To help illustrate how unwanted resistance in the primary circuit can be detected we've inserted a 0.3 ohm wire wound resistor before the coil(+) on this GM-HEI v8. Referring to figure #5, please note the voltages at 'Current X = 1743 ' near the center of the waveform. The voltage at the coil(-) during the current limiting period is less than normal at 7.9 volts. In a circuit free of unwanted resistance this voltage is normally 8 plus volts. When current flow is already being re duced by an unwanted resistance the HEI ignition module causes less voltage drop (resistance) between the coil(-) and ground.

(figure #5)

Meanwhile at the coil(+) the open circuit voltage of 13.3 vdc has dropped 3 volts to an applied voltage of 10.3 vdc. The coil(+) waveform shows us a short flat line while current limiting is in effect because current flow has stabilized. Normally on GM HEI we'd expect the difference between open circuit voltage and applied voltage at the coil(+) to be less than 1 volt.

The time based grid marks on the left of figure #5 each represents 0.005 seconds. Please note the downward sloping voltage drop ramp at the coil(+) coincides with the coil(-) turn-on at which time the ignition module provides a good ground to 'ban g on' the primary circuit. Once again the ramp at the coil(+) reflects the increase of current and build-up time (about 0.004 seconds) to reach the saturation point.

In figure #6 we've revved the engine to 1893 RPM and inserted a 1.5 ohm wire wound resistor between the ignition switch and the coil(+), then midway in the pattern the resistance value was increased to 5.0 ohms. As expected, revving the engine cau ses the dwell period to increase.

On the left side of the pattern the HEI system is still able to reach peak current flow because a small current limiting hump appears just before a spark is generated. The cursor indicates a small current limiting hump at the coil(-) of 4.13 volts . Again, because there is already resistance in the circuit the current limiting hump shows a low voltage drop at the coil(-) rather than the normally high voltage drop.

(figure #6) Note: Dwell is NOT marked in this illustration.

Meanwhile, the voltage applied to the coil(+) has dropped to 6.9 volts, a sure sign there is resistance in the circuit before the coil(+).

On the right side of the screen we can see the effects of very high resistance on the ignition patterns. At the coil(+) there is an even greater voltage drop than before. The coil(-) and coil secondary signals no longer show any current limiting h ump what-so-ever. The very high primary resistance is also now shortening the sparklines (burn-times) as indicated by both the coil output and coil(-) patterns. As the coil no longer has enough energy to sustain a spark we can see the voltage on the sparkline rise as current flow in the secondary comes to an end. The unused energy is dissipated as heat and shows up as 'ringing' in the intermediate (coil/condenser) section of our ignition scope patterns. Thusly, with a peak primary current flow o f about 2.5 amps as calculated with Ohms Law, not only do the burn-times get shorter, some of the limited energy available now gets wasted as heat.

Testing and recording the results both increases the likelihood of an accurate diagnosis and expands our knowledge base. Using a digital scope to document our time, labor, and diagnosis also helps with customer relations. In an age of increased c ompetition and regulation the wise and professional mechanic will test rather than guess.

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