Out of the many different aspects of the Pontiac hobby, ignition-system function is often the most misunderstood. Since it deals with an intangible (i.e., electricity), it's very easy to assume one thing is happening when it's actually another. But don't worry: Sit back and let HPP show you how to dazzle your buddies with ignition-system knowledge!
Myth 1:A Voltmeter Is BestReality:A Scope Is Much BetterThe reason ignition systems are so hard to understand is because the flow of electricity cannot be seen. A common voltmeter used by many to learn about the ignition event doesn't do much good because it only assigns a value to the electrical "pressure" in a circuit. To truly understand an ignition system and how the spark plug fires, an instrument called an oscilloscope (or scope), as shown in the lead photo, is required. This device displays an electrical picture of the ignition event as a function of voltage and time.
An oscilloscope is like a television. The viewing screen is the front side of a cathode ray tube (CRT), and the inside of the screen is coated with phosphorous; this is the anode or positive. So the displayed pattern is the result of the electrons (which have a negative charge) shot at the phosphorous. The phosphorous coating glows anywhere the electrons hit the surface, resulting in the image or pattern displayed on the CRT. Since the phosphorous continues to glow briefly after the electrons hit it, the image appears continuous and forms one single pattern.
An electron "gun" and two pairs of deflector coils sealed in a vacuum tube comprise the electronic portion of the CRT. Electrons shot from the gun are "steered" by an input signal to the scope, which causes the path of the electrons to bend as a result of the magnetic field created by the deflector coils.
The ignition scope connects to the engine's primary (negative post of the ignition coil), the coil wire for secondary output, and No. 1 spark plug wire as a synchronization signal. In addition, most automotive scopes require a 12-volt positive and negative connection.
Think of a scope as a high-speed visual voltmeter that shows not only the intensity of the voltage, but when the signal occurs. Instead of watching a voltmeter needle bounce up and down, the height of the scope pattern indicates the intensity of the voltage. The horizontal axis of a scope indicates the time period in milliseconds (ms) or 0.001 (one thousandth of a second).
With a scope, we cannot only see how many volts were required to light the spark plug, but how long it stayed lit, which is very important in diagnosing performance problems with the ignition, engine, and fuel system.
Myth 2:I Know All There Is To Know About Spark Plug FiringReality:There Is More To Spark Plug Firing Than Many RealizeOn any ignition system that uses a single coil, there are five distinct components of a scope pattern. Reference the illustration elsewhere in this primer while reading the explanation.
The secondary spike (section 1) is read in kilovolts (kV=1,000 volts) and is the energy required to overcome all secondary resistance and bridge the gap of the spark plug under compression. It is impacted by many factors, including the condition of the secondary wires, cap and rotor, the gap of the spark plug, the cylinder pressure in the bore, and the air/fuel ratio. The proper name for the secondary spike is the ionization voltage. It normally takes 5-10 kV (5,000-10,000 volts) to get the arc going at idle in a typical Pontiac V-8 engine.
More energy is always required to get the spark plug lit than to maintain the arc, in much the same manner that it is harder to get a car rolling from a stop than to keep it moving. Section 2 is identified as the firing line. It represents the amount of energy removed from the coil to keep the spark plug arcing, but also shows how long the spark plug remained lit in milliseconds. A good electronic-ignition system should have the capability to keep the plug burning at idle for at least 1.5 ms. When designing an ignition system, Pontiac engineers do not reference this reading, but instead look to maintain an arc for at least 20 degrees of crankshaft rotation.
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