The N.-E. Model O Distributor Ignition System is used on Dodge Brothers cars. This system provides high tension ignition for the engine by transforming the low voltage of the starter generator or the battery into a high voltage capable of jumping freely between the spark plug electrodes. This is accomplished through the agency of an induction coil, the primary winding of which, in series with an interruptor or contact breaker, receives current under normal running conditions from the starter generator. The starting and lighting battery, however, supplies this current instead of the generator whenever the engine is starting or running very slowly.

At each interruption of the primary current there is set up in the secondary winding of the coil a high tension current, and this current flows from the coil through a high tension cable to the distributor rotor from which point it is selectively conducted to the proper spark plug. Upon reaching the spark gap in the plug, it jumps from the inner electrode to the outer one, which is grounded, and then returns through the engine frame to the grounded end of the secondary winding on the ignition coil. The high tension spark thus produced in the cylinder ignites the gas and so brings about the necessary combustion.

Wiring (Fig. 62).—As will be evident upon reference to the accompanying wiring diagram, the primary circuit of the ignition system leads from the positive terminal of the battery through the charging indicator to the ignition switch binding post marked “Bat.,” thence, when the switch is turned on, through the switch to one of its binding posts marked “Ign. Coil.” Continuing on from this point through the ignition coil and the breaker contacts, it returns to the second switch binding post marked “Ign. Coil,” where it passes through the switch again. It then finally reaches the grounded negative terminal of the battery through the grounded terminal of the switch and the car frame.

The ignition switch is so constructed that it produces a reversal of polarity in the distributor circuit each time the switch is turned off and then on again. For this reason there is no necessity of making a distinction between the two wires leading from the distributor to the two switch binding posts marked “Ign. Coil,” because the operation of the system cannot be affected by the transposition of these wires. With this one exception, however, the ignition circuit connections must always be made exactly as indicated in the diagrams, if satisfactory operation of the system is to be maintained.

Ignition Distributor. (Fig. 63).—The model O ignition distributor is mounted on the right-hand side of the Dodge Brothers engine where it is held rigidly in position by means of four bolts. The horizontal shaft of the distributor is connected directly to the engine pump shaft through a flexible coupling, and runs, therefore, at engine speed. The vertical distributor shaft is driven from the horizontal shaft by means of spiral gears which reduce its speed to one-half that of the engine.

The complete distributor unit consists essentially of three self-contained assemblies: The ignition coil, the breaker box and distributor base assembly which include the automatic spark advancing mechanism. Each one of these three elements is so constructed as to be readily detachable from the distributor unit independently of the others.

Ignition Coil.—The ignition coil, which is contained in a separate housing, forming part of the distributor unit, is constructed for 12 volt service and operates directly on the starting and lighting circuit. The coil housing is attached to the distributor base by means of four screws and serves also as a cover for the automatic advance compartment. The high tension terminal located on the coil housing is designed to provide a safety spark gap, as well as to act as a binding post for the high tension cable which connects the coil to the distributor head.

Breaker Box and Distributor Head Assembly. (Fig. 64).—The breaker box and distributor head assembly is mounted in an upright position near the center of the distributor base and is secured in place by a large-headed screw in the vertical portion of the base. This screw projects into the annular groove in the vertical shaft bearing sleeve, thereby preventing the breaker box assembly from becoming detached from the distributor base and yet at the same time permitting it to turn freely from side to side. The short lug projecting downward from the manual control lever on the breaker box extends into the round hole near the middle of the distributor base and acts as a stop to limit the travel of the breaker box.

In case it should become necessary to remove the breaker box and distributor head assembly, the distributor head should first be detached from the breaker box and then, with the breaker box in the position of full retard, the exact location of the distributor rotor should be marked accurately on the edge of the box. This mark should be made with special care, because it has to serve as the sole guide for the correct position of the vertical shaft when the assembly is put back in place again on the distributor base. Moreover, while the breaker box assembly is separated from the base, the horizontal shaft in the base must not be turned from the position it occupied at the time when the location of the rotor was marked. If either of these precautions is neglected, the correct relationship between the several moving parts of the system will be likely to be disturbed to such an extent that the complete retiming of the distributor will become necessary.

Condenser.—The condenser, shunted across the breaker contacts to absorb the inductive surges that occur in the primary circuit at each interruption, serves to intensify the effect produced in the secondary circuit by these interruptions, and also to protect the breaker contacts from injurious arcing. It is contained in a sealed case which protects it against possible external injury, and is located in the breaker box close to the breaker contacts where its maximum effectiveness is obtained.

Being very substantially constructed, the condenser ordinarily requires no attention. If for any reason it should become inoperative, the best course is always to replace it with a new one, because condenser repairs are not economically practicable. The entire condenser unit can be easily removed, whenever desired, by disconnecting the two condenser leads from the breaker box binding posts, and then unscrewing the two nuts on the under side of the breaker box that hold the condenser case in place.

Breaker Contacts.—The breaker arm, which carries one of the two breaker contacts, is mounted on a pivot post from which it is thoroughly insulated by a fiber bushing. The helical spring, which is attached to the lug at the pivot end of the arm, holds it normally in such a position that the breaker contacts are kept closed. But the fiber block near the middle of the breaker arm lies in the path of the breaker cam and is consequently struck by each lobe of the cam as the vertical shaft revolves. Each of these blows from the cam cause the breaker contacts to be forced apart, and thereby produce the necessary interruptions in the primary circuit. The second contact is carried by the stationary contact stud, which is adjustably mounted in an arched support. With this stud properly adjusted the difference between the contact points when they are fully separated by the cam, is twenty thousandths of an inch (.020″).

If it should ever become necessary to renew the breaker contacts, a complete replacement of the entire breaker arm and the contact stud assemblies will in general be found to be the most effectual method of handling the work. The breaker arm can be removed by simply lifting it off its pivot bearing after its pigtail has been disconnected from the breaker box binding post. The spring attached to the breaker arm lug will slip off of its own accord as soon as the arm is raised sufficiently from its normal position. After the breaker arm has been taken off, the stationary contact stud can be removed by releasing its lock nut and unscrewing it from its support. To replace the breaker arm it is merely necessary to insert the lug in the spring, and then, with the spring held taut, to push the arm firmly down upon its pivot post until it snaps into position.

Breaker Cam.—The breaker cam, by which the interruptions in the primary circuit are produced has four projections on its working surface, so spaced that one of them strikes the breaker arm and causes the breaker contacts to be abruptly separated each time a spark is required. The cam is held in place on the upper end of the vertical shaft by means of a slotted nut and set of special lock washers. It should never be disturbed if avoidable, because its accurate setting is absolutely essential to the correct operation of the entire system. If, at any time, however, its position should become altered accidentally, it must be carefully reset at once in accordance with the timing directions given later on.

The breaker cam and the distributor rotor are both mounted on the vertical shaft and are rotated at exactly one-half engine speed. Accordingly, since the engine is of the usual four-cycle type requiring two revolutions of the crank shaft for one complete cycle of operation, the distributor rotor and breaker can make one revolution during the completion of each full cycle of the engine.

Distributor Head.—The distributor head contains five high tension terminals. The central terminal receives the current from the secondary winding of the ignition coil and transmits it to the rotor arm by which it is distributed to the four outer terminals. These outer terminals are numbered 1, 2, 3, 4 respectively, corresponding to the firing order of the engine, and are connected to the four spark plugs in accordance with their markings. The distributor rotor in completing one full revolution establishes contact successively between the rotor brush and each one of these four outer distributor terminals, each contact being made at the same moment that the primary circuit is interrupted by the action of the breaker cam. Thus when the spark plug leads are properly connected, the high tension current, as soon as produced in the secondary circuit, is conducted to the spark plug of the proper cylinder just at the moment when the gas in that particular cylinder is ready for firing. If, therefore, the spark plug leads ever have to be removed from the distributor head, they must always be attached again carefully in the correct order.

Automatic Advance Mechanism. (Fig. 65).—Combustion does not follow instantaneously upon the occurrence of the spark, however, because a small time interval is always needed for the gas in the cylinder to ignite. Consequently, unless some means are provided for offsetting the lag between spark and combustion, the explosion of the gas could not always be made to take place at exactly the correct moment under varying conditions of engine speed.

To compensate for this lag, therefore, there is incorporated in the distributor a centrifugally actuated mechanism, which is capable of automatically advancing or retarding the time of the spark in exact accordance with the rate of speed at which the engine is running.

The operating characteristics of the automatic advance are accurately proportioned to conform throughout the entire speed range with the requirements of the engine; and in order to insure the permanence of this relationship the device is so constructed as to be practically nonadjustable.

Manual Spark Control.—Besides this automatic advance there is also the usual manual control mechanism for changing the time of the spark independently of the centrifugal device. This manual control is for use principally for retarding the spark when starting or idling the engine or for facilitating carburetor adjustments. During normal operation of the engine, the spark lever on the steering wheel quadrant should be advanced as far as permissible without causing the engine to knock, and the actual regulation of the spark position be left entirely to the automatic advance mechanism. The arrangement of the manual control is such, provided the breaker cam is properly set, that when the spark lever is in the position of full retard, and the engine is running very slowly, the spark will occur in each cylinder at 5 engine degrees after the piston has passed the upper dead center of its compression stroke. With the spark lever advanced to the limit of its travel on the quadrant, the spark will occur 15 degrees before the upper dead center position has been reached by the piston on its compression stroke.

Timing the Distributor.—Whenever it becomes necessary to disconnect the distributor shaft from the engine pump shaft the exact relative positions of the two halves of the coupling joining these two shafts, as well as the location of the distributor rotor, should be carefully noted and marked. This is necessary in order to make possible the reëstablishment of the correct relations between the distributor shaft and the pump shaft when original conditions are being restored. Moreover, care must be taken to avoid turning the engine while the distributor is disconnected, because the proper timing relations can only be retained by keeping the position of the pump shaft unchanged during this time.

Should it ever happen, however, that the distributor has been taken off without the proper precautions having been observed, or that the timing arrangement has been disturbed in any other fashion, it will thereupon become necessary to make a complete readjustment of the timing relations of the distributor and the engine. This is to be done always after the distributor has been reconnected to the engine, the first step being to ascertain definitely the relative position of the engine pistons and valves. With this done, the positions of the breaker cam and the distributor rotor are then to be reset as directed below.

Since all the parts of the engine follow a regular sequence of operation, only the position of the piston and valves in the No. 1 cylinder need be considered in this process, and the three remaining cylinders may be practically disregarded. There are numerous methods, varying in their degree of accuracy, for locating the position of the engine pistons, but the most dependable one is that of removing the cylinder head so as to expose the pistons and valves to full view. With the head thus removed, the engine should be cranked slowly by hand until the No. 1 piston has risen to the top of its compression stroke and has just started to descend on its combustion stroke. At this moment the spark, when fully retarded, should normally occur in No. 1 cylinder.

Under circumstances where it is not convenient or desirable to remove the cylinder head the following approximate method for determining the location of No. 1 piston may be employed with a fair degree of success. Open the cocks of the priming cups on all the cylinders, and crank the engine slowly by hand until the No. 1 piston has just reached the top of its compression stroke. This can be ascertained by holding the thumb over the No. 1 priming cup and noting carefully the moment when the compression ceases to increase. After locating the dead center position of No. 1 piston in this way, turn the crank shaft a very slight distance further until the No. 4 exhaust valve is just at the point of closing. Under these conditions, provided the No. 4 exhaust valve lifter is in correct adjustment, the No. 1 piston should be approximately in the desired position of 5 engine degrees beyond dead center.

With the No. 1 piston thus carefully set in accordance with one of the above methods, preferably the former, bring the distributor into the position of full retard. To do this, disconnect the manual control attachment and turn the break-box as far as it will go in the direction in which the vertical shaft rotates. Then after making sure that the ignition switch is turned off, remove the distributor-head and the distributor rotor and the breaker box, and with a broad bladed screw driver back off the breaker cam nut until the cam is free to turn on its shaft. Next, replace the rotor temporarily, and turn the cam slowly until the breaker contacts just begin to open when the rotor occupies the position where it normally makes contact with the No. 1 distributor terminal. This adjustment can be made to the best advantage by turning the cam forward to separate the contacts then back again slowly until the contacts just come together, at which point the cam should be allowed to remain.

After the proper setting has thus been obtained, remove the rotor again and lock the cam securely in position by tightening the slotted nut that holds it. Finally, replacing the rotor, rock the vertical shaft backward and forward as far as the slack in the gears will permit, and note carefully the action of the break contacts. The setting of the cam must be so accurate that when the gears are rocked forward to take up the slack, the contacts will be just held apart and yet when the gears are rocked backward as far as the slack permits, the contacts will be actually closed.

A convenient method of verifying this adjustment is to turn on the ignition current and connect an ordinary 14 or 16 volt 2. c. p. lamp across the two binding posts of the breaker box. The lamp thus attached, will serve as a sensitive indicator for representing the action of the contact-points when the vertical shaft is rocked forward and backward to take up the slack in the gears. The moment the contacts begin to be separated, the lamp will light; but as soon as they are allowed to come together the lamp will at once go out again.

Should the test prove the first setting to be inaccurate, the cam must be readjusted, and the test repeated several times if necessary until the correct setting is finally obtained. Too much care cannot be employed in making this adjustment, because even a very slight inaccuracy in the setting of the cam will produce a considerably magnified effect upon the operation of the engine. This is due to the fact that the engine speed is twice as great as that of the vertical shaft.

General Care.—Under normal operating conditions the ignition system requires very little care aside from the usual precautions against moisture and dirt. There are, in fact, but three points of importance that need attention during service:

1. Lubrication.

2. Cleaning and adjustment of the breaker contacts.

3. Inspection of the wiring and the spark plugs.