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 1,600 times (2 x 800 = 1,600). And that is just at idle. What about at 3,000 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 Figures 1 and 2
The breaker points should be inspected and cleaned at 6,000 mile (9,600 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 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 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.
The points should be replaced every 12,000 miles (19,200 km), or if they are badly pitted, worn, or burned. To replace them, proceed as follows:
- If you have not already done so, perform Steps 1-3 of the preceding "Inspection and Cleaning" procedure.
- Unfasten the point lead connector.
- Remove the point retaining clip and unfasten the point holddown screw(s). It is a good idea to use a magnetic or locking screwdriver to remove the small screws inside the distributor, since they are almost impossible to find once they have been dropped.
- Lift out the point set.
- Install the new point set in the reverse order of removal. Adjust the points as detailed below, after completing installation.
Replace the condenser whenever the points are replaced, or if it is suspected of being defective. On Toyota trucks the condenser is located on the outside of the distributor (except the F, where it is inside the distributor). To replace it, proceed as follows:
- Carefully remove the nut and washer from the condenser lead terminal. On the F, you must first remove the distributor cap.
- Use a magnetic or locking screwdriver to remove the condenser mounting screw.
- Remove the condenser.
- Installation of a new condenser is performed in the reverse order of removal.
See Figures 3, 4, 5, 6 and 7
Perform the gap adjustment procedure whenever new points are installed, or as part of routing maintenance. If you are adjusting an old set of points, you must check the dwell as well, since the feeler gauge is really only accurate with a new point set. The points on 1975-77 trucks are adjusted in a slightly different manner than you may be familiar with so make sure that you follow the correct adjustment procedure below.
- Rotate the engine by hand (with a wrench on the crankshaft pulley nut) or by using a remote starter switch, so that the rubbing block is on the high point of the cam lobe.
- Insert a 0.46mm feeler gauge between the points; a slight drag should be felt.
- If no drag is felt or if the feeler gauge cannot be inserted at all, loosen, but do not remove the point holddown screw.
- Insert a screwdriver into the adjustment slot. Rotate the screwdriver until the proper point gap is attained. The point gap is increased by rotating the screwdriver counterclockwise and decreased by rotating it clockwise.
- Tighten the point holddown screw. Lubricate the cam lobes, breaker arm, rubbing block, arm pivot, and distributor shaft with special high temperature distributor grease.
The point set on this ignition system is covered by a piece of protective plastic shielding. Because of this, the gap must be checked between the point rubbing block and the distributor cam lobe instead of between the two points. Do not try to remove the plastic shielding as it will damage the point set.
- Using your hands, a wrench on the crankshaft pulley nut, or a remote starter switch, rotate the engine so that the rubbing block is resting on the low point (flat side) of the cam lobe.
- Insert a 0.46mm flat feeler gauge between the rubbing block and the cam lobe; a slight drag should be felt.
- If no drag can be felt or if the feeler gauge cannot be inserted at all, loosen, but do not remove, the point holddown screw.
- Insert a screwdriver into the point adjustment slot. Rotate the screwdriver until the proper gap is achieved. The gap is increased by rotating the screwdriver counterclockwise and decreased by rotating it clockwise.
- Tighten the point holddown screw. Lubricate the cam lobes, breaker arm, rubbing block, arm pivot and distributor shaft with special high temperature distributor grease.