ABS Activates at low speed

Hi,

The ABS system is set up using three sensors. Both front wheels have a speed sensor and the VSS on the rear of the transfer case functions as both the vehicle speed sensor and the ABS sensor. (See pic 1 below)

Since you are having trouble with the speedometer, that is the one I would focus on. First, if it hasn't been replaced, remove it and inspect it. This is a hall effect sensor which basically means it is a magnet that produces an AC current as a toothed ring (reluctor ring) spins past it.

Here is the thing. The sensor could be bad and can be tested the same as an ABS sensor, simply checking for AC current at low speeds (by turning the rear wheels so the drive shaft turns). I will provide a link explaining how to test an ABS sensor below.

If the sensor is good, then you need to confirm the toothed reluctor ring isn't damaged or dirty. It's tough because of the location. Take a flashlight and mirror if needed, to look into where the VSS mounts while slowly turning the drive shaft. Watch for broken cracks, missing teeth, dirt, or anything that would prevent the sensor from being accurate.

Next, I realize the front ABS sensors have been replaced, but the reluctor wheel on them can cause this as well and are not part of the hub assembly. See pic 2. I have arrows pointing to damaged points on the reluctor. Check this as well.

Overall, I suspect the VSS on the transfer case is the issue. That is where I would start my tests. Here is the link I mentioned above.

https://www.2carpros.com/articles/abs-wheel-speed-sensor-test

Let me know what you find or if you have other questions.

Take care,

Joe

See pics below.

Hi guys. Excuse the interruption. A real common cause of the "false activation" when slowing down is due to that wheel speed signal dropping out, as you were told. The most common cause of that is rust build-up under the front wheel speed sensors. These sensors develop rather wimpy signals to start with. Add to that the fact that speed affects signal strength, by the time you reach 15 - 20 mph one of the front signals becomes too weak for the computer to read it. It thinks that wheel locked up, so it activates to modulate brake fluid pressure to that wheel. That's the buzzing and vibrations you hear and feel.

The distance between the magnet and the coil of wire in the sensor, to the toothed tone ring on the axle shaft / wheel bearing also affects signal strength. The rust under the sensor pushes it away from the toothed ring. That results in a weaker signal that is more likely to drop out at lower speeds.

Most ABS systems are designed to not activate or to stop activating below roughly 9 - 15 mph because the wheel speed signals are always going to be really weak at that point. The function of all ABS systems is so you can maintain steering control. At real low speeds that is not an issue.

The arrow in this photo shows where that rust develops. Remove the sensor, then scrape off any rust you find there on the bearing housing.

Hi,

Did you confirm there isn't dirt or corrosion where the sensors mount? Also, are the tone rings in good condition, not loose, out of place, or damaged?

If you are not getting an AC signal, one of the aforementioned things has to be the issue. The sensor doesn't receive power to work. It produces it.

Let us know.

Joe

Hi,

Was it clean inside where the sensor pushes into the hub? Also, did you inspect the toner ring/exciter ring for dirt, damage, or anything that would cause issues?

As far as replacing the hub, I'm not sure that will take care of the issue. As the wheels spin, toothed rings interrupt magnetic fields in the wheel speed sensors. This causes each wheel speed sensor to generate an AC signal. Something is preventing that from happening. The toner ring/exciter is mounted on the half shaft, and I've had them loose, damaged, and corroded with dirt.

A bad hub bearing can cause the issue if it's loose enough. However, I question if that is the issue you're having.

Let me know.

Joe

Hi,

I'm glad to hear it's fixed. I knew the problem was at the sensor location and not an electrical circuit from the vehicle. It just makes it a little difficult to diagnose when the car isn't in front of me. I had thought it could have been the tone ring or a dirt issue. However, what you did makes perfect sense.

Regardless, I'm glad it's working. Please feel free to come back anytime in the future if you have questions. You're always welcome here.

Take care,

Joe