The ABS (Anti-lock Braking System) light is one of the most common dashboard warnings. Often, the culprit is a faulty wheel speed sensor. These sensors live in a harsh environment, exposed to heat, brake dust, and road salt.
Types of ABS Sensors
Before testing, you need to know which type of sensor you have:
- Passive (Inductive): These have two wires and generate their own AC voltage signal. They do not need external power.
- Active (Hall Effect/Magnetoresistive): These often have two or three wires and require a power source to operate.
Testing a Passive (2-Wire) Sensor
Passive sensors are the easiest to test with a basic multimeter.
- Resistance Check (Ohms): Unplug the sensor. Set your meter to Ohms. Measure across the two pins on the sensor side. A good sensor usually reads between 800 and 2000 Ohms. If it reads OL (Open Loop) or 0 (Short), it is bad.
- AC Voltage Output: Switch your meter to AC Volts. Connect the probes to the pins. Spin the wheel by hand. You should see a small voltage generation (usually 100mV to 1V AC) that increases as you spin faster. No voltage means no signal.
Testing an Active Sensor
Warning: Be careful measuring resistance on active sensors, as the meter current can sometimes damage them. The best way to test these is to check for power supply (usually 12V or 5V reference) at the harness connector, then use a scan tool to watch the live data stream.
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