152 Auto Engine Performance and Driveability Copyright by Goodheart-Willcox Co., Inc. Coil firing alternates with each crankshaft revolution. Note that both plugs in a cylinder fire at the same time, but are fired by different coils. Primary Wiring and Connectors Primary wiring carries relatively high current at low voltage. It has small-diameter conductors with light insula- tion. Primary ignition wiring resembles the other chassis wiring in the vehicle. Most primary ignition wiring is from 8 to 14 gage in size. On the other hand, battery cables are much larger in size, typically 4 to 6 gage. Primary wiring connectors are usually sealed, plug-in units, such as that shown in Figure 8-19. These connec- tors reduce the possibility of bad connections due to dirt, water, or corrosion. Plug-in connectors are shaped to fit one way, which eliminates the possibility of reversing the coil connection. Reversing connections was a common mistake made when installing older coil designs. Some connectors are part of their components. Parts that are installed together may have mating blade connec- tors and receptacles such as the ignition control module/ bracket and coil in Figure 8-20. Installing parts together eliminates the need for wiring and allows the connections to be effectively sealed. Resistor Some ignition systems include a resistor in the primary circuit. However, most vehicles with electronic ignition do not use a resistor in the ignition circuit. The majority of elec- tronic ignition systems use full battery voltage at all times. Electricity flows from the ignition switch to the resistor, which controls the amount of current reaching the coil. The resistor may be either a calibrated resistance wire built into the wiring harness or a temperature-sensitive, variable-ballast resistor. The resistor lowers battery voltage at the coil to around 9.5 volts during normal engine operation. When the engine is being cranked, the coil receives full battery voltage from a bypass wire. This full battery voltage comes from the igni- tion switch or starter solenoid. When the key is released, the coil receives its power through the resistance unit. Capacitor Some ignition systems include a capacitor in the pri- mary circuit. The capacitor absorbs voltage spikes, protect- ing the electronic components. The capacitor is usually integrated into the ignition control module. Ignition Timing Ignition timing is the sequencing of the spark to the movement of the piston on the compression stroke. On distributor-type ignition systems, the ignition timing can be adjusted. Systems without a distributor have no way to manually adjust timing. Crankshaft- and camshaft- position and engine-speed sensors provide input to the ECM to determine initial timing for starting the engine. Once the engine is running, the ECM uses inputs from the speed, MAP, MAF, and temperature sensors to determine timing. Initial timing and timing advance can be checked with the appropriate scan tool, although they cannot be adjusted. Ignition Timing Advance Systems As engine speed increases, it is necessary to ignite the air-fuel mixture earlier. If this is not done, the piston would Figure 8-19. Sealed plug-in wire connectors are used to keep out dirt and moisture, which can cause corrosion and increase circuit resistance. The low voltages common in computer- controlled systems can be altered by the increased resistance to the point where driveability problems occur. Figure 8-20. Some connectors are part of their components. The coil directly plugs into the bracket holding the ignition control module. This reduces the chance of water entry and the resulting corrosion. Also, there is no chance of wire or insulation damage.