Auto Service Professional

DEC 2018

Magazine for the auto service professional

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38 A S P D e c e m b e r 2 018 A B S performing a resistance test, the WSS connector needs to be disconnected and your DVOM set to the ohms scale. Measure across both terminals at the sensor. e spec for your sensor will vary but in your information system you will find the spec tolerances. e AC signal test is the same hook up at the sensor that you did for the resistance test, but re- member to change your DVOM scale to read AC volts. is is where a lab scope shines in that you can see this reading in a graph form. When performing this test, spin the wheel by hand and note your reading. You should see a sine wave showing you the AC pulses that the sensor is putting out. Depending on the vehicle, where the sensor is mounted in relation to the toner wheel that it gets its reading from will de- termine how your sine wave looks. If you have a toner wheel out of position, you will see an er- ratic sine wave. If you have a weak WSS you will see a small sine wave. Just a minor incorrect reading is enough to give the EBCM some bad information and quite possible set an ABS warning indicator light or trouble code. In some cases, the brake pedal will pulsate or kick back on your foot while depress- ing the pedal and not activate a trouble code but makes the driver aware that something is amiss. When that happens as it did on a Chevrolet Ex- press van that I was asked to look at, the WSS had a very slight dropout that the EBCM would catch. is happened to be caused by a front wheel bear- ing that had an excessive amount of play. e bearing was moving so much that the WSS sensor didn't have a direct reading on the ABS toner ring that was mounted on the inside of the brake rotor. For an ABS to work properly, the front suspension of the vehicle should be checked prior to diagnosing the ABS system. A failed or worn out suspension component could cause an ABS problem but in fact the vehicle might not have a brake problem at all. I mentioned earlier, the ABS works in con- junction with other systems on the vehicle. e Cadillac Escalade uses approximately 12 differ- ent control systems depending on the vehicle options it came with. Some of the systems used are traction control, stability control, hill decent control system, hill hold start assist, intelligent brake assist system, the trailer brake control sys- tem and a few more systems as well. As you can see, the ABS has changed im- mensely over the last decade or so and that's why you need to stay updated on your training and in- Figure 2: Wheel speed sensors are critical signal generators for the system.

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