Ignition Relays, Sensors, Switches & Control Units

While spark plugs, ignition coils and distributors are the recognizable “stars” of the ignition system, there would be no combustion and an engine wouldn’t run without the unseen and less glamorous utility players in the system – the relays, sensors, switches, and control units. These important components execute critical functions like safely conducting high levels of current, supplying essential data to ignition modules and engine control computers, and executing commands based on those inputs for accurate spark delivery. The electronic components in contemporary ignition systems generate higher voltage and more precise ignition timing than their mechanical forebears, which provides a more powerful spark for complete combustion and better performance, greater fuel economy and lower emissions. Modern ignition parts are also more reliable as well, but they too eventually wear out and fail, and when you have need of replacements, you’ll find them right here on our digital shelves.

The ignition switch is connected to battery voltage. When the key is turned or the start button is activated, power is routed to the ignition relay. A relay allows low current and lighter gauge wire to control the high current needed to supply the ignition coil, avoiding the danger and complexity of routing high current carrying, heavy gauge wire to the ignition switch. Inside the relay is a control circuit and a power circuit. When power is sent to the relay, current is applied to the relay coil, creating a magnetic field that closes the power circuit contacts and sends voltage to the ignition coil. If the ignition relay is faulty no power will be supplied to the ignition and the engine won’t start. When a properly functioning relay is energized, the closing of the internal switch contacts can be heard or felt by hand. Voltage and resistance checks with a DVOM (Digital Volt Ohm Meter) can also be performed to determine whether the relay or relay wiring is at fault.

When power is supplied to the ignition coil, the current flowing through the coil’s primary windings creates a magnetic field. When that current flow is interrupted the collapsing magnetic field produces an electric current in the secondary windings, which is stepped up to the 15,000 – 40,000 volts needed to jump the gap between the spark plug electrodes and ignite the air/fuel mixture in the cylinders. On distributor ignitions, current flow is interrupted by a triggering mechanism in the distributor – mechanical breaker points on older distributors and magnetic pickup coils or hall effect switches on electronic ignition distributors. There is a trigger wheel called a reluctor on the distributor shaft and as the shaft rotates and the reluctor passes the pickup coil or hall effect switch, a signal is sent to an ignition module, which contains a transistor that closes and opens the primary circuit to create and collapse the magnetic field and produce the voltage sent to the spark plugs.

Electronic distributor ignition offered several advantages over its mechanical predecessor: there were no breaker points to wear, the transistor in the module could open and close the primary circuit much faster, and higher voltage could be carried, resulting in more voltage to the spark plugs and more efficient engine operation. Nevertheless, although more reliable, electronic components are still subject to failure. If the vehicle won’t start and there is no spark at the ignition coil when checked with a spark tester, turn the ignition on and check for voltage at the positive terminal of the ignition coil. If voltage is present, connect a test light between the negative coil terminal and ground. The test light should flicker when the engine is cranked. If the light doesn’t flicker, the ignition module, pickup coil or hall effect switch, or circuit wiring is faulty. The pickup coil and hall effect switch can be tested with a DVOM. Some electronic systems are affected by the gap between the pickup coil and reluctor, which can be adversely affected by distributor bushing wear and excessive play between the distributor shaft and housing. If the gap dimension is incorrect the engine may misfire or fail to start.

The ignition coils in distributorless ignition systems have their primary circuits switched on and off by an ignition module or engine control computer based on the signal from the Crankshaft Position (CKP) sensor. This signal indicates the position and speed of the crankshaft for the greatest ignition accuracy, eliminating imprecise timing that can be caused by timing chain, distributor drive gear and distributor wear and play. There are 2-wire permanent magnet and 3-wire hall effect sensors and the sensor may be mounted on the timing cover, or at the side or rear of the engine depending on whether the reluctor is mounted on the crankshaft damper, the crankshaft itself or the flywheel/flexplate. If the vehicle won’t start, runs rough, stalls, and has poor acceleration and fuel economy, the CKP sensor may be faulty. A defective CKP sensor may set a fault code and illuminate the CHECK ENGINE light. Both sensor types can be tested with a DVOM, and a scan tool connected to the vehicle’s diagnostic port will reveal fault code(s). Some engines also employ a Camshaft Position (CMP) sensor, which functions in a similar manner, for ignition timing purposes.

On electronic ignition systems primary circuit on/off control is by an ignition module, sometimes called an igniter, and/or the engine control computer. That was the sole task of the modules used with early electronic distributors, where initial timing was manually set using an inductive timing light, and ignition advance was determined by centrifugal and vacuum advance mechanisms. However, on distributorless ignition systems timing is not adjustable, and both initial timing and advance are computer controlled based on internal programming and inputs from sensors indicating engine speed, engine load, throttle position, and engine coolant temperature. A knock sensor is also employed to detect spark knock and signal the computer to retard timing. If an ignition module fails, the engine won’t start; troubleshooting is described above. If there is a problem with an engine control computer or with one of the sensors, actuators, or circuits in the engine control system, the CHECK ENGINE light should illuminate. A scan tool connected to the vehicle’s diagnostic port can be used to find trouble codes.

No matter what vehicle you’re working on or the type of ignition system, we have the replacement relays, sensors, switches and control units needed to restore proper performance, reliability and fuel economy. We offer components that are manufactured to original equipment specifications, so when repairs are completed you can count on the performance your vehicle was designed to deliver. In addition to the components described above, we also offer parts that are often needed to achieve a complete repair including harnesses, connectors, mounting brackets, and more.