See Figures 1, 2, 3, 4 and 51970-73 Gasoline Engines Only
The dual point distributor has two sets of breaker points which operate independently of each other and are positioned with a relative phase angle of 5° (1970), 10° (1971) or 7° (1972-73) apart. This makes one set the advanced points and the other set the retarded points.
The two sets of points, which mechanically operate continuously, are connected in parallel to the primary side of the ignition timing, depending on whether or not the retarded set of points is energized.
When both sets of points are electrically energized, the first set to open (the advanced set, 4° or 1° sooner) has no control over breaking the ignition coil primary circuit because the retarded set is still closed and maintaining a complete circuit to ground. When the retarded set of points opens, the advanced set is still open, and the primary circuit is broken causing the electromagnetic field in the coil to collapse and the ignition spark is produced.
When the retarded set of points is removed from the primary ignition circuit through the operation of a distributor relay inserted into the retarded points circuit, the advanced set of points controls the primary circuit. The retarded set of points is activated as follows:
When the ambient temperature is below 34°F (1°C), the retarded set of points is removed from the ignition circuit no matter what switch is on.
In the case of an automatic transmission, the retarded set of points is activated at all times except under heavy acceleration and high speed cruising (wide open throttle) with the ambient temperature is above 50°F (10°C).
There are three switches which control the operation of the distributor relay on 1972-73 models and five switches on 1970-71 models. All of the switches must be ON in order to energize the distributor relay thus energizing the retarded set of points.
The switches and their operation are as follows:
A transmission switch located in the transmission closes an electrical circuit when the transmission is in 3rd gear (1970-72) and in any gear except 4th (1973). On 1970-71 models only, there is a transmission neutral switch which ON when the transmission is in all gears except Neutral.
A clutch switch mounted against the clutch pedal (1970-71) is ON when the clutch pedal is released (clutch engaged).
A throttle switch located on the throttle linkage at the carburetor is ON when the throttle valve is moved within a predetermined angle: up to 35° on 1970-71 models, 40° on 1972 models, and 45° on 1973 models.
An accelerator switch mounted to the accelerator pedal linkage (1970-72) is ON when the accelerator pedal is nearly completely released and OFF when the pedal is opened farther.
The temperature sensing switch on 1972-73 models is located near the hood release lever inside the passenger compartment. The temperature sensing switch comes ON between 41°F (5°F) and 55°F (13°C) when the temperature is rising and goes OFF at about 34°F (1°C) when the temperature falls.
The distributor vacuum advance mechanism produces a spark advance based on the amount of vacuum in the intake manifold. With a high vacuum, less air/fuel mixture enters the engine cylinders and the mixture is therefore less highly compressed. Consequently, this mixture burns more slowly and the advance mechanism gives it more time to burn. This longer burning time results in higher combustion temperatures at peak pressure and hence, more time for nitrogen (N) to react with oxygen (O 2 ) and form nitrogen oxides (NOx). At the same time, this advance timing results in less complete combustion due to the greater area of cylinder wall (quench area) exposed at the instant of ignition. This cooled fuel will not burn as readily and hence, results in higher unburned hydrocarbons (HC). The production of NOx and HC resulting from vacuum advance is highest during idle and moderate acceleration in lower gears.
Retardation of the ignition timing is necessary to reduce NOx and HC emissions. Various ways of retarding the ignition spark have been used in automobiles, all of which remove vacuum to the distributor vacuum advance mechanism at different times under certain conditions. Another way of accomplishing the same goal is the dual point distributor system.
INSPECTION AND ADJUSTMENTS
See Figures 6, 7, 8 and 9Phase Difference
- Disconnect the wiring harness of the distributor from the engine harness.
- Connect the black wire of the engine harness with the black wire of the distributor harness with a jumper wire. This connects the advanced set of points.
- With the engine idling, adjust the ignition timing by rotating the distributor.
- Disconnect the jumper wire from the black wire of the distributor harness and connect it to the yellow wire of the distributor harness. The retarded set of points is now activated.
- With the engine idling, check the ignition timing. The timing should be retarded from the advanced setting as follows: 5° for 1970; 10° for 1971; and 7° for 1972-73.
- To adjust the out-of-phase angle of the ignition timing, loosen the adjuster plate set screws on the same side as the retarded set of points.
- Place the blade of a screwdriver in the adjusting notch of the adjuster plate and turn the plate as required to obtain the correct retarded ignition timing specification. The ignition timing is retarded when the adjuster plate is turned counterclockwise. There are graduations on the adjuster plate to make the adjustment easier; one graduation is equal to 4 degrees of crankshaft rotation.
- Replace the distributor cap, start the engine, and check the ignition timing with the retarded set of points activated (yellow wire of the distributor wiring harness connected to the black wire of the engine wiring harness).
- Repeat the steps above as necessary to properly set the retarded ignition timing.
Disconnect the electrical leads at the switch and connect a self-powered test light to the electrical leads. The switch should conduct electricity only when the gearshift is moved to the corresponding gear for that particular model year vehicle: 3rd gear on 1970-72 models and 4th gear on 1973 models. The neutral switch on 1970-71 models should conduct current when the transmission is shifted into N .
If the switch fails to perform in the above manner, replace it with a new one.Clutch Switch
Test the clutch switch on 1970-71 models in the same manner as the transmission switch (self-powered test light). The switch should conduct current when the clutch pedal is released (clutch engaged).Accelerator Switch
The accelerator switch is mounted on the accelerator pedal linkage (except 1973). It is checked with a self-powered test light in the same manner as outlined for the transmission switch. The switch should conduct current when the accelerator pedal is nearly completely released.Throttle Switch
The throttle switch located on the throttle linkage at the carburetor is checked with a self-powered test light. Disconnect the electrical leads of the switch and connect the test light. The switch should not conduct current when the throttle valve is closed or opened as follows: The throttle valve opened up to 35° on 1970-71 models; 40° on 1972 models; and 45° on 1973 models. When the throttle is fully opened, the switch should conduct current.Temperature Sensing Switch
See Figure 10
The temperature sensing switch mounted in the passenger compartment (near the hood release lever) should not conduct current when the temperature is above 55°F (13°C) when connected to a self-powered test light as previously outlined for the throttle switch.