below are some tests you can perform with a multimeter. The more specialized tools are used by non dealers also.
1000 to 1500 ohms when you have the wheel rotating by hand.
a wheel speed sensor is out of the acceptable resistance range, the ABS warning should turn on and remain on when the ignition is turned on. If the warning lamp cycles on then back off after a few seconds, that usually means the wheel senor circuits are within the acceptable resistance range. This power-on test is intended to check for speed continuity and shorts.
If all the wheel speed sensor circuits pass the resistance test, you can test the output of each sensor. Connect your AC voltmeter to the sensor leads. Spin the hub or wheel at about 1 revolution per second (approximately 5 MPH). Read the AC voltage produced by each sensor. Rear sensors should produce a minimum of about 0.6VAC, front sensors should produce a minimum of about 0.9VAC. Lack of adequate signal from a sensor will set a code as soon as the vehicle is moving about 5-7 MPH. The EBCM actually reads the frequency of the sensor signals, but the AC output voltage is a good indication of sensor performance.
Each wheel sensor harness should also have a DC bias voltage when the ignition is on. The wheel speed sensor connector (vehicle harness end) should have between 2.5-5VDC at each wheel. Lack of a bias voltage can indicate a wire harness or EBCM problem.
The EBCM also relies on a VSS signal from the PCM. If the wheel sensors check out as normal, and the ABS warning lights at about 5-7 MPH, that can indicate the lack of a VSS signal (the EBCM is receiving all wheel signals but no vehicle speed signal).
Other things to check are the brake fluid level sensors, brake pedal switches, and the rear/center brake lamp. Some systems use the brake pedal switch and filament of the center brake lamp as redundant inputs, and a failed center brake lamp can cause the ABS warning to light (as well as cause the cruise control to be inoperative).
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