Ignition diagnostics in the 1960s was simple. By replacing the distributor’s points, rotor and cap as part of a tune up, ignition problems were solved as long as the technician set up the points and timing to factory specifications.
In the early 1970s, solid-state semiconductor components reached a point where they were cheap and robust. Transistors and other electronic components replaced ignition points. With no moving parts, they require zero maintenance while providing a more powerful spark.
This miracle component, made of aluminum, silicon and other metals, acts like a switch. Transistors allow a lower voltage and current signal to control a higher current circuit. Most transistors have three prongs. Two prongs are the power and ground. The center prong is the trigger, command or signal connection.
In a modern ignition system, this switch lets a sensitive microprocessor drive voltage going to the primary side of the coil. The driver circuit can control timing, dwell and burn time of the spark. Some manufacturers are even using a second firing of the spark plug to improve emissions.
When the transistor turns off the power to the primary and the secondary fires, there is a sudden spike in voltage, called an inductive kick. On a scope, it looks like a spike with several tight oscillations after it. On points systems, it was the job of the condenser to absorb the inductive kick. In a solid-state ignition circuit, diodes and capacitors filter and limit the inductive kick.
The kick can be damaging to the transistor switch if the voltage is too high. The components that control and filter the inductive kick are typically packaged together with the transistor.
The ground going to the coil is not your typical ground. The ground used by the primary and transistor typically runs back to the module and can be called a signal or low-reference. Some coils might have a chassis ground in the connector, while others use a fourth wire to sense coil performance. This is where things can get confusing and a wiring diagram might be necessary.
If you have a two-wire coil, the two wires are power and ground, and the switching of the voltages happens in a module. To scope this signal, you can tap into the circuit, but use an attenuator so the inductive kick does not damage the scope. You can also use a current clamp on one of the wires to see the switching of the driver circuit and the primary saturating the secondary. On two-wire systems, chances are the ignition module is measuring dwell and burn time through the power and ground in the module.
If you have a three-wire coil, you will have power, ground and, depending on the manufacturer, a third wire called the command, signal or IGT. In this type of coil setup, the transistor is inside the coil.
If you measure the power and ground, you will see 12 volts with the key on. With the engine running, you will see small voltage drops as the coil fires. If you use a current clamp, you will see the switching of the coil with greater definition.
The signal wire will typically have a voltage of 5- to 7-volts that will switch on or off as the primary is energized. If you have a two-channel scope, graph the current and signal wire.
If you get a coil with four wires, get a wiring diagram to figure out what the manufacturer is doing with the fourth wire. The majority of the time, it is a ground. But, some ignitions have an inductive coil to measure coil performance by the engine control module.
What Kills an Ignition Circuit?
What damages ignition circuits is current that is higher and where it should not be, like a short to ground or short to power. The increased current causes heat, which can damage the transistor and coil control circuit.
If you have a crank and no-start condition, you should check for a control signal to the coil and power. On some engines, verifying the coil inputs is easier than pulling a coil and plug and awkwardly laying it on a piece of metal on the hood that is grounded. It can also provide you with more diagnostic information.
In the case of a misfire or dead cylinder, you first need to use a scope and meter. Swapping coils may lead to additional damage to other damaged driver circuits on the vehicle. If the resistance is lower or zero, or the meter shows infinite resistance, it should be replaced. If you are unsure, check the resistance on another coil.
Low resistance or a short in the primary can cause the transistor to carry more current that can damage the control circuit. If you swap a bad coil and the driver for the coil is in a module, you could damage another circuit and have two bad drivers instead of just one.