The passive speed sensor creates its own AC signal that changes frequency with wheel speed. This signal is only present while the wheel is turning at a rate fast enough to create the AC signal.
Passive wheel speed sensors generally do not read at or below 2 mph coming from a dead stop. During a wheel spin condition or when one wheel is turning slower than the other three wheels, the system will reduce the hydraulic brake pressure (under braking) to allow that wheel to regain its traction with the road surface.
The signal can be affected by a number of different problems. From a weak sensor, damaged tone ring, debris, rust, wiring issues or the gap between the sensor and the tone wheel. One of the most common problems found is when someone replaces the sensor but does not push it in all the way, causing a fault code for that sensor. In most cases, any variation in the preset values not reaching the ABS control will set a code for that particular sensor.
Some of the drawbacks of the passive-type wheel speed sensor are its inability to operate at very low speeds or in reverse. This is one of the many reasons why most manufacturers have gone to active wheel speed sensor systems.
Determining which type of speed sensor is on the vehicle you’re working on can be done several ways. Obviously, using a scan tool or checking the manufacturer specs are good ways, but looking at the sensor itself won’t give you any clues. Both are two-wire sensors, and they both look basically the same.
The main difference would be the resistance readings. The typical reading f
or a passive wheel speed sensor is between 1,000 and 2,500 ohms, while the active speed sensor is 5 to 6 mega ohms or more. Don’t rely on this as the ultimate answer in case there is a problem with the sensor itself.
Some vehicles will send a DC voltage to the sensor through its ground wire. The signal will travel up the leads just as the generated AC signal does. This allows the sensor to be tested by the ABS system without the vehicle moving. This can set a code shortly after the key is turned on if there is a problem with the wiring, connection or the sensor.
This DC signal is not generated by the sensor but by the controller, and is only used as a way to verify whether the sensor is there, not the actual ability of the speed sensor to create the AC signal. That’s entirely up to the rotation of the wheel.
BIAS voltage can detects any high resistance or an open circuit. Anytime the BIAS (DC) voltage is not received back at the ABS controller, the controller will assume the sensor is not functioning. To measure this BIAS voltage, a lab scope is going to be your best choice. Ground the lead of the scope on a good battery ground, and connect the positive lead to the signal wire at the sensor. Without the wheel spinning, you can observe the DC voltage being supplied to the circuit.
Testing the Sensor
If the signal is viewed on a scope, the voltage will rise above the zero volt line when the key is turned on. A typical BIAS voltage is about 2.5 volts (some manufacturers use a slightly higher voltage). You might be able to read this with your DVOM, but some prefer using a scope. If you use a pin or needle, never stab directly into the sealed wiring of the sensor leads as this can cause problems down the road when water, salts and other debris start to seep into the opening you just created.
On some vehicles, the ABS controller turns off the BIAS voltage when the sensor is unplugged. So if you are trying to read the voltage, you will have to do it without disconnecting the sensor. Basically, any disturbance in the two wires from the cont- roller to the sensor can be detected by the controller as a problem and can cause the loss of the BIAS voltage. The controller will “self-save” itself from any damage this way. In these cases, if you do not see the BIAS voltage at the sensor, it’s a good time to trace for a bad connection or broken lead.
Practice, More Practice
With a little practice, you can diagnose these sensors quickly and efficiently. Understanding how they work and how to properly test these sensors will make your job easier and more efficient.
A good method of increasing your skills is to practice hooking up the scope or scanner to a speed sensor on those slow days at the shop. That way when the need arises, you’ll be the sharpest tool in your shop’s diagnostic toolbox. Captions: Wheel speed sensor signals can be triggered by a number of problems such as rust, a weak sensor or the gap between the sensor and the tone wheel. Readings for a passive wheel speed sensor tend to be between 1,000 and 2,500 ohms, while the active speed sensor is between 5 and 6 mega ohms or more. It is possible to read passive wheel sensor signal with a DVOM, but others prefer to use a scope to diagnose problems.