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

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
AD

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.
Nice advice Caradiodoc, well written!

I read about it a lot, yet not a whole of explanation explains why rust, metal shavings, and possibly corrosion diminish, or even kill the wheel speed sensor output voltage.

Basically and simplistically speaking, the sensor is a two pole magnet and as the tones pass over it, a signal is generated. The strength, amplitude, and frequency of this signal is dependent on the laws of generating electricity which to name a few are, speed on the tones passing over the poles, the distance between the poles and tones, and how "squarely" the tones ride above the poles. But all of these variables are dependent on the constant magnetic flux lines from the WSS pole pieces. If the magnetic field created by these pole pieces is week, so will be the signal. Rust any kind of magnet metal shaving stuck to the WSS pole piece effectively "shorts out" the magnetic flux field required to generate electricity (or signal).

Think of it taking a metal bar and placing across the two poles of a horseshoe magnet, then try to pick something up with the magnet in this condition. You will notice the magnetic pulling power is greatly diminished. Remove the metal bar and viola! The magnet is back to "grabbing" potential, trying to attach to anything magnetic friendly. Hence you can see how important it is to keep these poles clean.

Tiny
Sentrec
Mar 25, 2015.
Yup. I've never really seen the metal particles cause that many problems. The issue is the air gap and the resulting signal strength. We experimented with this on a friend's Suburban a few years ago. One wheel speed sensor was generating a 0.7 volt signal and the other was developing a 0.5 volt signal. Neither of these are anything to be proud of, and it's impossible to know if both wheels were turning at the same speed because we were doing this with the truck jacked up in his garage, not running down the road. The additional variable, as you mentioned, is speed. Just as generating a current in a generator requires "movement" between a magnet, (electromagnet), and the coil of wire, less movement means a loss of efficiency when it slows down. That causes the wheel speed sensors' signals to get weaker too at lower speeds and is why the false activation always occurs at low speeds.

The next issue is what signal voltage will the ABS Computer accept? With some common sensors like crankshaft and camshaft position sensors, they develop a square wave that goes from approximately 5.0 volts to 0.0 volts, or "high" and "low" in computer terms. What will the computer interpret it to be if the signal hits 2.5 volts? There is no "sorta" or "about". It has to be "high" or "low".

Speed sensors are expected to generate signals of varying voltage, but in this case the computer isn't looking for a "high" state, then a "low" state. It's looking for frequency, but there comes a time when the signal voltage gets too low for the computer to read it. Normally there is plenty of signal voltage down to three miles per hour at which most ABS systems stop activating. But as the air gap increases, (or the metal particles pile up), the signal gets weak enough that they drop out at low speeds above three miles per hour. In my buddy's case, cleaning the rust under the left front sensor got that voltage up to over 3.0 volts. We were so excited that on the right side we also cut off about 1/64" of plastic so the sensor would sit even closer to the tone ring. As I recall, that didn't accomplish much. We got that one up to around 3.0 volts too, but you also have to remember that digital voltmeters are only accurate on the AC volts scales at 60 Hz. The voltage they display will drop off rapidly as frequency increases.

The proper way to read sensor voltages is with a scanner that will do that function. It will show the AC voltage the computer is seeing. I can do that on Chrysler products with my Chrysler DRB3 scanner, but for other cars I only have an outdated Monitor 4000, and at the time I didn't have the ABS cartridge for it. I don't know what normal is on other car brands, but I do know only GM has trouble with weak signals caused by the design of two styles of their wheel speed sensors.

Caradiodoc
Mar 25, 2015.

AD