What is a Vehicle Speed Sensor? A Vehicle Speed Sensor is used to convert the signal from a spinning gear in the transmission to an digital signal (pulsed square wave) that can be used throughout the car for devices that utilize this input.
There are 4 basic types of vehicle speed sensors found on a Subaru. With all of these variations the thing to remember is that the gauge cluster determines the speed sensor type you will use. For all versions of VSS there is a signal transmitted to the Engine Control Unit (ECU) so that it knows how fast you are going.
Types of Vehicle Speed Sensors
97 and older - Mechanical type with cord that runs from the transmission up to the cluster to directly drive the speedometer. The cluster will convert a mechanical input to a electrical signal that is sent to the ECU.
97-98 - 2 wire input that comes out as a pulse that the cluster uses to display speed and also converts that pulse into a voltage that the ECU can process. 97-98 VSS Plug replacements here.
99 and newer - 3 wire input where the conversion from a mechanical drive to an electrical signal happens inside the sensor itself and then gets sent over to the ECU as a voltage read out (just like the other two types of VSS). This signal also gets sent to the cluster where a microcomputer moves the needle to show speed based on the same voltage the ECU receives. 99+ VSS Plug replacements here.
For models that use CANBUS (05+ LGT, 08+ Impreza, etc) - Wheel speed is pulled from each individual wheel into the ABS module. From there the module does the conversion to a CAN signal (think 0s and 1s) where all the modules (not just the ECU) can see it and use it as needed. The microcomputer in the cluster will take this information and drive the needle to register speed on the cluster.
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