Active sensors, on the other hand, are smarter. Many use Hall effect wafers exposed to a magnet. As the wheel turns, the magnet’s north and south poles cause the Hall effect sensor to produce a “hall voltage.” An operational amplifier in the sensor then converts this into a higher voltage signal. Smart active sensors can even determine if the vehicle is stationary or moving backward. They produce a DC square wave signal, typically with two wires (the third for ground being chassis-mounted). These sensors often send out a periodic “I’m okay” pulse, and different pulse trains for forward or backward movement.
Bosch typically makes these for European cars, and they produce unusual pulse trains that require a lab scope to analyze, showing small voltage shifts. This two-wire smart sensor technology is not new and is also seen in airbag sensors, where they pulse current (drawing more or less current) to communicate their status. This allows them to send different square wave patterns, even for a crash.
Testing these smart active wheel speed sensors beyond a scan tool is crucial, as scan tools can have flaws, miss intermittents, or not display PIDs fast enough. Using a meter and scope allows for more accurate diagnostics.
ABS Stability Controls and WSS
