamber. When the valve is opened, there is not a full head of water, because it takes some time to set a body of water in motion. During this time energy is stored in this volume of water, because of its speed of movement. If the valve is suddenly closed the pressure on the line plus the momentum of the moving body of water has to be stopped. This is many times greater in pressure than the normal line pressure. The surge pressure forces the water into the air chamber compressing the air. The air then offers a counter pressure which slows down the momentum of the water. A moment later the air pressure in the air chamber overcomes the surge pressure in the line, and forces the water back into the line. This action absorbs the surge without damage to the system. The condenser in the ignition system, Fig. 51, acts in exactly the same manner. When the ignition switch is closed, it takes a fraction of a second to establish the full current value flowing in the circuit, just the same as it required some time to set the water in motion. When the breaker points open the current tends to keep on flowing, just the same as the water tends to keep on flowing after the valve is closed. If there was no condenser, this surge of current would arc across the distributor points and burn them, just the same as the velocity of the water would carry away the valve in the water line. The current surges into the condenser attracting a positive charge on one side and a negative charge on the other, building up a counter voltage in the condenser, just the same as the water surging into the air chamber, compressing the air, built up a counter pressure in the air chamber. A fraction of a second after the points open, the counter pressure in the condenser overcomes the surge pressure on the line, and the condenser discharges from the positively charged side back through the primary winding of the coil, through the battery to the ground and back to the negative