Brake wheel sensor

  • 4 CYL
  • FWD
  • 56,000 MILES
Stability track and service brake assist light came on yesterday. Stopped car, turned back on. Light was out. Then it came on again today but did not go out with stopping and restarting engine. Where is the wheel sensor located to test with ac volt meter?
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have the same problem?
Wednesday, May 31st, 2017 AT 7:16 PM

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Digital voltmeters are only accurate when measuring a 60 Hz sine wave, meaning house current. Wheel speed sensors put out a distorted square wave of varying frequency. Some meters require the sine wave to go positive, then negative, like house current does. Three-wire crankshaft position sensors and camshaft position sensors develop signals that go from 0 volts to typically 5.0 or 8.0 volts, so you're measuring a pulsing DC voltage. Some digital meters set on the AC Volts scale will read 0 volts, falsely indicating there is no signal voltage being developed. Some meters will indicate there is something vs. Nothing, but the actual voltage will change as speed changes. Wheel speed sensors are two-wire magnetic sensors with signals that do go positive, then negative, but again, all they are good for is to see if you are getting something. It is impossible to compare the signal from a good sensor to the signal from one that is suspect or is setting a fault code.

Basically, what I am saying is using a voltmeter to test a sensor is a huge waste of time and will leave you questioning the results, if you are lucky, and will lead you down the wrong diagnostic path if you are unlucky. We do not have time to waste experimenting when customers are paying us by the hour. Instead, let the ABS Computer detect a missing signal and set a fault code. If you get a code related to an electrical problem with the circuit, use your ohm meter to verify a break or short in the circuit, but there is no value in doing voltage tests. You can also observe the wheel speeds on a scanner. That is all that's important to the computer. The computer is not looking at signal voltage. It is looking at frequency. Some of the more expensive digital meters can measure frequency, but after twenty five years as a brake system specialist, I have never done that, so I do not know what to tell you to expect, or what value that would have. The computer can detect the extra pulse per wheel revolution if the tone ring is cracked. You cannot do that even if you used two meters at the same time. Instead, just read the wheel speeds on the scanner.

You did not list the fault code so I do not even know if you have an electrical problem with the sensor circuit. You can also have a mechanical problem, and GM is famous for that with their front-wheel-drive cars. Most of their front wheel bearings have the speed sensors built in. They develop real wimpy signals on a good day, and with a very little normal wear, the bearing can become sloppy enough that the tone ring can move away from the sensor ring, resulting in the signal going too low for the computer to read. That gets worse at lower vehicle speeds when the sensors' signals naturally get weaker anyway. The computer thinks that wheel locked up, so it pulses the valves to try to get it back up to speed. That is referred to as "false activation". That can occur to a bearing with as little as 15,000 miles on it. The dealer's scrap metal bins are full of these bearing assemblies.

The first thing you need to do is read and record the fault code number, then determine if it is referring to an electrical problem or a mechanical problem with that sensor. Either one can trigger a fault code that turns on the yellow ABS warning light, but the diagnostic steps are quite different. You can get a clue by observing when the warning light turns on. An electrical problem, such as a cut wire or corroded connector terminal will be detected as soon as you turn on the ignition switch and the system goes through its six-second self-test sequence. The yellow warning light may flicker off, but it will turn right back on again. If the light turns off after that self-test, everything is okay electrically. That is further proof there is nothing to be learned from trying to take voltage readings, or resistance readings for that matter. The computer just did that for you.

Mechanical problems are detected once the car starts moving. The computer expects to see the same wheel speeds from each wheel. You cannot go by frequency here either. Most cars have different numbers of teeth on their tone rings for the front and rear wheels. If no signal is seen from a sensor that is okay electrically, the problem is caused by something mechanical. That is where the sloppy wheel bearing becomes the common suspect. On other car brands, a missing tone ring will be seen as a missing signal when three good signals are showing up from the other wheels. A cracked tone ring can also be detected.

The third group of fault codes relate to pressure switches in the hydraulic controller. Those codes can be set by a defective switch, and by a good switch that is not turning on or off when it is expected to do so. Some of those defects may not be detected until ABS operation is initiated during a skid.
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Wednesday, May 31st, 2017 AT 11:14 PM

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