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 and onto 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 coil. When the cam in the distributor turns, the points open, breaking the primary circuit. The magnetic field in the primary circuit of the coil then collapses and cuts through the secondary circuit windings around the iron core. Because of the physical principle 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 tries to jump 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 start to change rapidly.
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.
If you have ever wondered why it is necessary to tune your engine occasionally, consider the fact that the ignition system must complete the above cycle each time a spark plug fires. On a 4-cylinder, 4-cycle engine, two of the four plugs must fire once for every engine revolution. If the idle speed of your engine is 800 revolutions per minute (800 rpm), the breaker points open and close two times for each revolution. For every minute your engine idles, your points open and close 1600 times (2 x 800 = 1600). And that is just at idle. What about at 3000 rpm-
There are two ways to check breaker point gap; with a feeler gauge or with a dwell meter. Either way you set the points, you are adjusting the amount of time (in degrees of distributor rotation) that the points will remain open. If you adjust the 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 meter, you are measuring the number of degrees (of distributor cam rotation) that points will remain closed before they start to open as a high point of the distributor cam approaches the rubbing block of the points.
If you still do not understand how the points function, take a friend, go outside, and remove the distributor cap from your engine. Have your friend operate the starter (make sure that the transmission is not in gear) as you look at the exposed parts of the distributor.
There are two rules that should always be followed when adjusting or replacing points. The points and condenser are a matched set; never replace one without replacing the other. If you change the point gap or dwell of the engine, you also change the ignition timing. Therefore, if you adjust the points, you must also adjust the timing.
INSPECTION AND CLEANING
See Figure 1
The breaker points should be inspected and cleaned at 6000 mile (9600 km) intervals. To do so, perform the following steps:
- Disconnect the high tension lead from the coil.
- Unsnap the two distributor cap retaining clips and lift the cap straight up. Leave the leads connected to the cap and position it out of the way.
- Remove the rotor and dust cover by pulling them straight up.
- Place a screwdriver against the breaker points and gently pry them open. Examine their condition. If they are excessively worn, burned, or pitted, they should be replaced.
- Polish the points with a point file. Do not use emery cloth or sandpaper; these may leave particles on the points causing them to arc.
- Clean the distributor cap and rotor with alcohol. Inspect the cap terminals for looseness and corrosion. Check the rotor tip for excessive burning. Inspect both the cap and rotor for cracks. Replace either if they show any of the above signs of wear or damage.
- Check the operation of the centrifugal advance mechanism by turning the rotor clockwise. Release the rotor; it should return to its original position. If it doesn't, check for binding parts.
- Check the vacuum advance unit, but removing the plastic cap and pressing on the octane selector. It should return to its original position. Check for binding if it doesn't.
- If the points do not require replacement, proceed with the adjustment section below. Otherwise perform the point and condenser replacement procedures.
REMOVAL & INSTALLATION
- Mark or tag then remove the coil high tension wire from the top of the distributor cap. Remove the distributor cap and place it out of the way. Remove the rotor from the distributor shaft by pulling up.
- On single point distributors, remove the condenser from the distributor body. On early dual point distributors, you will find that one condenser is virtually impossible to reach without removing the distributor from the engine. To do this, first note and mark the position of the distributor on the small timing scale on the front of the distributor. Then mark the position of the rotor in relation to the distributor body. Do this by simply replacing the rotor on the distributor shaft and marking the spot on the distributor body where the rotor is pointing. Be careful not to turn the engine over while performing this operation.
- Remove the distributor on dual point models by removing the small bolt at the rear of the distributor. Lift the distributor out of the block. It is now possible to remove the rear condenser. Do not crank the engine with the distributor removed.
- On single point distributors, remove the points assembly attaching screws and then remove the points. A magnetic screwdriver or one with a holding mechanism will come in handy here, so that you don't drop a screw into the distributor and have to remove the entire distributor to retrieve it. After the points are removed, wipe off the cam and apply new cam lubricant. If you don't, the points may wear out in a few thousand miles.
- On dual point distributors, you will probably find it easier to simply remove the points assemblies while the distributor is out of the engine. Install the new points and condensers. You can either set the point gap now or later after you have reinstalled the distributor.
- On dual point models, install the distributor, making sure the marks made earlier are lined up. Note that the slot for the oil pump drive is tapered and will only fit one way.
- On single point distributors, slip the new set of points onto the locating dowel and install the screws that hold the assembly onto the plate. Don't tighten them all the way yet, since you'll only have to loosen them to set the point gap.
- Install the new condenser on single point models and attach the condenser lead to the points.
- Set the point gap or dwell (see the following sections).
ADJUSTMENT OF THE BREAKER POINTS
With A Feeler Gauge
SINGLE POINT DISTRIBUTOR
See Figures 2, 3, 4, 5 and 6
- If the contact points of the assembly are not parallel, bend the stationary contact so that the points make contact across the entire contact surface. 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 center bolt which holds the crankshaft pulley to the crankshaft.
- Place the correct size feeler gauge between the contacts (see the Tune-Up Specifications chart). Make sure that it is parallel with the contact surfaces.
- With your free hand, insert a screwdriver into the eccentric adjusting screw, then twist the screwdriver to either increase or decrease the gap to the proper setting.
- Tighten the adjustment lockscrew and recheck the contact gap to make sure that didn't change when the lockscrew was tightened.
- Replace the rotor and distributor cap, then connect the high tension wire which connects the top of the distributor and the coil. Make sure that the rotor is firmly seated all the way onto the distributor shaft and that the tab of the rotor is aligned with notch in the 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 clips 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.
See Figures 7 and 8
The two sets of breaker points are adjusted with a feeler gauge in the same manner as those in a single point distributor, except that you do the actual adjusting by twisting a screwdriver in the point set notch. Check the Tune-up Specifications chart for the correct setting. Both are set to the same opening.
See Figure 9
The dwell angle or cam angle is the number of degrees that the distributor cam rotates while the points are closed. There is an inverse relationship between dwell angle and point gap. Increasing the point gap will decrease the dwell angle and vice versa. Checking the dwell angle with a meter is a far more accurate method of measuring point opening than the feeler gauge method.
After setting the point gap to specification with a feeler gauge as described above, check the dwell angle with a meter. Attach the dwell meter according to the manufacturer's instruction sheet. The negative lead is normally grounded while the positive lead is connected to the primary wire terminal which runs from the coil to the distributor. Start the engine, let it idle and reach operating temperature, then observe the dwell on the meter. The reading should fall within the allowable range. If it does not, the gap will have to be reset or the breaker points will have to be replaced.
Using A Dwell Meter
Dwell can be checked with the engine running or cranking. Decrease dwell by increasing the point gap; increase by decreasing the gap. Dwell angle is simply the number of degrees of distributor shaft rotation during which the points stay closed. Theoretically, if the point gap is correct, the dwell should also be correct or nearly so. Adjustment with a dwell meter produces more exact, consistent results since it is a dynamic adjustment. If dwell varies more than 3° between idle speed and 1,750 engine rpm, the distributor is worn.SINGLE POINT DISTRIBUTOR
- Adjust the points with a feeler gauge as previously described.
- Connect the dwell meter to the ignition circuit as according to the manufacturer's instructions. One lead of the meter is usually connected to a ground and the other lead is 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 meter does not have a scale for 4 cylinder engines, multiply the 8 cylinder reading by two.
- If the reading is above the specified range, the breaker point gap is too small. If the reading is 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.
Adjust the point gap of a dual point distributor with a dwell meter as follows:
- Mark and disconnect the wiring harness of the distributor from the engine wiring harness.
- Using a jumper wire, connect the black wire of the engine side of the harness to the black wire of the distributor side of the harness (advance points).
- Start the engine and observe the reading on the dwell meter. Shut the engine off and adjust the points accordingly as previously outlined for single point distributors.
- Disconnect the jumper wire from the black wire of the distributor side of the wiring harness and connect it to the yellow wire (retard points).
- Adjust the point gap as necessary.
- After the dwell of both sets of points is correct, remove the jumper wire and connect the engine-to-distributor wiring harness securely.