As we continue our study of voltage drops and build-up times in primary circuits let us take a look at a normal Ford TFI-IV waveform which is virtually identical to the GM HEI in terms of fast build-up time, low resistance, current li miting, variable dwell, and high energy.

Starting at the left of the waveform (figure #3, A to B) in the firing section we can observe that the sparkline is similar to a secondary pattern. On most modern ignition systems we can measure the burn-time and cylinder characteristics using jus t the primary pattern. This is a bonus especially when it comes to distributorless systems because it provides an alternative hookup.

On this six cylinder we see that the dwell is only 23.9 degrees (39% dutycycle) at idle. This means plenty of cooling time and reduced energy consumption as compared to older ignition systems.

At the beginning of dwell (figure #3, point C) the ignition module provides a good ground and the voltage at the coil(-) drops straight to zero. Please observe that with the passing of time the voltage at the coil(-) creeps up as current flow thro ugh the circuit increases. About 0.004 seconds (4 ms) after the coil turn-on, the ignition module inserts resistance in the circuit (figure #3, point D) to limit and stabilize current flow. This point is the beginning of the current limiting hump we see in secondary patterns. It took only 0.004 seconds (figure #3, C to D) to reach the saturation point of this low resistance/high energy coil. While in the current limiting mode (figure #3, D to E), we'll typically find 8 to 11 volts at the coil(- ).

(figure #3) Note; the equally spaced gridmarks = 0.002 seconds each.

BONUS! RPM, dwell, build-up time (current ramp) can all be viewed from the coil(+) with just one hookup. Because there is no such thing as a perfect conductor in automotive wiring and because manufacturers will use the smallest gauge wire suitab le, there is normally a 0.5 to 1.5 volt drop at the coil(+) while peak current is being sustained (figure #4, C to D). And, because voltage drop across a fixed resistance changes with current flow, we can see the current ramp or build-up time (figure #4, B to C) in a primary circuit by attaching a scope to the coil(+). The higher voltage on the left (figure #4, A to B) is the open circuit voltage at the coil(+).

(figure #4)

How does this help us? As one example, if we set our scopes up properly, we should be able to monitor ignition module activity on a GM distributorless by back-probing the power feed at the ignition module. Some fuel injector circuit problems can also be detected by hooking up your scope to the injector positive.

Next we'll look at some HEI waveforms with defects.

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Copyright Dan Buckley 4/2000

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