Gerard yes you are correct . The speed is read by the module the faster or slower each wheel turns will change voltage. when the computer see`s one wheel turning slower then the others it will turn on the abs light or while braking it will hold some brake pressure from the slower wheel trying to match the slower wheel to the other wheels. You can read this speed with a DVOM set to the AC scale. My problem is rust the air gap has changed from sensor to reluctor [notched ring on axle] The computer see`s this as different speeds from each wheel .I would like some one to give me the AC voltage at 70km per hr from each off the three sensors from the same car on a working system. I will then adjust mind to match. Thanks Dale