Ask a Mechanic.....The Phantom Menace

Every week, we answer your questions about what is going on with your vehicle. Today we’re discussing a Sierra with a brake concern.

Dear Ask a Mechanic,

I have a 2006 GMC Sierra pickup, and sometimes just as I come to a stop the brake pedal kicks back and it sounds like the ABS (anti-lock brake) is coming on. This happens even on nice days on dry pavement, and there’s no warning lights showing on the dash. What’s going on? – Buzzing Brakes

What you’re describing sounds like classic “phantom ABS,” a condition where the anti-lock brakes activate unnecessarily at low speeds just prior to stopping. It can also manifest the opposite way, briefly activating the traction control when accelerating away from a stop. Phantom ABS is a well-known occurrence in a number of vehicles from a variety of automakers.

The root cause is usually the same thing: a faulty signal that tricks the anti-lock brakes into believing either that a wheel has stopped turning or that it has slowed at too fast of a rate relative to the others.

Corrosion, age, or physical damage can all result in damaged sensor components. Depending on the degree of damage and the sophistication and sensitivity of the system itself, the fault may or may not be recognized. Typically, if the system does catch the glitch, the ABS (and related systems such as traction and stability control) will be disabled for at least the duration of that ignition cycle. After a restart, it’s common that the ABS will resume functioning until the fault is detected again.

In the case of your Sierra, the likely cause is that corrosion on a front hub assembly (commonly referred to as a “wheel bearing”) has pushed the wheel speed sensor fractions of an inch outwards and away from the toothed ring that it reads inside the hub. This small increase in the sensor gap is enough that the signal becomes too weak to read at low speed; the ABS system sees this zero input as a locked wheel and tries to prevent the skid. Because it only occurs at such low speed, the ABS systems in these models normally don’t detect it as a failure.

It’s possible to remove the sensor from the hub and grind the corrosion away, however the sensor frequently breaks off in the hub during removal and you risk contaminating the bearing inside with metal debris. The surest course of action is replacement of the affected hub assembly. A scan tool able to read ABS data will be required to verify that this is indeed the problem and determine which wheel’s speed sensor is at fault.

Ask a Mechanic is written by Brian Early, a longtime Wheels contributor and a working Red Seal-certified automotive technician with over 25 years’ experience. You can send your questions to wheels@thestar.ca. These answers are for informational purposes only. Please consult a certified mechanic before having any work done to your vehicle.