Primary Waveform Analysis

16-01 Investigate primary circuit faults and operation.

As complex as modern ignition systems have become, the primary circuit remains fairly basic. Primary circuit testing is often forgotten about on newer systems since multi-wire coils (three and four-wire coils) do not have access to the primary circuit. However, primary ignition system testing is available, and it’s a valuable test on distributor ignition systems, distributorless ignition systems (DISs), and COP ignition systems that use an external ignition control module (ICM) or applications where the ICM is inside the powertrain control module (PCM). The primary voltage waveform is typically more reliable than the secondary, when testing COP systems, because it provides the most information with the least effort. The primary circuit is accessible by tapping into the control (the negative side of the coil) circuit anywhere between the COP and the PCM/ICM, for testing.

Since the secondary output is a direct result of the primary, the primary circuit must be operating correctly. If there is an issue in the secondary circuit, do not forget to check the primary. For every 1 volt lost in the primary circuit, another 10,000 volts may be lost in the secondary.

Primary Voltage Test

For a coil to work, it requires two connections, regardless of the application: battery power and a switchable ground. The primary winding circuit includes both the power and ground circuits and a power transistor (breaker points on conventional systems) to control current flow. The positive feed is usually from the ignition switch, a fuse or relay that supplies electrical energy to the B+, or positive, terminal. The switching side is the negative, or ground, side of the coil, and it marks the difference among systems. Coil control varies depending on the system. Ground side switching may be from opening and closing a set of breaker points, an ICM/igniter that includes a transistor to switch current flow on and off, or a switching transistor inside the PCM. All modern ignition systems use a transistor to interrupt current flow. The circuit between the transistor (switching device) and the coil is where access to the primary voltage signal or primary current measurement is made. On coils (all two-wire COP systems, DISs, and single-coil systems) where the switching ground circuit is accessible, the primary circuit voltage can be diagnosed by using an oscilloscope to probe the negative (ground) side of the connector.

Some coils may have more than two wires or terminals. On distributor systems, manufacturers may run an additional wire from the negative terminal for a tachometer input to the PCM or instrument cluster. This does not restrict access to the coil primary, so scope use is still available. On the other hand, a growing number of original equipment manufacturers (OEMs) are using multi-wire COPs (three- and four-wire COPs) that move the switching transistor from the PCM to inside the ignition coil. The pulsing ground control is internal to the coil, eliminating access to the coil primary and primary circuit testing on three- and four-wire COPs. The accessible ground wire in the coil’s connector is for transistor operation, not the switching ground that interrupts current flow in the coil primary. In these systems, testing the primary coil voltage is not available, so use primary current instead. These multi-wired configurations require a third signal terminal for coil operation. The signal from the PCM tells the transistor when to toggle the coil primary circuit on and off. This circuit is known as ignition control or electronic spark timing (EST) control.

Besides multi-wire COPs, other coils that may restrict access to the primary circuit include distributor systems that house the ignition coil inside the distributor cap and DISs that mount the ICM under the coils. For systems lacking a connection to the primary circuit, either due to coil design or placement, try accessing the coil secondary for a waveform.

Before connecting the scope to the negative terminal of the coil, use an attenuator to reduce the voltage spike that reaches the coil. Attenuators are available in 10:1 and 20:1 filters. Most scopes have a preset that matches the attenuator in use. Using the preset adjusts the scope’s vertical scaling to match the attenuator. In other words, if the primary system is producing 300 volts, a 10:1 attenuator will reduce the voltage to 30 volts. Likewise, a 20:1 attenuator drops the voltage surge from 300 to 15 volts. If the scope’s scaling is not adjusted to match, it could lead to a misdiagnosis. Check the scope’s owner’s manual for its maximum voltage input. Remember that coil primary voltage can reach 300 to 400 volts. If the scope’s protection level can’t be determined, it is better to error on the side of caution and instead use an attenuator in-line with the scope lead that being connected to the coil negative terminal than to forgo it and risk damaging the scope.

An ignition waveform, primary or secondary, can provide a window into the combustion chamber. An adept scope user can determine an ignition system, fuel delivery, or mechanical fault in a cylinder. All modern ignition systems are built to maintain spark as long as hydrocarbons (fuel) are present in the cylinder. The duration and angle of the spark line are representative of in-cylinder conditions. (FIGURE 16-1) Since the primary waveform is a mirror image of the secondary, learning what the scope results are applies to both. Either waveform can be used for diagnostic purposes. Choose whichever is accessible or produces the best results.

FIGURE 16-1 Diagram showing the relationship between the primary voltage and primary current flow. When the transistor turns on, voltage is pulled low and current begins to flow and build the magnetic field. In some COP applications, when the coil is fully saturated, the PCM controls current to prevent excessive flow. This control (switching) can be seen in both the voltage and current waveforms. When the transistor interrupts current flow, voltage spikes (inductive kick) from 12 volts to as much as 300 to 400 volts, inducing the increased voltage into the coil secondary.

Connecting the Scope for a Primary Waveform

Connecting the scope is straightforward. Decide which channel to display the primary waveform on, and attach the attenuator to that channel’s input. Connect the scope’s lead to the attenuator. Connect the scope’s black lead to a good ground. On two-wire COPs or DIS coil packs, the PCM directly controls the coil’s dwell by using a transistor to pulse the coil negative wire to ground. The circuit between the transistor and coil is where the primary ignition voltage or current is measured. A primary ignition waveform can be accessed at each coil or at the PCM connector—whichever is easier. For distributor-based ignition systems attach the scope’s lead to the coil’s negative terminal.

If all the cylinders are being checked at once from a fuse or relay, use a trigger to identify the position of the cylinders on the screen. For systems that use plug wires, place a capacitance probe around any plug wire for a trigger signal that syncs the waveform to the firing order for cylinder identification. In COP systems, either back probe the control wire from an individual coil or, if a coil is accessible, install an accessory high-tension (HT) lead between the coil boot and spark plug and connect a sync probe. Attach the sync probe to a second channel on the scope. Start the engine and observe the waveform.