There are many options on the market when it comes to vehicle speed sensors. This guide will help you choose the best speed sensor for your application and budget. Today’s vehicles mainly use either inductive and hall effect RPM sensors to measure the speed (RPM) of the vehicle. The RPM sensors are most commonly inductive Variable Reluctance or Hall Effect type. While the basic operation of these sensors is very similar, to measure rotational speed, there are some very clear differences in the signal generated by these sensors.
Variable Reluctance Inductive Sensors – 2 Wire Sine Wave
The inductive variable reluctance (VR) sensor is composed of a transducer combined with a moving ferrous metal or magnetic object as a source of signal input. The VR sensor creates an oscillating AC voltage, sine wave signal as result of inductive effect. The AutoMeter part number 5293 is a 2 wie sine wave generator.
When the magnet or ferrous trigger wheel passes in enough close distance to the transducer of the sensor, the magnetic field around the coil is changed. The result of the magnetic field changing causes the coil voltage to be induced, which is proportional to the strength and rate of change of the magnetic field. Each tooth/magnet that passes beside to the sensor causes a singular oscillation, or pulse, in the signal output. The signal amplitude produced by the sensor is dependent upon the speed of the trigger and the number of turns in the coil.
VR Sensor Advantages and Disadvantages
While VR sensors are based on mature technology, they still offer a few advantages over a modern Hall Effect style sensor. The first major benefit is low cost – coiled wire and magnets are relatively inexpensive to manufacture and in turn lead to a lower unit price. VR sensors are a low cost solution that satisfy the majority of applications. This problem can be noticeably prevalent on applications with a low rear end ratio, a very high tooth count on the speedometer driven gear, and/or a larger than stock driven tire size.
Hall Effect Sensors – 3 Wire Square Wave
Hall Effect (HE) sensors are a relatively new technology, which many of the OE manufactures are utilizing in place of the aging VR sensors. The single largest advantage of a Hall Effect sensor is that the signal generated is not effected by the rate of change of the input, which provides a very clean signal, especially at low speed. The Hall Effect sensor effectively operates by pulsing the supply voltage on and off, producing a 12v square wave digital waveform. The amplitude of the sensor output remains constant, only the pulse frequency increases with RPM. In contrast to inductive VR sensors, which generate a voltage signal itself, Hall Effect sensors must be supplied with an additional voltage input wire. AutoMeter part numbers 5290, 5291, and 5292 are Hall Effect syle speed sensors.
Hall Effect Sensor Advantages and Disadvantages
Hall Effect sensors produce a well-defined digital signal that is less susceptible to nearby RF noise on a vehicle over other pulse sensor options. The 12v square wave signal produced from a Hall Effect sensor operates exceptionally well at low speeds, where as other types of sensors can have difficulty producing a strong signal on certain conditions. (Low final drive ratio, larger than OE tire size, etc.) The Hall Effect’s square wave signal is the cleanest signal available for speed.
Alternative Vehicle Speed Signals
PCM/ECU Vehicle Speed Output
Late model vehicles utilize VR or Hall Effect vehicle speed sensor (VSS) to control various devides on the vehicle; fuel/ignition maps, transmission shift points, cruise control, etc. Depending on the year/make/model, the PCM may also supply a VSS output signal to drive a factory speedometer. This signal is most commonly a 5v square wave signal. AutoMeter electric speedometers are compatible with most factory output signals, however, AutoMeter recommends verifying the signal type before making the connection to the speedometer.
GPS Interfaces
Very similar to Hall Effect sensors, the AutoMeter 5289 GPS interface generates a 12v square wave signal. The GPS interface provides all of the same benefits as a HE sensor, but without the need to install a vehicle speed sensor in the transmission. When selecting a GPS module, the refresh rate is the most critical factor to note to ensure a quick pointer response with maximum accuracy. The AutoMeter GPS module outputs an ultra-fast 10 Hz signal refesh rate, providing a smooth, responsive speedometer.