October 2021

BY Frank Massey I left off from part one with two important points I wanted to clarify. At the point of primary current interruption two essential things will happen. First, a prompt drop in current flow and a sharp rise in voltage. Prior to measuring current, I used two references to confirm good induction properties. The back EMF should reach approximately 450 volts. This would depend on peak current flow, coil impedance and how efficiently the circuit was interrupted. See Fig.1. This is the voltage pull-up above the ground reference during the coil saturation. Next, refer to Fig.2, which shows the Fiat Punto’s normal ignition profile. Then, refer to Fig.3 which show’s the Fiat Punto’s high slew rate and poor current. This has now been replaced by observing the current ramp and current interruption at the point of induction, the current should fall promptly and vertically and if efficient! Have a negative overshoot, i.e., fall below the 0 amps base line. 26 AFTERMARKET OCTOBER 2021 TECHNICAL www.aftermarketonline.net IGNITION DIAGNOSTICS Frank’s two-part look at potential causes for ignition issues continues apace Any curvature during the current interruption will indicate one of two problems. It could be curvature with no coil ringing, which would mean faulty power stage, i.e., ignition amplifier, PCM, or coil pack. This depends entirely on where the coil current is controlled. Alternatively, it could be a secondary coil shunt, or a partial short in the secondary coil windings. This fault can exist even when a spark is delivered to the plug. It will however result in a reduction in burn time. Good current interruption with no coil ringing, which would equate to a secondary coil winding fault. Please refer to Fig.4, which shows the Fiat Punto’s normal primary with a secondary coil shunt. Essence The second point referred to in part one was the spark burn slope. As the expression suggests, it is the angle or profile of the spark propagation within the cylinder. This is the PART TWO