DESCRIPTION & OPERATION
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 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. 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 also collapses and cuts through the secondary circuit windings 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 a spark plug 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, 4 spark plugs fire once for every two revolutions of the crankshaft. That means that in one revolution, 4 spark plugs fire. So, when the engine is at an idle speed of 800 rpm, the points are opening and closing 3200 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 meter, 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 changed the ignition timing. So, if the point gap or dwell is changed, the ignition timing must be adjusted also.
REMOVAL & INSTALLATION
See Figures 3 through 15
The usual procedure is to replace the condenser each time the point set is replaced. Although this is not always necessary, it is easy to do at this time and the cost is negligible. Every time you adjust or replace breaker points, the ignition timing must be checked and, if necessary, adjusted. No special equipment other than a feeler gauge is required for point replacement or adjustment, but a dwell meter is strongly advised.
- Remove the distributor cap. You might have to unclip or detach some or all of the plug wires to remove the cap.
- Clean the cap inside and out with a clean rag. Check for cracks and carbon paths. A carbon path shows up as a dark line, usually from one of the cap sockets inside terminals to a ground. Check the condition of the carbon button inside the center of the cap and the inside terminals. Replace the cap as necessary.
- Pull the rotor up and off the shaft. Clean off the metal outer tip if it is burned or corroded. Don't file it. Replace the rotor as necessary or if one came with your tune-up kit.
- The factory says that the points don't need to be replaced if metal transfer from one contact to the other doesn't exceed 0.020 in. (0.51 mm). However, sad experience show that it is more economical and reliable in the long run to replace the point set while the distributor is open, than to have to do this at a later (and possible more inconvenient) time.
- Pull off the two wire terminals from the point assembly. One wire comes from the condenser and the other comes from within the distributor. The terminals are usually held in place by spring tension only. There might be a clamp screw securing the terminals on some older versions. Loosen the point set hold-down screw(s). Be very careful not to drop any of these little screws inside the distributor. If this happens, the distributor will probably have to be removed to get at the screw. If the holddown screw is lost elsewhere, it must be replaced with one that is no longer than the original to avoid interference with the distributor workings. Remove the point set, even if it is to be reused.
- If the points are to be reused, clean them with a few strokes or a special point file. This is done with the points removed to prevent tiny metal filings getting into the distributor.
- Loosen the condenser hold-down screw and slide the condenser out of the clamp. This will save you a struggle with the clamp, condenser, and the tiny screw when you install the new one. If you have the type of clamp that is permanently fastened to the condenser, remove the screw and the condenser. Don't lose the screw.
- Attend to the distributor cam lubricator. If you have the round kind, turn it around on its shaft at the first tune-up and replace it at the second. If you have the long kind, switch ends at the first tune-up and replace it at the second.
Don't oil or grease the lubricator. The foam is impregnated with a special lubricant.
If you didn't get any lubricator at all, of if it looks like someone took it off, don't worry. You don't really need it. Just rub a match head size dab of grease on the cam lobes.
- Install the new condenser. If you left the clamp in place, just slide the new condenser into the clamp.
- Replace the point set and leave the screw slightly loose. Replace the two wire terminals, making sure that the wired don't interfere with anything.
- Check that the contacts meet squarely. If they don't, bend the tab supporting the fixed contact.
- Turn the engine until a high point on the cam that opens the points contacts the rubbing block on the point arm. You can turn the engine by hand if you can get a wrench on the crankshaft pulley nut, or you can grasp the fan belt and turn the engine with the spark plugs removed.
On a stick shift car, you can push it forward in high gear. Another alternative is to bump the starter switch or use a remote starter switch.
- There is a screwdriver slot near the contacts. Insert a screwdriver and lever the points open or closed until they appear to be at about the gap specified in the Tune-Up Specifications.
- Insert the correct size feel gauge and adjust the gap until you can push the gauge in and out between the contacts with a slight drag, but without distributing the point arm. This operation takes a bit of experience to obtain the correct feel. Check by trying the gauges 0.001-0.002 in. (0.025-0.051mm) larger and smaller than the setting size. The larger one should disturb the point arm, while the smaller one should not drag at all. Tighten the point set hold-down screw. Recheck the gap, because it often changes when the screw is tightened.
- After all the point adjustments are complete, pull a white business card through (between) the contacts to remove any traces of oil. Oil will cause rapid contact burning.
You can adjust dwell at this point, if you wish. Refer to Step 18.
- Push the rotor firmly down into place. It will only go on one way. Tighten the V8 rotor screws. If the rotor is not installed properly, it will probably break when the starter is operated.
- Replace the distributor cap.
- If a dwell meter is available, check the dwell.
See Figures 16, 17 and 18
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° from idle speed to 1750 engine rpm, the distributor is worn.
If the engine won't start, check:
- That all the spark plug wires are in place.
- That the rotor has been installed.
- That the two (or three) wires inside the distributor are connected.
- That the points open and close when the engine turns.
- That the gap is correct and the hold-down screw is tight.
- After the first 200 miles or so on a new set of points, the point gap often closes up due to initial rubbing block wear. For best performance, recheck the dwell (or gap) at this time.
- Since changing the gap affects the ignition point setting, the timing should be checked and adjusted as necessary after each point replacement or adjustment.