Wheel Speed Sensors: ABS and TCS Signal Use

Wheel Speed Sensors: ABS and TCS Signal Use

The signals from the Wheel Speed Sensors (WSS) are the most important sensor input to the Brake Control Module (BCM) for ABS activation.

Data from the scan tool provides information from the BCM as it processes the speed signal. Other sensors involved in the activation of the ABS system are the yaw, lateral acceleration and steering wheel position sensors.

A signal from the sensor can contain errors based on the condition of the reluctor or magnetic ring. Signal problems can cause the system to activate when it is not needed or set Diagnostic Trouble Codes (DTC). Information stored in the controller, called algorithms, is used to detect signal problems as well as a circuit open, short and/or ground.

Most vehicles in use today use two types of wheel speed sensors — variable reluctance and magnetoresistive. The wheel speed sensors’ input is key to the operation of the Electronic Stability Control (ESC) and Traction Control (TC) programming that can activate the ABS system and override the Electronic Throttle Control (ETC). The ESC, TC and ETC are programs that are located in different controllers that share information on the Controller Area Network (CAN) buss. This sharing of data input between controllers can produce problems and DTC that may appear to be unrelated to a WSS input. A scan tool is your resource to the operation of the CAN buss.

Sensor Types

Variable Reluctance
The variable-reluctance magnetic sensor has a permanent magnet pickup core and coil surrounding the magnet. The sensor is mounted at a specific gap from a notch-and-tooth reluctor ring. The rotation of the reluctor ring increases and decreases the magnetic field between the notch and the tooth to generate a voltage in the coil. As the gap between the notch and the ring changes with the rotation of the reluctor, the sensor generates an analog sine wave voltage signal. The voltage signal will increase in amplitude with an increase with the speed of the reluctor ring. The change in amplitude can affect the switching toggle in the BCM where a cracked reluctor ring may produce an extra switching toggle. The variable-reluctance sensor is a two-wire sensor mounted in the steering knuckle and rear suspension or axle.

Magnetoresistive
A Magnetoresistive sensor has a sensing element that contains an output module mounted on a silicon substrate and magnetic material. The magnetoresistive sensor’s main advantage is the ability to sense direction and operate with a wide air gap. The sensor is more stable in high vibration conditions and operates more efficiently than the reluctance sensor.

The sensor can detect the direction of rotation of the reluctor ring and zero speed. The sensor’s compact size can allow it to be incorporated into a hub bearing utilizing a alternating magnetic pole reluctor ring that is built into the rubber seal. A magnetoresistive sensor can be a two- or three-wire sensor. The three-wire sensor has a power, ground and signal connector pin.

The two-wire sensor has a power and signal connector pin. The ground is a part of the mounting for the sensor. The Philips magnetoresistive wheel speed sensor power input can range from 12 to 5 volts from the ABS controller.

The sensor produces a square wave signal with a high signal of 1.93 Volts, and a low signal of 0.64 Volts. The high signal must cross 1.29 Volts and the low signal must cross 0.97 Volts to low. The main advantage is that the amplitude of the signal does not change.

Diagnostic Procedures

The diagnosis of a trouble code or problem condition requires an investment in information and tools. The information and scan tool are expensive and are a good reason to charge for diagnostics. Diagnosing a DTC or failure condition should be performed in the following order. Technical Service Bulletins (TSB) search from your information data base. The condition may already exist and there is a known fix that can save time and money.

Scan Tool Retrieval of Diagnostic Trouble Codes and Data

The scan tool is the most efficient tool for diagnosing ABS sensor problems. If you are servicing three and four year old vehicles, your shop cannot function well without it. A Digital Volt Meter (DVM) and oscilloscope are used to verify a DTC or problem. An oscilloscope is the ultimate DVM. It measures voltage and time and many scan tools have a built-in scope. You can survive without a scope, but you have to have a good DVM that can measure frequency. Frequency can provide information that a signal is present.

Oscilloscope
The use of an oscilloscope can be a time consuming process that requires a certain amount of information to accurately display a wheel speed sensor wave pattern.

The following contains information for setting up an oscilloscope. The average automotive tire ranges from 24 to 28 inches in diameter. At one revolution per second, the vehicle would be traveling at a speed of four to five miles per hour.

The frequency of the sensor would give you an estimate of the number of teeth on the reluctor. This information will help you set the time base for your scope. The time base is set to allow all of the signals per revolution to appear on the display. If a wheel rotates at one revolution per second, the sensor and tone ring produces a 32-Hertz (Hz) frequency signal, the time base is calculated by dividing the frequency by 1.

This would be 0.03125 second, or 30 millisecond (mS) rounded down to the nearest mS. Set the time base for 30 to 40 mS. The amplitude of the signal for a inductive sensor will change as the speed of the reluctor increases. This will also affect the oscilloscope display.

A low amplitude signal can indicate an air gap or high resistance connector condition. If the oscilloscope you are using has a feature that will allow you to capture a series of frames, it will allow you examine the trace of one revolution of the reluctor. A cracked, damaged or missing reluctor can produce a signal that may not be processed by the BCM. The square wave signal does not need to be processed by the BCM; it is a direct input. Trouble Codes C1141 thru C1144 can be caused by a missing tooth and can be verified with a scope.

The aftermarket technician has to be more proficient in diagnosing system failures. He or she has to be familiar with more than one manufacturer and be able to understand their methods of diagnosis verses the dealership technician who deals with one manufacturer. The scan tool and service information are the primary tools, but a through knowledge of fundamental mechanical and electronic component operation is also an important tool in affecting the right fix the first time.

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