Wheel speed sensors are critical components of your vehicle’s Antilock Braking System (ABS), Traction Control System (TCS), and Electronic Stability Control (ESC). These sensors monitor the rotational speed of each wheel and send this data to the car’s computer. If a wheel speed sensor malfunctions, it can trigger warning lights on your dashboard and affect the performance of these safety systems. Knowing how to check a wheel speed sensor is a valuable skill for any car owner or DIY mechanic looking to diagnose and resolve ABS related issues. This guide will walk you through the process of checking your wheel speed sensors using a multimeter, ensuring you can confidently identify if a sensor is faulty.
Understanding Wheel Speed Sensor Testing
Before you begin, it’s important to understand the basic tests you can perform to assess a wheel speed sensor. The most common methods involve checking the sensor’s resistance and its AC voltage output. Additionally, verifying the DC bias voltage supplied to the sensor is crucial. These tests can be done with basic tools and provide valuable insights into the sensor’s condition.
Tools You Will Need
To check a wheel speed sensor effectively, you will need the following tools:
- Multimeter: This is essential for measuring resistance, AC voltage, and DC voltage.
- Automotive Jack and Jack Stands (Optional): Lifting the vehicle can make accessing and rotating the wheels easier, but it’s not always necessary depending on your vehicle and sensor accessibility.
Step-by-Step Guide to Check Your Wheel Speed Sensor
Follow these steps to test your wheel speed sensor:
1. Resistance Test
This test checks the internal coil of the sensor for continuity and proper resistance.
- Locate the Wheel Speed Sensor Connector: Find the wheel speed sensor connector for the wheel you want to test. These are typically located near the wheel hub, often behind the brake rotor or caliper.
- Disconnect the Sensor: Carefully disconnect the electrical connector from the wheel speed sensor.
- Set Your Multimeter to Ohms (Ω): Turn on your multimeter and set it to measure resistance (Ohms). Select a range that is suitable for measuring around 1000 to 1500 ohms.
- Connect the Multimeter Leads: Place one multimeter lead on one terminal of the sensor connector and the other lead on the other terminal.
- Rotate the Wheel by Hand: Slowly rotate the wheel or hub by hand.
- Observe the Resistance Reading: As you rotate the wheel, you should see a resistance reading on your multimeter. A typical reading for a functioning wheel speed sensor, while rotating, is around 1000 to 1500 ohms. If you get a reading significantly outside this range, or no reading at all (infinite resistance or an open circuit), the sensor may be faulty.
2. AC Voltage Output Test
This test verifies if the sensor is generating an AC voltage signal as the wheel rotates, indicating it’s properly detecting wheel speed.
- Set Your Multimeter to AC Voltage (VAC): Change your multimeter setting to measure AC voltage. Select a low voltage range, such as 2V or 20V AC.
- Reconnect the Sensor Connector: Reconnect the electrical connector back to the wheel speed sensor.
- Connect the Multimeter Leads (Backprobing): Carefully backprobe the sensor connector from the wiring harness side. This means you will insert the multimeter probes into the back of the connector while it is still connected to the sensor. Be careful not to damage the wires or terminals.
- Rotate the Wheel: Spin the wheel or hub at approximately one revolution per second (roughly 5 mph).
- Read the AC Voltage: Observe the AC voltage reading on your multimeter.
- Rear Sensors: Should typically produce a minimum of about 0.6 VAC.
- Front Sensors: Should typically produce a minimum of about 0.9 VAC.
If you do not get a sufficient AC voltage reading while rotating the wheel, the sensor might be failing to produce an adequate signal.
3. DC Bias Voltage Test
This test checks if the Electronic Brake Control Module (EBCM) is supplying the sensor with the necessary power.
- Set Your Multimeter to DC Voltage (VDC): Switch your multimeter to measure DC voltage. Select a range suitable for measuring low DC voltages, such as 20V DC.
- Disconnect the Sensor Connector Again: Disconnect the electrical connector from the wheel speed sensor.
- Turn the Ignition ON (Do Not Start the Engine): Turn your vehicle’s ignition key to the “ON” position, but do not start the engine. This will power up the ABS system.
- Connect the Multimeter Leads to the Vehicle Harness Side: Connect the multimeter leads to the terminals on the vehicle harness side of the connector (the side that does not go to the sensor).
- Read the DC Bias Voltage: You should read a DC bias voltage between 2.5 to 5 VDC. If you do not measure a bias voltage, it could indicate a problem with the wiring harness or the EBCM itself.
Interpreting Your Results and Further Diagnosis
- Resistance Test Results: A reading outside the 1000-1500 ohm range or no reading indicates a likely sensor fault.
- AC Voltage Output Test Results: Low or no AC voltage output while rotating the wheel suggests a sensor issue.
- DC Bias Voltage Test Results: Lack of bias voltage points to a potential wiring or EBCM problem, not necessarily the sensor itself.
If all wheel speed sensor tests appear normal, but you are still experiencing ABS warning lights, consider these additional checks:
- Vehicle Speed Sensor (VSS) Signal: The EBCM also relies on a VSS signal. If wheel speed sensors are fine, but the ABS light comes on around 5-7 mph, it might be a VSS signal issue.
- Brake Fluid Level Sensor: Check the brake fluid level and sensor.
- Brake Pedal Switch: A faulty brake pedal switch can sometimes cause ABS issues.
- Center Brake Lamp: In some systems, a failed center brake lamp can trigger the ABS warning light.
Conclusion
Checking your wheel speed sensors is a straightforward process that can save you time and money by allowing you to diagnose ABS issues at home. By systematically performing resistance, AC voltage output, and DC bias voltage tests, you can effectively determine if a wheel speed sensor is functioning correctly. Remember to consult your vehicle’s repair manual for specific values and procedures as they may vary slightly between makes and models. If you are uncomfortable performing these tests or the problem persists, it is always best to consult a qualified mechanic.
