See Figures 1 and 2
The points function as a circuit breaker for the primary circuit of the ignition system. The ignition coil must boost the 12 volts of electrical pressure supplied by the battery to as much as 25,000 volts in order to fire the plugs. To do this, the coil depends on the points and the condenser to make a clean break in the primary circuit.
The coil has both primary and secondary circuits. When the ignition is turned on, the battery supplies voltage through the coil to the points. The points are connected to ground, completing the primary circuit. As the current passes through the coil, a magnetic field is created in the iron center core of the oil. As the cam in the distributor turns, the points open and the primary circuit collapses. The magnetic field in the primary circuit of the coil cuts through the secondary circuit winding around the iron core. Because of the scientific phenomenon called electromagnetic induction, the battery voltage is increased to a level sufficient to fire the spark plugs.
When the points open, the electrical charge in the primary circuit jumps the gap created between the two open contacts of the points. If this electrical charge were not transferred elsewhere, the metal contacts of the points would melt and the gap between the points would start to change rapidly. If this gap is not maintained, the points will not break the primary circuit. If the primary circuit is not broken, the secondary circuit will not have enough voltage to fire the spark plugs.
The function of the condenser is to absorb excessive voltage from the points when they open and thus prevent the points from becoming pitted or burned.
It is interesting to note that the above cycle must be completed by the ignition system every time spark fires. In a V8 engine, all of the spark plugs fire once for every two revolutions of the crankshaft. That means that in one revolution, four spark plugs fire. So when the engine is at an idle speed of 800 rpm, the points are opening and closing 3,200 times a minute.
There are two ways to check the breaker point gap: it can be done with a feeler gauge or a dwell meter. Either way you set the points, you are basically adjusting the amount of time that the points remain open. The time is measured in degrees of distributor rotation. When you measure the gap between the breaker points with a feeler gauge, you are setting the maximum amount the points will open when the rubbing block on the points is on a high point of the distributor cam. When you adjust the points with a dwell mete, you are adjusting the number of degrees that the points will remain closed before they start to open as a high point of the distributor cam approaches the rubbing block of the points.
When you replace a set of points, always replace the condenser at the same time.
When you change the point gap or dwell, you will also have the ignition timing. So, if the point gap or dwell is changed, the ignition timing must be adjusted also.
See Figure 3
- Disconnect the high tension wire from the top of the distributor and the coil.
- Remove the distributor cap by prying off the spring clips on the sides of the cap.
- Remove the rotor from the distributor shaft by pulling it straight up. Examine the condition of the rotor. If it is cracked or the metal tip is excessively worn or burned it should be replaced.
- Pry open the contacts of the points with a screwdriver and check the condition of the contacts. If they are excessively worn, burned or pitted, they should be replaced.
- If the points are in good condition, adjust them, and replace the rotor and the distributor cap. If the points need to be replaced, follow the replacement procedure given below.
- Remove the coil high tension wire from the top of the distributor cap. Remove the distributor cap from the distributor and place it out of the way. Remove the rotor from the distributor shaft.
- Loosen the screw that holds the condenser lead to the body of the breaker points and remove the condenser lead from the points.
- Remove the screw that holds and grounds the condenser to the distributor body. Remove the condenser from the distributor and discard it.
- Remove the points assembly attaching screws and adjustment lockscrews. A screwdriver with a holding mechanism will come in handy here so you don't drop a screw into the distributor and have to remove the entire distributor to retrieve it.
- Remove the points. Wipe off the cam and apply new cam lubricant. Discard the old set of points.
- Position the new set of points with the locating peg in the hole on the breaker plate, and install the screws that hold the assembly onto the plate. Do not tighten them all the way.
- Attach the new condenser to the plate to the ground screw.
- Attach the condenser lead to the points at the proper place.
- Apply a small amount of cam lubricant to the shaft where the rubbing block of the points touches.
ADJUSTMENT WITH A FEELER GAUGE
See Figures 4 and 5
- If the contact points of the assembly are not parallel, bent the stationary contact so they make contact across across the entire surface of the contacts. Bend only the stationary bracket part of the point assembly, not the movable contact.
- Turn the engine until the rubbing block of the points is on one of the high points of the distributor cam. You can do this by either turning the ignition switch to the start position and releasing it quickly (bumping the engine) or by using a wrench on the bolt that holds the crankshaft pulley to the crankshaft. Be sure to remove the wrench before starting the engine!
- Place the correct size feeler gauge between the contacts. Make sure it is parallel with the contact surfaces.
- With your free hand, insert a screw driver into the notch provided for adjustment or into the eccentric adjusting screw, then twist the screw driver to either increase or decrease the gap to the proper setting.
- Tighten the adjustment lockscrew and recheck the contact gap to make sure that it didn't change when the lockscrew was tightened.
- Replace the rotor and distributor cap, and the high tension wire that connects the top of the distributor and the coil. Make sure that the rotor is firmly seated all the way onto the distributor shaft. Align the tab in the base of the distributor cap with the notch in the distributor body. Make sure that the cap is firmly seated on the distributor and that the retainer springs are in place. Make sure that the end of the high tension wire is firmly placed in the top of the distributor and the coil.
ADJUSTMENT WITH A DWELL METER
- Adjust the points with a feeler gauge as described above.
- Connect the dwell meter to ignition circuit according to the manufacturer's instructions. One lead of the meter is connected to a ground an the other lead is to be connected to the distributor post on the coil. An adapter is usually provided for this purpose.
- If the dwell meter has a set line on it, adjust the meter to zero the indicator.
- Start the engine.
Be careful when working on any vehicle while the engine is running. Make sure that the transmission is in Neutral and that the parking brake is applied. Keep hands, clothing, tools, and the wires of the test instruments clear of the rotating fan blades.
- Observe the reading on the dwell meter. If the reading is within the specified range, turn off the engine and remove the dwell meter.
- If the reading is above the specified range, the breaker point gap is too small. If the reading gets below the specified range, the gap is too large. In either case, the engine must be stopped and the gap adjusted in the manner previously covered. After making the adjustment, start the engine and check the reading on the dwell meter. When the correct reading is obtained, disconnect the dwell meter.
- Check the adjustment of the ignition timing.