The first thing is to have the diagnostic fault codes read and recorded. Based on your description of the symptoms, it is pretty likely a wheel speed signal is being lost from one of the front wheel speed sensors. This has been a real common and recurring problem for GM front-wheel-drive cars. The front sensors develop rather wimpy signals to start with due to their design, then, a little normal play develops in the wheel bearings that lets the sensor move away from the toothed tone ring. That results in the speed signal becoming so weak the computer can't see it. At first this results in false activation when braking normally. It occurs at slow speeds because this type of sensor signal naturally gets weaker as speed slows down. Eventually the signal gets so weak it can't be detected at any speed. The computer will turn the system off, set a diagnostic fault code, and turn the yellow warning light on to tell you.
There's a couple of important facts to be aware of. First, this is so common, it can happen to replacement wheel bearings within as little as 15,000 miles. Don't be angry with your mechanic. It's just the nature of the design. The bigger issue is in how the computer determines when to set a diagnostic fault code. To do so, there is always a long list of conditions that must be met, and that includes certain other codes can't already be set. The computer compares all four wheel speeds to each other. When one signal goes missing, anything that uses that signal as a reference will have its test suspended since there's nothing to compare it to. If you have the problem addressed in a reasonable amount of time, that is usually the end of the story.
The problem comes in when you wait months or years. It's pretty likely a second problem is going to develop in that time, but since many of the system tests run by the computer have been suspended due to the first problem, that second one never gets detected, and no fault code is set. When you finally go to see your mechanic, all he has to go on when preparing a repair estimate is that first fault code that's in the computer's memory. Once that repair is completed and the fault code is erased, the computer resumes the rest of the tests it runs continuously, and that's when the next problem gets detected. Now the mechanic has to start the diagnosis all over, then tell you more parts and service are needed. We hate having to do that, and it's real frustrating for car owners. You incorrectly assume your mechanic didn't diagnose the original problem properly, or didn't fix it properly, when in reality, there were already two or more problems there; we just only knew about one of them. Most of this frustration can be avoided by just not waiting a real long to have the original problem repaired.
As for "is it safe to drive", ... When the ABS Computer turns the system off, you are left with the base brake system, often called the "foundation" brake system, which is basically identical to what every car has had for the last century. The anti-lock feature is an option added on to the standard brake system, and you are allowing that option to go unused. The problem is in how it affects the aftermath of a crash, particularly one caused by someone else. More on that in a minute.
It's important to understand what anti-lock brakes are designed for. A skidding tire has no traction so it will slide a lot further than one that is just at the edge of starting to skid. That means if you lock up the brakes in a panic situation, there's a good chance you're going to slide into the car that just ran the red light. If you're a calm driver and fast thinker, you'll let off the brake pedal a little to keep the tires turning so the car stops sooner.
Here's where the ABS function shines. Suppose you suddenly need to panic stop as quickly as possible. You push harder and harder on the brake pedal until eventually one tire starts to skid. Knowing it's going to lose traction, you release a little pressure from the brake pedal. Sure enough, the skidding stops, but the problem is three tires were not yet up to their maximum braking potential, yet you let off the brake pedal. Those three tires were not even close to their maximum stopping potential. This is what we had to work with since almost forever. Also, since most drivers didn't have the reaction time, we just skidded the tires into the crash, then there were skid marks for the police to measure.
The ABS Computer watches all four wheel speeds, then, during brake pedal application, it watches for the wheel with the lowest speed. First it blocks additional brake fluid pressure to that wheel even if you push harder on the brake pedal. If the wheel doesn't pick up speed, the computer bleeds off some fluid pressure, then, once it does speed up, it opens another valve to reapply fluid pressure. This "block, bleed, apply" is the same thing we were taught to do by pumping the brake pedal years ago, only the computer can do it up to 30 times per second, (that's the horrendous buzzing noise you hear), and most importantly, it does it for each wheel totally independent of the others. That means that while in a panic stop you would release brake pedal pressure as soon as one wheel gets close to skidding, (and three weren't yet up to their maximum stopping potential), the computer keeps all four tires right at the edge of their maximum stopping power.
To prove how valuable this is, I've just about torn the seat belts off their hinges when trying to do a panic stop on sand-covered pavement with my '93 Dodge Dynasty. That ABS system is extremely effective to the point that I have to worry about being rear-ended. This is the "Bendix-10" ABS system. Some GM models used the similar "Bendix-9" system that ran the two rear wheels together. If either one was about to lock up, it reduced brake fluid pressure to both rear wheels at the same time. There was more difference to it than that because on the Caprice Classic police cars, the stopping distance was much longer on those cars with that system than on those with no ABS. That brings me to the true purpose of anti-lock brakes. It is not to shorten stopping distances even though that was the result on a lot of car models. It's true purpose is to allow you to maintain directional control. In simple terms that means you can steer around an obstacle. Since a skidding tire has no traction, you can turn the steering wheel all you want to, but the car is going to go straight. ABS lets the tires turn and maintain traction so you can steer and stop without having to consciously try to control all of that in a split second.
That brings me back to my point of great value that I said I'd get to in a minute. This involves cars with altered ride height, trucks that are raised, anything that has non-standard wheels and tires, and especially cars with safety systems that are known to be non-functional, even if they were optional. You can easily be involved in a lawsuit after the other guy ran the red light. All of these modifications adversely affect the vehicle's steering response, directional stability, and braking distances. The other guy's lawyer or insurance investigator will convince a jury that you were partially at fault for the crash because you were less able to avoid it, and they will be right.
Given all the intended benefits and unexpected side benefits of anti-lock brakes, and especially the risks of finding out your failure to maintain a safety system may have led to a crash or the severity of it, I hope you will see why I'm strongly suggesting you don't ignore the repairs. Chrysler developed the first ABS system for some 1969 models based on those used on fighter planes landing on aircraft carriers. That wasn't much of a system, but it led the way to what we have now, and most of the systems do a real good job.
Tuesday, July 28th, 2015 AT 6:03 PM