Many late model cars- use a resistor or resistance wire connected in the primary circuit between the ignition switch and the coil primary.
Before its function is described, we will discuss some conditions in the ignition system which affect its operation. At low RPM, the ignition points stay closed long enough and allow enough current to flow to establish a good strong spark at the spark plug gap.
But as RPM increases, the points are closed for a shorter period, less current flows, and a weaker spark is generated. In other words, the efficiency of the ignition system, as we have studied it so far, decreases rapidly with increasing speed.
To overcome this problem, the resistance wire is placed in the primary circuit as we have already mentioned.
The behavior of an electrical resistor is such that the hotter it gets, the higher becomes its resistance which reduces current flow.
Also, the opposite is true; the cooler it becomes, the less is its resistance and the greater the current flow.
Therefore, at low speeds, the primary circuit current flows through the resistor for a relatively long period causing it to heat up, raising its resistance and reducing the current flow.
This action keeps the coil cool and reduces the load of the distributor contact points. At higher speeds, the current flows through the primary circuit for a shorter period allowing the resistor to cool and thus permitting more current to flow.
This action increases the secondary voltage at high engine speed where it is needed.
During starting, because of the heavy drain of the starting motor, the resistor is shorted out of the circuit to compensate for the lower battery voltage available.
The starting switch and associated circuits are designed to bypass the resistor, thus shorting it out of the circuit during actual cranking.
Primary and Secondary Circuits
From all our previous discussions, it should be clear as to why we refer to the ignition system as having a primary and a secondary circuit.
The entire primary circuit is that which includes all the circuitry which handles the battery voltage: battery, ignition switch, resistor, points, condenser, the heavy primary coil winding and all associated wiring.
The secondary is that which carries the high voltage produced in the coil winding: the fine coil winding, coil high tension lead, rotor, spark plug leads and spark plugs.
The primary is actually the first or primary handler of the current in the generation of the high voltage for sparking.
Just one more time now; we’ll take it from the top. The engine is presumed running. The distributor shaft is being turned. The points are closed.
Current flows into the coil primary. The distributor cam is turning; it opens the points. Current from the battery through the coil primary stops.
The condenser is charged to prevent arcing and to collapse the magnetic field. The coil secondary has a voltage induced.
At the same time the rotor comes under the proper spark plug lead. The high voltage from the coil secondary flows out of the coil high tension lead into the center of the distributor cap into the rotor and out through the proper spark plug lead, through the plug, across the gap and ignites the mixture.
The fuel/air mixture burns. The tremendous expansion of the hot gasses forces the piston down and we’ve initiated a power stroke , swinging!
Article Source : This article courtesy should goes to Auto Mechanics Autodology – Technical instruction manual by System Operation Support.