How Capacitors Protect Ignition Circuits

How Capacitors Protect Ignition Circuits

A capacitor is a two-terminal electrical device that stores and releases energy as an electric charge.

If you look at some ignition system wiring diagrams, you may notice something unexpected on the ground connection for the coils on some BMW models: it is a capacitor. 

I was stumped! So, I emailed Philip Austin, NGK’s Manager of Technical Training. He quickly shot back: “Since it’s on the power feed side, it plays the role of keeping the energy constant as the coil is fired and/or ready to fire during each cycle of firing.”

A capacitor is a two-terminal electrical device that stores and releases energy as an electric charge. It consists of two electrical conductors that are separated by a distance. The space between the conductors may be filled by a vacuum or an insulating material known as a dielectric.

I started looking for other ignition systems with capacitors on the ignition system wiring diagram. I ran across a similar coil arrangement from a 2011 Nissan Frontier. Instead of calling it a capacitor, Nissan calls it a “condenser.”

In most electrical engineering textbooks, capacitor and condenser are interchangeable terms. But, condensers are typically associated with points ignition systems.

The function of a condenser in a points ignition circuit is to reduce the sparking at the contact points to minimize the burning and pitting of the points. This arcing is caused by the effect of induction in the coil as the points interrupt the flow of current. 

A points ignition system is the oldest “pull to ground” circuit. The primary side of the coil has positive and negative connections. The positive side is connected to battery voltage. The negative side of the circuit is connected to the points and then to ground. The condenser is in parallel with the points on the negative side of the coil.

When the points close, the voltage saturates the windings of the primary. When the points open, the magnetic field collapses, and the windings in the secondary use induction to generate thousands of volts at the spark plug’s electrodes. When the points open, the primary voltage spikes, very briefly, around 400 volts. This spike can burn up the tungsten contacts on the points. Many old systems required new points or a tune-up every 3,000 to 7,000 miles.

To extend the life of the points, engineers placed a condenser in parallel with the points on the negative side of the primary of the coil. A condenser offers the voltage spike an easier path to ground than the air gap between the points. This helped to reduce the arcing when the points opened.

The condenser will store and release the energy and prevent the excessive voltage from damaging the points. It also stabilizes the voltages in the circuit. If a condenser fails in some points distributor systems, the engine might not start or have a bad misfire.

So why is this ancient technology being used on late-model ignition systems? The ignition circuit for a coil-over-plug operates like a points ignition system by turning the power on and off to the primary side of the coil. 

Instead of mechanical points, a modern ignition system has a transistor that performs the switching. A solid-state transistor has no moving parts that can become worn or go out of adjustment. But like the tungsten pads, the silicon circuitry of the transistor does not like 400-volt power spikes or the generated heat.

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.

If a capacitor or condenser is not functioning on a modern vehicle, it could generate diagnostic trouble codes for the primary and secondary coil circuits. The most common generic codes are P0350-P0359. Some manufacturers will have proprietary DTC for the ignition system circuits. 

How do you test a condenser or capacitor? First, the unit must be removed from the circuit and discharged for testing. This might be difficult if the capacitor is integrated into the ignition or engine control module. Some manufacturers have made the part accessible for testing. Nissan has the condenser mounted on the wiring harness for the ignition coils. If you can remove the capacitor, you can measure resistance, capacitance and voltage.

Measuring resistance can be inconclusive. When a capacitor is connected to a meter, the resistance should start soaring without stopping or until your meter reaches infinite resistance. This means the capacitor is functional. This doesn’t necessarily mean that a capacitor is as good as new; it is only functional. Voltage measurement involves applying a voltage to the capacitor. The voltage should not drop when the power is connected.

Capacitance is measured in Farads. Most capacitors have the Farad rating printed, cast or stamped into the body of the component. Comparing the reading on your meter is far more accurate than resistance testing.

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.

Kick Back

As mentioned earlier, the inductive kick that occurs when the transistor turns off the power to the primary and the secondary fires, looks like a spike with several tight oscillations after it. The condenser in a points system or the, diodes and capacitors in a solid-state ignition circuit 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. This can be a capacitor or even a diode.

Scoping Coils

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.

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. 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 module.

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.In the case of a misfire, 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.

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