Active Speed Sensor
There are two "types" of sensors
generally found on the modern car, the
passive speed sensor and the active
speed sensor. They both perform the
same function but work entirely different.
The passive speed sensor uses a magnet
with fine copper wire wrapped around it to
create its own alternating magnetic field. The polarity changes from positive to negative as the tone ring passes by the magnetic field. This frequency changes with wheel speed.
The newer active sensor uses a digital signal created by the ABS controller. This type of sensor uses a hall effect or a variable reluctance signal with a square wave pattern. The sensor consists of two wires; one is used as the positive (DC) voltage from the controller and the other lead as the return (Ground) to it. The advantages of the active speed sensor are their ability to read more accurately at slow speeds than the passive speed sensors. It doesn’t have to self-generate the needed voltage by the spinning action of the wheel. Also, since these sensors use a DC voltage they can detect not only the speed of the wheel but the direction of travel. This allows the controller the ability to calculate not only wheel speed but can also be used for the hill holding and hill descent control features found on some vehicles.
How Does It Work
The active speed sensor or “Magneto Resistive Sensor” consists of two parallel resistors and a magnetic material located at a precise distance from a permanent magnet. The resistors are about 1.4k Ohms each, however if you were to measure the resistance at the wire ends you would probably see 5 to 6 Mega Ohms. (That’s because you are reading the resistance values of the not only the resistors but the magnetic material inside the sensor.) These parallel resistors work together to create the voltage changes you’ll see on your lab scope as the tone wheel passes by the internal magnet. The tone wheel (if flattened out) looks like the square wave pattern it creates. As the tone wheels high part of its tooth is near the sensor a higher voltage is created while the opposite is done when the lower part of the tone wheel is near the sensor. Even though it is close to battery voltage at the sensor (10.6v is what I’ve found) you won’t be able to use a test light to read it. A test light consists of a ground lead, a bulb (resistance), and the positive end, which is basically the same thing the sensor has internally minus the magnets. If you try stabbing a test light on the positive lead to the sensor the computer sees that as a possible short to ground. Meaning, it thinks your test light is the sensor, because the return voltage on the ground side of the sensor has drastically changed. In fact, in most of these ABS systems if you do “short” the lead the processor shuts that sensor off (0.0v) until the next key cycle. This way it avoids any harm coming to the internal circuits of the processor. But, if you continued to check the circuit without turning off the key you’d end up back at the processor and more than likely come to the conclusion the processor is bad. It’s not… just cycle the key and recheck it.
Testing an Active Speed Sensor
The best tool for testing the active speed sensor is a good scanner, preferably one that can show the separate speed sensors in graph form. A lab scope is probably the best for a more precise answer and can be attached directly at the sensor to read the individual sensors or wires rather than the entire system as with the scanner. But if you’re looking for a quick test to determine if just the speed sensor is in working order you can make a quick little tester yourself.
You'll need a 220 ohm resistor, an LED and a 12v battery source. Disconnect the sensor from the car, (DO NOT TRY THIS WITH IT HOOKED UP!) hook up the battery to one side of the sensor leads, the other lead from the sensor is hooked up to your resistor and LED, the remaining lead from your LED should return to the battery. An LED is polarity conscious, be sure to have the current flowing in the right direction or it won’t light up at all. (The sensor doesn’t care which direction just the LED does). What you’ll see as you spin the wheel is your LED blinking on and off with an increasing and decreasing blink as the wheel speed changes. This is one of those times when the old school test light isn’t going to help you out at all; in fact, it might create more of a problem. Your scope or your scanner on these types of sensors is the better choice. But, this quick little LED tester might help you with your diagnostics; it won’t test the entire system but will confirm if the speed sensor is functioning. Testing is always better when you have more than one method to determine the condition of a component. In this case, a scanner, a scope, or a homemade LED tester can help you in diagnosing these active speed sensors. Always test first, test second… and when still in doubt… test a third time.