Mechanics

BRAKES WORK FINE UNTIL LOW SPEEDS

2000 GMC Sierra • 180,000 miles

My brakes work fine till im at low speeds or almost a stop then the anti lock brakes kick in. I've replace master cylinder and nothing changed. I drive a 2000 TNX SIERRA 1500 4WHEEL DRIVE. ANY IDEAS

I don't have any idea what it could be. I'm more than capable of doing the work. Just not diagnosing
Avatar
BRITT SAPP
April 26, 2012.




Could it be a brake line, caliper, or even my power booster. Or does it sound like it is my anti lock brakes
AD

Tiny
BRITT SAPP
Apr 26, 2012.
This is very co min. What you have is called false abs activation. This will happen when one speed sensor is reading slightly lower voltage or frequncy than the others. To check it you mill need to graff the a c voltage and frequncy of all 4 sensors. My best gess is one of the front speed sensors has a larger air gap than the other. Rust can get under the sensor and rase it up a hare. You car try removing the front to and cleaning up the hole it might work. Or you have. To replace the sensor.

Djcl
Apr 26, 2012.
This is a real common problem caused by a signal dropping out from one of the front wheel speed sensors. The fix is to replace the sensors but there's a less expensive approach. Once you remove them, look for rust buildup under the mounting ears. That pushes the sensors up away from the tone ring and reduces the voltage of the signal being produced. One of the ingredients in developing that signal voltage is movement between the magnet in the sensor and the tome ring. When that movement slows down, the signal strength drops until it's unreadable by the computer, and it thinks that wheel stopped turning due to locking up.

If you see that rust buildup on the mounting ears, scrape it off, and consider filing just a little of the ear off so the sensor sits a little deeper in the mounting hole. 1/64" is plenty.

You can also use a digital voltmeter to measure the signal strength. The readings won't be accurate but all you're looking for is the one with the weakest signal.

Caradiodoc
Apr 26, 2012.
So I replaced both of my front wheel sensors. And the problem persisted. What would me my next step

Tiny
BRITT SAPP
Apr 28, 2012.
Is the another wheel speed sensor that I Should try to replaced

Tiny
BRITT SAPP
Apr 28, 2012.
Then you need a scanner that can access the ABS Computer to see what it's responding to. One of the signals has to be dropping out when it gets weaker at low speeds. As an experiment on the last one I worked on, we measured the AC voltage being produced by the two front sensors. The weak one developed.30 volts at about 10 mph, and the other one developed over.40 volts. Once the rust was removed and a little of the sensor's mounting flange was ground away, it developed over.70 volts and the false ABS activation stopped.

The scanner will not display the output voltage. Digital meters are inaccurate too because they are designed to measure 60Hz voltages. All you can do is use it to compare one side to the other to view the relative signal strength. The scanner will display miles per hour for each wheel. You're looking for the one that drops suddenly to 0 mph before the others do. That's when the signal is too weak to register.

GM has also had a real big problem with play that develops in the front wheel bearings in as little as 15,000 miles but I've only heard of that happening on the front-wheel-drive cars, not on the trucks. The play is normal but the tone rings inside the bearing assembly are so puny that very little signal is developed to begin with. You didn't list whether you have a 1500, 2500, or two or four-wheel-drive. The sensors are the same but the wheel bearings are different. The 2500 has a pretty significant tone ring similar to what's used by other manufacturers, not like what GM used in their cars.

Once you know which signal is dropping out, I would switch the bearing assemblies side-to-side, then see if the other side is losing the signal. If it is, replace that bearing assembly.

If the same side is losing its signal, you have to find why it's too weak. Rust under the mounting ear and metal filings on the tip of the magnet are common suspects but those would have been solved by installing the new sensors. Also look in the sensors' connectors for signs of corrosion. That can reduce the strength of the signal.

If both wheel speed sensors are generating a solid signal when the false activation occurs, suspect a cracked tone ring. Usually that will be detected while driving, and the computer will turn on the warning light and turn the anti-lock function off. The crack is detected as an extra pulse per wheel revolution making the computer think that wheel is turning faster than the other one. If that crack is wide enough, there will be a momentary dropout of that signal each time it passes under the sensor. The scanner won't show that because it occurs too quickly, but the computer is supposed to react fast enough to that loss of signal. To identify that, you would need an oscilloscope to view the waveform. That can even be done with the truck raised off the floor and by spinning each wheel by hand. You would see a nice steady sine wave, rising in amplitude as you spin the wheel faster, but it's the drop to 0 volts once per revolution you're looking.

The tone ring is available separately but to save time and labor cost, and to insure the quality of the repair, most mechanics will elect to replace the entire bearing assembly. The tone rings don't crack often, but you're looking for an uncommon cause. You've already addressed the common cause by replacing the sensors.

Caradiodoc
Apr 28, 2012.

AD