Ignition triggering devices are the components responsible for signaling exactly when a spark should fire in an internal combustion engine. They detect crankshaft or camshaft position and send that signal to the ignition coil or control module, which then delivers high voltage to the spark plug. Over the decades, these triggering components have evolved from simple mechanical switches to precision electronic sensors, but they all serve the same core function: telling the system “fire now.”
Breaker Points (Contact Points)
The oldest and simplest ignition triggering device is the contact breaker point, a mechanical switch inside the distributor. When the ignition key is turned on, low-voltage current from the battery flows through the primary windings of the ignition coil and through the closed breaker points, building a magnetic field around the coil. As the engine rotates, a cam on the distributor shaft pushes the points apart. The instant the points separate, current flow stops, the magnetic field collapses, and this collapse induces a high-voltage spike in the coil’s secondary windings. That spike is strong enough to jump the gap at the spark plug and ignite the fuel mixture.
The points then close again to restore current flow, rebuild the magnetic field, and repeat the cycle for the next cylinder. This design is purely mechanical, which makes it easy to understand and repair but also prone to wear. The contact surfaces erode over time, changing the gap distance and degrading timing accuracy. A tach/dwell meter and timing light are the standard tools for checking whether points are still triggering correctly.
Magnetic Pickup Coils (Reluctor Systems)
Electronic ignition systems replaced breaker points with a magnetic pulse generator, sometimes called a reluctor or variable reluctance sensor. This setup uses a toothed metal wheel (the reluctor) mounted on the distributor shaft and a stationary pickup coil wound around a permanent magnet. As each tooth on the reluctor sweeps past the pickup coil, it changes the magnetic field and induces a small voltage pulse in the coil winding.
The signal from a reluctor is analog, meaning it produces a waveform that rises and falls as each tooth approaches and passes. The amplitude of this signal increases with engine speed, so at low cranking speeds the pulse can be weak. The ignition control module reads these pulses to determine engine position and fires the coil at the correct moment. Because nothing physically touches, there’s no mechanical wear, which makes reluctor systems far more reliable than breaker points over the long term.
Hall Effect Sensors
A Hall effect sensor is a solid-state triggering device that produces a clean digital square-wave signal each time a magnet or ferrous metal tab passes its tip. Unlike a reluctor, the signal strength doesn’t depend on how fast the trigger wheel is spinning. Whether the engine is cranking at 200 RPM or running at 6,000 RPM, the output is a consistent on/off voltage pulse. This makes Hall effect sensors especially reliable for low-speed starting.
Hall effect sensors typically require an external power supply of 5, 8, or 12 volts depending on the model, plus a ground connection and a signal wire. The ignition control module or engine computer reads the signal wire and uses each voltage transition as its firing cue. These sensors are compact and easy to mount, though they can be sensitive to high temperatures, and some models may miss closely spaced trigger teeth at very high speeds.
Many distributor-equipped electronic ignition systems from the 1980s and 1990s used Hall effect sensors internally. The ignition control module interprets the distributor’s Hall sensor signals and precisely controls coil firing timing, eliminating mechanical breaker points entirely.
Crankshaft and Camshaft Position Sensors
In modern distributorless ignition systems, the primary triggering devices are the crankshaft position sensor (CKP) and the camshaft position sensor (CMP). These are considered two of the most important sensors in any current engine. The CKP reads a toothed ring on the crankshaft to track its exact rotational position and speed. The CMP does the same for the camshaft, which tells the engine computer which cylinder is on its compression stroke and ready to fire.
Together, these two sensors give the engine control unit everything it needs to calculate precise spark timing for each cylinder individually. If the CKP sensor fails, the engine typically will not start at all or will cut out while running, because the computer has lost its primary timing reference. The CMP sensor failure is slightly less catastrophic in some designs but still causes misfires or prevents starting. Both sensors can use either reluctor or Hall effect technology depending on the manufacturer.
Magneto Triggering Components
Small engines like those on lawnmowers, chainsaws, and many small aircraft use magneto ignition systems, which generate their own electricity and don’t need a battery. A magneto triggering system consists of permanent magnets embedded in the engine’s flywheel, a U-shaped armature, a primary coil of about 200 turns of thick wire, and a secondary coil of roughly 20,000 turns of fine wire. The triggering mechanism itself is either a set of breaker points with a capacitor or a simple electronic control unit often marketed as “electronic ignition.”
As the flywheel spins, the magnets pass the armature and build a magnetic field in the primary coil. When that field reaches its peak, the electronic control unit (or the breaker points) breaks the circuit, collapsing the field and generating a voltage spike of around 200 volts in the primary. The secondary coil multiplies this into the thousands of volts needed to fire the spark plug. Aircraft like the Cessna 152 use magnetos specifically because they’re self-contained and extremely reliable, with no dependency on the electrical system.
Analog vs. Digital Trigger Signals
All ignition triggering devices produce one of two basic signal types. Reluctor-style magnetic pickups produce an analog signal, a continuously varying voltage wave whose peak height changes with engine speed. The ignition module must interpret the slope and level of this wave to determine the exact firing point, typically triggering on a specific voltage threshold as the signal crosses through zero.
Hall effect sensors and many modern CKP/CMP sensors produce a digital signal, a clean square wave that snaps between a low voltage (near zero) and a high voltage (typically 5 volts). The ignition system triggers on either the rising or falling edge of each square pulse. Digital signals are easier for engine computers to process accurately, which is one reason they’ve become dominant in modern vehicles. The consistency of the signal at all speeds eliminates the low-RPM weakness that analog reluctor signals can have during cranking.
How Triggering Failures Show Up
When an ignition triggering device starts to fail, the symptoms follow a predictable pattern. A simple test with a 12-volt test light connected to the primary circuit should show the light flashing on and off as the engine cranks, confirming the switching mechanism inside the distributor is working. If the light glows steady or doesn’t light at all, the triggering device has failed.
Intermittent failures are more common than total failures, and they tend to show up as random misfires, rough idle, or an engine that dies at certain temperatures. Heat is a frequent culprit with electronic sensors. A failing triggering device can also stress other components in the system. Misfires caused by erratic trigger signals can damage an electronic ignition module or, in older systems, burn out the condenser in the low-voltage circuit. If plug wires show resistance above 5,000 ohms, they can compound the problem by forcing the coil to work harder, but the root cause of most no-start or intermittent-misfire conditions traces back to the triggering device itself.