How to Test a Vehicle Speed Sensor with a Multimeter: A Comprehensive Guide

A malfunctioning vehicle speed sensor (VSS) can wreak havoc on your car’s performance, leading to issues with speedometer readings, cruise control, transmission shifting, and even anti-lock braking systems (ABS). Fortunately, testing a VSS is a relatively straightforward process that you can often do yourself with a multimeter and a little know-how. This comprehensive guide will walk you through the steps involved, helping you diagnose whether your VSS is the culprit behind your vehicle’s problems.

Understanding the Vehicle Speed Sensor (VSS)

Before diving into testing procedures, let’s briefly understand what a VSS does and why it’s so crucial. The VSS is essentially a small generator that produces an electrical signal proportional to the speed at which your vehicle’s wheels or transmission output shaft are rotating. This signal is sent to the car’s engine control unit (ECU) or powertrain control module (PCM), which uses the information to:

• Display Speedometer Readings: The most obvious function is providing an accurate reading on your speedometer.
• Control Cruise Control: The system relies on VSS data to maintain a set speed.
• Manage Transmission Shifting: Modern automatic transmissions use VSS information for optimal gear selection.
• Assist ABS: In vehicles equipped with ABS, the system uses the VSS data to determine wheel lockup and engage the brakes accordingly.
• Engine Management: The ECU can use the VSS signal for various engine management tasks, such as fuel cutoff during deceleration.

A faulty VSS can cause a range of issues. Some common symptoms include:

• Inaccurate or fluctuating speedometer readings.
• Cruise control malfunctions.
• Rough or erratic transmission shifting.
• ABS light illumination.
• Check engine light activation with VSS-related codes.

It’s important to note that symptoms can vary depending on your car’s make, model, and specific VSS type. The location of the sensor will also vary, which we will touch upon later.

Types of Vehicle Speed Sensors

There are several types of VSS used in vehicles, but the two most common are:

• Reluctance (Magnetic) Sensors: These are the most widely used type. They consist of a toothed reluctor wheel (attached to a rotating component) and a coil of wire. As the reluctor wheel rotates, the teeth pass near the coil, inducing a magnetic field and generating a signal. These sensors output an AC voltage signal that varies in frequency with speed.
• Hall Effect Sensors: These sensors use a Hall effect integrated circuit (IC) to detect changes in a magnetic field. They typically have a magnet inside the sensor body and a rotating magnet attached to the shaft being measured. They output a digital square wave signal, usually with a consistent voltage amplitude.

Knowing the type of sensor you have is helpful because it will affect the readings you expect to see while testing with a multimeter.

Tools You’ll Need

To test your VSS effectively, gather the following tools:

• Multimeter: A digital multimeter (DMM) is the most versatile option, allowing you to measure voltage, resistance, and continuity.
• Vehicle Service Manual: Having access to your vehicle’s service manual (either physical or digital) is invaluable. It provides the correct wiring diagrams, sensor location, and the manufacturer’s specifications.
• Optional: Safety glasses, a set of car ramps or jack and jack stands, gloves, and basic hand tools might be needed depending on the location of the sensor and the type of testing required.

Locating the Vehicle Speed Sensor

Before you can test the VSS, you need to find it. The location of the VSS varies greatly depending on the vehicle’s make, model, and drivetrain (front-wheel drive, rear-wheel drive, all-wheel drive). Here are some common locations:

• Transmission Housing: Many VSS are located on the transmission housing, near the tailshaft or the output shaft. This is a common location for rear-wheel drive vehicles.
• Transaxle Housing: In front-wheel drive cars, the VSS is often found on the transaxle, near the differential.
• Wheel Hub: Some vehicles, especially those with advanced ABS systems, have wheel speed sensors located at each wheel hub. These sensors may serve as VSS for some systems.
• Differential Housing: In some vehicles, the VSS is located on the differential housing.

Consult your vehicle’s service manual for the specific location of your VSS. You may need to remove some undercarriage panels or other components to access it. Always remember to disconnect the battery for safety precautions before any work.

Testing the Vehicle Speed Sensor with a Multimeter: Step-by-Step

Now, let’s get to the actual testing procedure. Here’s a breakdown of the steps:

1. Visual Inspection

Before even grabbing your multimeter, perform a thorough visual inspection. Look for:

• Damaged Wiring: Check the VSS wiring harness and connector for any cuts, abrasions, or corrosion. Damaged wiring can often cause intermittent faults.
• Loose Connections: Ensure the VSS connector is securely plugged into the sensor. A loose connection can lead to a faulty signal.
• Physical Damage: Look for any cracks or physical damage to the sensor itself.
• Contamination: Check for debris, grime, or other contaminants that might interfere with the sensor’s operation.

Clean the connector and the sensor body if needed. If you find any visible issues, address them before proceeding with electrical testing.

2. Understanding Your Sensor’s Wiring

Before you start testing electrically, it’s critical to know which wires you’ll be working with. Vehicle speed sensors will typically have two or three wires. This information should be in your vehicle’s wiring diagram. Here’s what the wires usually do:

• Two-Wire Reluctance Sensor: Usually, these will be two signal wires. One wire will be the positive signal from the sensor while the other will be a low reference signal path.
• Three-Wire Hall Effect Sensor: These usually consist of a power, a ground, and a signal output wire. The power wire will typically have battery voltage when the ignition is on.

Again, always consult your vehicle-specific wiring diagram or service manual. Identifying the wires correctly will prevent damage.

3. Basic Continuity Test

First, we will perform a continuity test to verify the sensor wiring is not broken or shorted out. Here’s how to perform the test:

• Turn the vehicle and ignition off: Disconnect the vehicle’s battery to avoid any damage and make sure the ignition is off.
• Access the sensor connector: Locate the sensor and the connector, and then disconnect it from the sensor.
• Set your Multimeter: Set your multimeter to measure continuity (the symbol usually looks like a sound wave or a diode).
• Perform continuity test: Place the multimeter probes in either of the pins. A good connection will result in the meter showing a reading near zero ohms or an audible sound. If the meter reading indicates an open circuit (often signified as “OL” on digital multimeters), it indicates a break in the wiring and a repair or replacement should be required.

4. Testing for Power (For 3-Wire Hall Effect Sensors)

If you have a three-wire Hall effect sensor, you can check if it’s receiving power. Here’s how:

• Reconnect the VSS connector.
• Set your multimeter to measure DC voltage.
• Turn the ignition switch on (but do not start the engine).
• Locate the power wire: According to your vehicle’s service manual, find the pin or wire that supplies power to the sensor.
• Connect your multimeter probes: Connect the red (positive) probe of your multimeter to the power wire pin and the black (negative) probe to a suitable ground (e.g., a clean metal point on the chassis).
• Read the voltage: You should typically read battery voltage (around 12V or 14V) if the circuit is working correctly. If you do not receive any voltage, there is likely an issue with the wiring or the power supply to the sensor. If voltage is present proceed to the next test.

5. Testing the Signal Output (Reluctance Sensors – AC Voltage Test)

For a two-wire reluctance sensor, you’ll test the AC voltage signal that the sensor produces when the wheels rotate:

• Set your multimeter to measure AC voltage. You may need to select a relatively low AC voltage range (e.g., 2V, 10V).
• Disconnect the sensor connector: Disconnect the connector from the sensor.
• Connect your multimeter probes to the sensor pins: Connect the multimeter probes to the two wires of the sensor.
• Lift the vehicle: Use jack stands or car ramps to safely raise the driven wheels off the ground.
• Spin the wheel by hand: With the ignition switch on, have an assistant turn a driven wheel.
• Observe the reading: Your multimeter should display an AC voltage that fluctuates as the wheel turns. The voltage will vary depending on the speed of rotation. The faster the wheel turns, the higher the reading. If there is no reading, then the sensor could be defective, or the sensor wheel could be damaged.

Note: The AC voltage reading will be relatively small at hand-speed rotation. You are looking for a changing, rather than large, value. As you spin the wheels faster, the voltage reading should increase.

6. Testing the Signal Output (Hall Effect Sensors – DC Voltage/Frequency Test)

For a three-wire Hall effect sensor, you’ll be looking for a pulsed DC voltage signal when the wheels are rotating:

• Reconnect the VSS connector.
• Set your multimeter to measure DC voltage or, preferably, frequency. If your multimeter has a frequency measurement function, this will be more accurate, otherwise use a DC voltage reading.
• Access the signal wire: Identify the signal output wire according to your vehicle’s service manual.
• Connect your multimeter probes: Connect the red (positive) probe to the signal output wire and the black (negative) probe to a good ground on the chassis or the ground pin of the sensor.
• Lift the vehicle: Use jack stands or car ramps to safely raise the driven wheels off the ground.
• Spin the wheel by hand: With the ignition switch on, have an assistant turn a driven wheel.
• Observe the reading:
  • DC Voltage: You should see a changing DC voltage that fluctuates as the wheel turns. The voltage readings should transition between a relatively low and a high value. As you spin the wheels faster, the frequency of the pulse will increase. The amplitude should remain relatively constant between the high and low value.
  • Frequency: You should see a frequency reading that changes depending on the speed of rotation. The faster the wheel turns, the higher the frequency.

If you are getting no frequency change or no consistent fluctuation in the DC voltage, then you likely have a faulty sensor. If your multimeter has a frequency measurement function, use this as this is the more accurate reading that you can get.

7. Interpreting the Results

Now that you’ve completed the tests, let’s interpret the results:

• Good continuity: Indicates that the wires are not broken or shorted.
• No continuity: Indicates a broken wire and wiring should be repaired or replaced.
• Proper power to a 3 wire sensor: A reading of battery voltage to a 3 wire sensor indicates the wiring is good and should be considered during troubleshooting.
• No power to a 3 wire sensor: indicates a break in the wiring to the sensor.
• AC Voltage on a 2 wire sensor with turning wheels: Indicates a sensor that is outputting a signal. If there is no voltage, the sensor likely has failed.
• Pulsed DC Voltage or Frequency on a 3 wire sensor with turning wheels: This indicates that the sensor is generating a signal and is functional.
• No Pulsed DC Voltage or frequency change on a 3 wire sensor: Indicates the sensor is faulty.
• Any of the other common symptoms still exist: If you have confirmed that the VSS is working correctly, then the fault you are experiencing is likely caused by something else.

If your tests suggest a faulty VSS, replacing it is the next step. If the sensor tests good, further troubleshooting will be required.

VSS Replacement

If you’ve determined your VSS is faulty, replacement is necessary. While the exact steps will vary depending on your car’s make and model, here’s a general overview:

• Disconnect the battery: Always disconnect the negative terminal of your battery before beginning any electrical work.
• Locate and access the sensor: Refer to your service manual for the sensor location.
• Disconnect the VSS connector: Unplug the electrical connector.
• Remove the old sensor: The sensor is usually held in place by one or more bolts or screws. Carefully remove these fixings and then gently extract the old sensor. Be aware that the seal may be tight and require some force.
• Install the new sensor: Install the new sensor, ensuring that any O-rings or seals are also replaced. Tighten the bolts or screws to the correct torque.
• Reconnect the electrical connector.
• Reconnect the battery.
• Test: After installing, clear any trouble codes with an OBD2 scanner and test drive the car to ensure the issue is resolved. If the issue persists, then some more troubleshooting is needed.

Always refer to your vehicle’s service manual for the precise replacement procedure. If you are not confident in your mechanical abilities, it’s best to have the work done by a professional mechanic.

Additional Tips

• Consult Vehicle Specific Information: Always use your vehicle’s service manual and wiring diagrams when performing electrical tests. This will ensure you are testing the correct components and pins.
• Be careful when working around electrical components: Never work with power on, and make sure you disconnect the negative terminal from the car’s battery first.
• Use the right tools: Do not try and modify your testing tools, use the tools that are required.
• Take note of the values: When testing, record any voltage, resistance or continuity readings that you measure. This will assist with any troubleshooting that you may need to do.
• Take your time: Never rush electrical repairs, it can be dangerous and cause serious damage to you or your car.
• If in doubt, see a professional: If you are unsure about testing or replacement procedures, take your car to a qualified mechanic.

Conclusion

Testing a vehicle speed sensor with a multimeter is a useful skill that can save you money and time. By following the steps outlined in this comprehensive guide, you can effectively diagnose potential issues with your vehicle’s VSS. Remember, accuracy and safety are paramount when working with automotive electrical systems. If you encounter any challenges, consult a trusted automotive professional for assistance. This knowledge empowers you to better understand your vehicle and address minor repairs yourself, allowing you to keep it running smoothly for years to come.