Component Speed Sensors

The speed of moving components on an automobile can be monitored with a permanent magnet generator.

A permanent magnet (PM) generator is comprised of the following parts:

• A stationary coil of wire wrapped around a permanent magnet.

• A permanent magnet.

• An external toothed reluctor wheel will rotate very close (typically 0.060") to the PM generator. 

As the reluctor wheel rotates, the magnetic field strength will increase and decrease with each passing tooth.  The fluctuating magnetic field induces AC voltage/current into the coil of wire.  The faster the rotational speed of the reluctor wheel, the higher the AC output voltage.  The AC voltage/current is read by a computer and translated into a rotational speed based upon the frequency of the AC voltage signal.

Examples of PM generators on an automobile are:

• Wheel Speed Sensors (WSS) - Wheel speed sensors used by the Anti-lock Brake System (ABS), Traction Control System (TCS), Vehicle Stability Control System (VSC), and Powertrain control module (PCM). The PCM uses the WSS signal to disable engine misfire detection when rough roads are detected. 

• The PM generator is located inside the gold colored bearing cap shown on the right. 

• The reluctor wheel (toothed ring), shown on the left, is located on the hub assembly on the left.

• Output Shaft Speed Sensor (OSS) and Input Speed Sensor (ISS) of a transmission or transaxle.  The OSS and ISS are used to calculate which gear the transmission is currently in, detect any slippage of transmission components, and monitor torque converter clutch slippage. 

• The PM generator, shown on the left, screws into, or is bolted into, the transmission or transfer case housing. 

• The reluctor wheel, shown on the right, is attached to the output shaft of the transmission or transfer case.

• Crankshaft Position Sensors (CKP).  The CKP is used for a reference signal to trigger the ignition and fuel injection systems.  the CKP is also used for engine misfire detection.

• The PM generator, shown on the left, screws into, or is bolted into, the engine housing. 

• The reluctor wheel, shown on the right, is attached to the crankshaft.

• Camshaft Position Sensors (CMP). The CMP is used to synchronize the sequential fuel injection system on an engine.  the CMP is also used to monitor camshaft timing on engines with variable cam timing.

• The PM generator, shown on the left, screws into, or is bolted into, the cylinder head or cam cover housing. 

• The reluctor wheel, shown on the right, is attached to the camshaft.

• Distributor pole piece/pick-up coil. - This was the equivalent of a crankshaft position sensor inside of the distributor.  It was used to trigger the ignition module to fire each spark plug at the appropriate time.