Point type ignition was used through the 1978 model year. The following information applies only to these model years. Beginning in 1979, electronic ignition became standard equipment. Testing and adjustments on 1979 and later models is given a little later in this section.
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.
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.
INSPECTION OF THE POINTS
- 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 or by turning the screwheaded fasteners.
- 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.
REMOVAL & INSTALLATION All Models
Because of engine design, the rotary engine uses 2 sets of points in the distributor. Removal and installation is basically the same as for a single set of points.
- Raise the hood and locate the distributor. It is on the front of the engine.
- Scribe an alignment mark on the distributor cap and the distributor body.
- Remove the distributor cap and rotor. The rotor goes on the shaft only one way.
- Disconnect the primary and condenser wires from the breaker point terminal. Note the position of the wires before removing them from the terminal.
- Remove the screws attaching the breaker points to the base plate. If possible, it is best to use a magnetic screwdriver to do this. The screws are very small and can be dropped easily.
- Lift the breaker point(s) assemblies from the distributor. Remove the condenser.
- Place the breaker point(s) assemblies on the base plate. Install the attaching screws, again using a magnetic screwdriver, if you have one.
- Install the condenser. It is always best to install a new condenser each time you replace the points. It is just cheap insurance against condenser failure.
- Connect the primary and condenser wires to the point(s) terminal and tighten the connection.
- Be sure that the points are aligned.
- Set the point gap or dwell angle and install the rotor and distributor cap. Use the alignment marks made previously and install the cap correctly.
See Figures 1, 2, 3, 4 and 5
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.
When setting ignition contact points, it is advisable to observe the following general rules:
- If the points are used, they should not be adjusted using a feeler gauge. The gauge will not give an accurate reading on a pitted surface.
- Never file the points. This removes their protective coating and results in rapid pitting.
- When using a feeler gauge to set new points, be certain that the points are fully open. The fiber rubbing block must rest on the highest point of the cam lobe.
- Always make sure that a feeler gauge is free of oil or grease before setting the points.
- Make sure that the points are properly aligned and that the feeler gauge is not tilted. If points are misaligned, bend the fixed contact support only, never the movable breakable arm.
A dwell meter virtually eliminates errors in point gap caused by the distributor cam lobes being unequally worn, or human error. In any case, point dwell should be checked as soon as possible after setting with a feeler gauge because it is a far more accurate check of point operation under normal operating conditions. The dwell meter is also capable of detecting high point resistance (oxidation) or poor connections within the distributor.
The dwell meter, actually a modified voltmeter, depends on the nature of contact point operation for its usefulness. In this electro-mechanical system, a fiber block slides under tension, over a cam. The angle that the block traverses on the cam, during which time current is made available to the coil primary winding, is an inverse function of point gap. In other words, the wider the gap, the smaller the dwell (expressed in degrees); the closer the gap, the greater the dwell.
Because the fiber block wears down gradually in service, it is a good practice to set the dwell on the low side of any dwell range (smaller number of degrees) given in specifications. As the block wears, the dwell becomes greater (toward the center of the range) and point life is increased between adjustments.
To connect the dwell meter, switch the meter to the appropriate cylinder range, as the case may be, and connect one lead to ground. The other lead should be connected to the coil distributor terminal (the one having the wire going to contact points). Follow the manufacturer's instructions if they differ from those listed. Zero the meter, start the engine and gradually allow it to assume normal idle speed. See the Tune-Up Specifications earlier in this section. The meter should agree with the specifications. Any excessive variation in dwell indicates a worn distributor shaft or bushings, or perhaps a worn distributor cam or breaker plate.
It is obvious from the above procedure that some means of measuring engine rpm must also be employed when checking dwell. An external tachometer should be employed. Hook-up is the same as for the dwell meter and both can be used in conjunction. Most commercial dwell meters have a tachometer scale built in and switching between them is possible.
Single and Dual Point Distributors
There are two methods to adjust the breaker point gap, the feeler blade method and the dwell meter method, of which, the latter is preferred.FEELER BLADE METHOD
- Remove the distributor high tension lead and ground it. Remove the distributor cap and rotor.
- Check the breaker point alignment (refer to the accompanying illustrations). If necessary, align the contact points by bending the stationary contact. Never bend the moveable arm(s).
- Crank the engine in short bursts, until the rubbing block on the breaker arm rests on a high point of one of the distributor cam lobes. In this position, the gap between the contact points is largest.
- Insert a feeler blade of the specified thickness between the breaker points. The feeler blade will slide through the contact points with a slight drag when the gap is correct. If your truck is equipped with a dual point distributor, this step and the following steps should be repeated for each set of points.
- If adjustment is required, loosen the breaker point attaching screws and move the stationary contact and base until the correct gap is obtained.
- Tighten the attaching screws and recheck the gap.
- Install the rotor and distributor cap. Reconnect the high tension lead.
- Disconnect the vacuum line from the distributor and plug it.
- Connect the dwell meter in accordance with the manufacturer's instructions.
- Start the engine, and run at idle until normal operating temperature is reached.
- Observe the dwell meter reading. If the reading is within specifications, turn the engine OFF and skip to Step 5. If it is not within specifications, adjust the dwell as follows:
If dwell angle is above the specified amount, the point gap is too small; if it is below the specified amount, the gap is too large.
- Turn the engine OFF. Remove the high tension lead and ground it. Remove the distributor cap and rotor.
- Loosen the breaker point attaching screws and, while observing the dwell meter, crank the engine. Move the stationary contact(s) and base(s) until the correct dwell reading is obtained.
- Tighten the attaching screws. Install the rotor and distributor cap. Reconnect the high tension leads.
- When the dwell angle check is completed, disconnect the dwell meter and reconnect the vacuum hose to the distributor.