reluctor

A reluctor is a ferromagnetic toothed ring or wheel that rotates with a shaft and passes by a fixed magnetic sensor. As the teeth move past the sensor, they change the magnetic flux in the sensor's coil, generating a variable voltage signal that an engine control unit can detect and measure. This signal timing allows the engine to know the exact rotational speed and position of the crankshaft or camshaft, which is essential for ignition timing, fuel injection control, and other critical functions.

Most reluctors are mild steel rings pressed or keyed onto a crankshaft or camshaft, with teeth that are typically 2 to 3 millimeters wide and spaced at equal intervals around the circumference. Common tooth counts range from 58 to 60 on a crankshaft reluctor (with one or two teeth deliberately omitted to create a timing reference), or 4 teeth on a camshaft reluctor. The variable reluctance sensor itself is a stationary coil-and-magnet assembly positioned a few millimeters away from the rotating teeth; as each tooth approaches and passes, the changing gap modulates the magnetic field and induces a voltage pulse in the coil.

Reluctor-based systems are favored in automotive and small engine applications because they require no external power source, are mechanically simple, and work reliably across a wide temperature range. They are cheaper to manufacture than optical encoders and less vulnerable to contamination than Hall effect sensors in harsh engine compartment environments. However, reluctor sensors can degrade if the air gap between sensor and reluctor grows too large due to wear or debris buildup, or if the reluctor teeth become damaged by impact or corrosion.

The name comes from the principle of magnetic reluctance, the resistance to magnetic flux flow through a material. When a ferromagnetic tooth moves toward the sensor coil, reluctance decreases and flux increases; when a gap approaches, reluctance increases and flux decreases. This oscillation of reluctance is what generates the alternating signal. Early versions were developed for mechanical engine timing systems in the 1960s and 1970s; modern systems use the reluctor signal as the primary input for computerized engine management.

Reluctors work in tandem with reluctance sensors (also called variable reluctance pickups or inductive sensors) to form a passive speed and position feedback loop. They are found on crankshafts to provide crank position signal (CPS) and on camshafts to provide camshaft position signal (CPS), though the latter is becoming less common as dual overhead cam engines rely more heavily on variable valve timing actuators. Technicians must always check reluctor air gap when diagnosing rough idle, misfire, or no-start conditions, as a gap over 0.050 inches can prevent reliable signal generation.