To accurately measure the current in a Firing Coil, you will need a 5Watt 0.1Ohm resistor and an oscilloscope. The process involves placing the resistor in series with the power wire connected to a single firing coil. By using an oscilloscope to measure the voltage drop across this resistor, you can determine the current flowing through the coil. Utilizing Ohm’s Law (I = V/R), the current is calculated by dividing the measured voltage by the resistance value, 0.1Ohm. This calculation simplifies to multiplying your measured voltage by 10 to get the current.
Alt text: An automotive oscilloscope screen displaying a waveform, likely illustrating firing coil current measurement during a diagnostic procedure.
The waveform displayed on the oscilloscope will show the current through the firing coil increasing linearly during the coil’s dwell period. Following the dwell, the current will sharply drop back to zero, and this cycle will repeat. Because this current increase is linear, you can determine the average current by taking the value at the midpoint of the rise during the dwell period. This midpoint value represents the constant coil current throughout the dwell duration. It’s important to remember that current only flows through the coil periodically during operation. The rest of the time, the coil wire is inactive, allowing any accumulated heat to dissipate.
By considering the dwell time and the engine’s maximum RPM, you can calculate the maximum duty cycle of the firing coil current on the wire. This duty cycle allows you to determine the average current along that wire, which is crucial for proper wire sizing selection.
Alt text: A close-up view of a 0.1 Ohm resistor integrated into a vehicle’s wiring harness, set up for measuring firing coil current during automotive electrical testing.
In practical applications, detailed calculations may be bypassed for simplicity. Using 20AWG M22759/32 wire for powering the firing coils is often a sufficient and robust solution.
The original equipment manufacturer (OEM) fuse typically protects all firing coils collectively, which explains the common 15A fuse rating. The 12AWG wire often used in these systems likely reduces in gauge as it branches out to each individual coil. When modifying or upgrading, a similar approach can be taken: using 16AWG wire from the power supply to the distribution point, and then 20AWG wire to each individual firing coil should provide ample capacity and safety.
