Last month's article began diagnosing a problem with my 1946 Packard Standard 8, 282 CID straight 8 (see archives section). The problem is loss of power at full throttle with backfiring through the tail pipe. This month, I will continue the overview of the ignition system from the Packard Service Training Program booklet, with my personal notes added. This is relevant for most old cars with points, plugs and condenser systems, and specific for 19th (1941), 20th (1942) and 21st (1946-47) Series Packards. Our '46 is an Autolite, 6 volt, positive ground. We are assuming a good battery, spark plugs and distributor.
The ignition system of Packard cars consists of the following essential units:
A- A battery and generator as the source of power
B- An ignition switch to control the starting and stopping of the engine.
C- An ignition coil to step up the voltage
D- A distributor to direct the spark to the proper spark plugs at the correct time.
E. Ignition cables to carry the current to the spark plugs.
F. Spark plugs to ignite the mixture.
SEQUENCE OF OPERATION:
With the ignition key turned on, closing the distributor contact points completes the ignition primary circuit. The current flows from the battery, through the ammeter, the ignition switch, the primary winding of the coil, and the contact points to ground. This flow of current through the primary winding creates a magnetic field around the coil and through the core. The current flow and magnetic field, however, do not increase to their peak instantly. It takes a small fraction of a second, called the build-up time, for the current flow and the magnetic field to reach their peak. This is due to the counter voltage induced in the winding by the increase of magnetism. The battery voltage, which forces the current through the primary winding, is opposed by the counter voltage.
Coil characteristics are so balanced with build-up time that, even at top engine speed when the contact points remain closed for the minimum time, the coil will build up sufficiently for good ignition. The amount of cam rotation between the closing and opening of the contacts controls the build-up time, and is known as the cam angle or dwell angle.
When the distributor contacts open breaking the primary circuit, the current attempts to continue to flow and tends to cause an arc across the points. The condenser prevents the arc by absorbing the sudden shock of the current caused by the opening of the contacts. Therefore, the magnetic field around the primary winding, induced by current flow, quickly collapses. It is this sudden collapse of the magnetic field that induces a high voltage in both the primary and secondary windings.
The voltage induced in the secondary winding causes current to flow through the coil to distributor cable, the distributor cap center contact and rotor to the brass contact lined up with the rotor at this time, then through the spark plug cable to the spark plug and across the gap of the spark plug electrodes to ground. The voltage induced is proportional to the turns of wire in the windings and the resistance at the spark plug gap. The induced voltage in the primary winding may be as high as 250 volts; and, consequently, the induced voltage in the secondary winding may go as high as 25,000 volts.
The voltage necessary to cause an arc at the spark plug gap is somewhere between 4,000 and 18,000 volts. The extra voltage represents the electrical reserve built into the ignition system. The voltage required to fire the plug varies with conditions including engine compression, engine speed, mixture ratios, spark plug gap, temperature, and many other conditions. These conditions must be taken into consideration when diagnosing ignition troubles and servicing the ignition system.
SERVICING THE IGNITION SYSTEM
Although good ignition is one of the requirements for good engine performance, it must also be remembered that good carburetion and good compression are equally essential. The ignition system is often blamed for engine failures and poor performance when the source of the trouble might be either the carburetor and fuel system or compression. To make a correct diagnosis, the procedure for checking the ignition system is outlined as follows:
1. Check the ignition primary and secondary wiring and connections. Replace any leads that are frayed, have broken strands, or have defective or deteriorated insulation. Clean and tighten all connections.
2. Check the distributor vacuum advance on the Six and Eight by rotating the complete distributor in the direction opposite that of the normal (counter clockwise) rotation of the breaker cam. To loosen the distributor, back off the screw on the spark retard/advance plate where the vacuum advance connects to the distributor. The distributor should turn freely and the vacuum advance spring should return the distributor to its original position. Another test is to disconnect the vacuum line at the carburetor and connect a vacuum/brake bleeder gauge. Pull vacuum, and the needle should stay steady at 20". If it falls, the vacuum advance diaphragm is bad.
On the Super Eight distributor, rotate the breaker plate clockwise to check the vacuum advance operation. The plate should rotate without binding. It should not wobble, and should return to its original position when it is released. On this type unit, be sure to check carefully the ground leads, known as "pigtail" leads, for fraying, broken strands, or broken terminal connections.
The centrifugal advance can be checked by rotating the breaker cam in the direction of its normal rotation. It should rotate freely and the centrifugal advance springs should return the cam and rotor to their original position without binding.
These quick checks indicate whether the advance mechanisms are working or not. However, for accurate testing of these units, the distributor should be removed and tested with a reliable distributor tester and instruments under various speeds and vacuum conditions.
3. While the distributor cap is off, wipe out the cap with a soft cloth. Inspect the cap and rotor for chips, cracks and carbon paths which would allow secondary current leakage to ground. Examine the contacts by holding them apart with the finger or thumb. Contacts that have been in service will not appear to be smooth and bright. This does not necessarily mean that they are not operating satisfactorily. On the contrary, they may be making contact over a greater area than could be obtained with new contacts. Clean the contact points with a fine-cut file. Blow out all dust particles. Make sure none remain on the contact surfaces. If the contacts are burned or pitted, they should be replaced.
Caution: Never use emery cloth or sandpaper to clean the contacts as the particles of these abrasives will be embedded in the contact surfaces and cause the contacts to burn. Use a point file instead.
4. Test breaker arm spring tension using the spring scale. The spring tension should be 19 to 23 ounces on the Delco-Remy distributor and 17 to 20 ounces on the Auto-Lite distributors. The correct tension may be obtained by shifting the breaker arm spring in its slot.
5. When the contacts require cleaning or replacement, remove the distributor from the engine, since the job can be done more easily and more accurately on the bench. Notice the position of the rotor and the distributor before removing it, so that it can be easily installed in the same approximate position and will require only a slight adjustment to complete the timing.
After the contacts are cleaned or replaced, adjust them by setting the cam angle using a feeler, to the correct opening of .020 inch on the Six distributor, and .017 inch on the Eight and Super Eight distributors. Be sure all contacts are aligned and have maximum contact area.
On the late-type distributors, the stationary contact is adjustable by loosening the lock screw and turning the eccentric screw for adjustment. The earlier type contacts are adjusted by loosening the lock nut and turning the contact in or out.
Whenever a synchroscope or some other good distributor tester is available, check the operation of the centrifugal advance throughout the entire distributor speed range. There are several reliable makes of synchroscopes and testers, any of which will test distributors satisfactorily.
6. After the distributor is tested accurately and the condenser is reinstalled, install the distributor on the engine with the rotor and distributor in the same position they were when the distributor was removed. Make sure the distributor is all the way down in its mounting and that the hold-down clamp or screw is tight. Be sure the vacuum line is tightened securely. Turn the grease cup in one turn. The ignition timing should then be adjusted correctly.
7. With the distributor cap removed and using the timing marks on the vibration damper to check crankshaft rotation, rotate the crankshaft in the direction of normal rotation until #1 piston comes up on the compression stroke with the intake and exhaust valves closed, and the marks on the vibration damper align with the pointer. Loosen the clamp screw and rotate the distributor base clockwise to advance the timing, or rotate it counterclockwise to retard the timing, until the breaker contacts just start to open with the rotor aligned with the #1 electrode of the distributor cap. Be sure to tighten the clamp screw when timing is completed.
The timing may be checked by means of a test lamp connected from the distributor primary terminal to ground. When the ignition switch is turned on, the light will not burn when the contacts are closed, and will go on just as the contacts break. Press the distributor cam lightly against the direction of normal rotation to remove all backlash.
An ignition timing light which operates on the current from the distributor to the #1 spark plug may be used to adjust ignition timing. By putting a chalk mark on the pointer and the correct timing mark on the vibration damper, the stroboscopic effect of the timing light with the engine idling will cause these two chalk marks to appear to line up when the ignition timing is correct. Caution: Be sure that the engine is idling no faster then 450 to 500 rpm when setting the timing with a timing light. An engine that is operating above 500 rpm may cause the centrifugal advance to come into operation and give inaccurate timing setting. With the correct timing, there may be a slight trace of spark ping when accelerating with wide open throttle from ten to thirty miles per hour in high gear.
8. Oil on the contact surfaces is the most common cause of burned contacts. The carbon formed by the burning of the oil will embed in the contact surfaces. This will cause the contact points to arc, and, consequently, to burn. Clean or replace the contact points and adjust the gap. Locate and eliminate the source of the oil. Do not over lubricate the breaker cam surface or the cam wick.
9. High voltage in the electrical system will cause excessive current to flow through the contacts. This results in a blue scale formation on the contact surfaces. When this condition is found, all electrical connections of the ignition circuit should be tightened and the voltage regulator should be checked and reset if necessary.
10. Inspect the insulation of the secondary ignition cables and the primary wiring for cracks, worn insulation, frayed insulation, and brittleness. Check the wires and cables for broken strands. Examine the terminals and connections for corrosion and looseness. Replace the wiring and clean, repair or replace the terminals if necessary.
11. The spark plugs should be inspected, cleaned and adjusted or replaced if necessary. Gap should be .028 inch and is adjusted by bending side electrode only.
I will continue next month with more on the ignition system. Keep warm and keep 'em driving!