I guess it's hard for me to draw a decent conclusion with so many variables in the braking system. A sticking caliper is not common on GM cars, but it CAN happen. Any caliper sticking intermittently is not common, but it CAN happen. When it sticks, it can reduce the effectiveness of the anti-lock function, but the computer will just assume the tire is sliding on ice or is finding extra traction compared to the other three tires. That won't result in a fault code. Also, the ABS doesn't even do anything unless the brake pedal is pressed AND one wheel is spinning slower than the two other slowest wheels.
I suspect any fault code in the computer will be related to an overheated sensor and that will be the result of a mechanical problem, (sticking caliper). The question is if it was overlooked during an inspection or test drive, was it sticking intermittently and why?
Rusted pins, bolts, or slides that a caliper mounts on will also allow it to stick but I don't believe that would be so bad as to melt a wheel cover. Usually that problem just results in a slight pull to one side during or after braking. It can also keep only one pad on that wheel partially applied so you might see an inner pad, for example, half worn out while the outer pad looks like new.
I suppose a leaking valve in the ABS hydraulic controller could be leaking and partially applying the brake but I've never really heard of that on any car. I don't think that is something that is even monitored by the ABS computer. The place to start is by having the codes read. Their description will provide a good starting point. By the way, when the yellow ABS warning light is on, the ABS system is disabled and shouldn't even be a factor. If they're still using the same system they had in the '90s, your system doesn't even pump and store high pressure fluid like other systems, so internal leakage can't even be considered. Three rotating gears run screws rapidly up and down that pressurize and depressurize fluid in their cylinders many times per second that feed either front wheel or the rear pair of wheels as necessary to control the braking force to that wheel. With the system disabled, there will be no pressurized fluid to cause a brake to apply on its own. That leads back to a sticking caliper.
If I understand your complaint correctly, it isn't so much that there is a problem, it's the fact someone should have caught it earlier during an inspection. That might have saved overheating and melting parts, but I can think of a few scenarios where it could be overlooked even during a thorough inspection. Some hints or clues that might have been visible could also be present when there is no other underlying problem. One example is uneven pad wear from side to side or between the inner and outer pads on either side.
I should probably clarify too, after rereading your first post, that the computer only memorizes fault codes for electrical problems, not physical problems with the base brake system. By that I mean worn pads, rotors that are too thin or grooved, sticking calipers, even fluid leaks are not monitored. What IS monitored is the continuity to the four wheel speed sensors, the four signals from those sensors, the continuity to the three motorized actuators and the voltage supplied to the brake light switch. One real common problem only GM has been having for a few years is "dropouts" from one wheel speed sensor due to the tiniest amount of play in a wheel bearing. Only they build the wheel speed sensor into the bearing assembly, and while the bearing itself is not defective or even excessively worn, replacing it is the only way to get the ABS sensor working properly again. While that IS indeed a mechanical problem, it shows up to the computer as an electrical problem in the form of a loss of signal from that one wheel.
Another mechanical problem that should be mentioned for the benefit of other readers with different brands of cars, is a cracked toothed tone ring on the cv joint, rotor hub, or brake drum hub. Water can seep in under it, freeze and expand causing the ring to crack. The sensor will pick up that crack as another gap between two teeth and end up with one extra pulse per wheel revolution. While that doesn't sound like a lot, it can be enough to turn on the warning light and disable the system. Even though the ABS doesn't do anything unless the brake light switch is turned on, (brake pedal is pressed), the computer still monitors wheel speeds while driving, in part, to identify if one or two tires are a different side than the rest. This is how the system works on Chrysler products. Based on distance traveled, usually less than a mile, the computer can determine the difference between a cracked tone ring, (causing 48 pulses per tire revolution instead of the normal 47), a slightly different size tire, and the vehicle going around a gentle curve on the highway. Part of the strategy is on a curve, all four wheels will be going different speeds while if three wheel speeds are the same, it knows you're going straight and the fourth wheel is supposed to going the same speed.
Sorry to get so long-winded with that description but it might help someone else looking for a solution to their problem. Your car can not suffer from the cracked tone ring. It uses a stamped steel disc inside the wheel bearing that can't get water in it and it won't crack. Only the signal dropouts have been real common, but that won't lead to melted wheel covers either.
One simple thing you can look at quickly is the rubber bladder seal under the cap on the fluid reservoir. If it balloons up and is mushy and hard to reinstall, the fluid is contaminated with petroleum product. If that's the case, we have other, serious issues to discuss. That WILL lead to sticking brakes eventually and replacing just a caliper won't be the final fix. By the way, the most common clues to contaminated fluid is a car that won't move, (brakes stay applied), and melted wheel covers, usually on multiple wheels. If that seal looks normal and pops back into the cap with light finger pressure, (be sure your fingers are clean and have no grease on them), I suspect the whole problem stems from nothing more than a sticking caliper. They are normally replaced in pairs to insure even braking. Also, if the mechanic says you need rotors too, that is real common on today's cars because they are made so thin to start with to save weight and improve fuel mileage. The good news is most rotors that used to cost near $100.00 are now often less than $25.00. Many shops elect to replace them during routine brake service rather than machine the old ones because it saves time, ($$ for both of you), and eliminates the cost of machining that you would ultimately pay for. The other advantage to replacing both rotors is they will heat up and cool down at the same rate. The friction characteristics of the brake linings change with temperature, so the rotors being the same thickness promotes even braking all the time. This has become a real big issue with larger, heavier pickup trucks. It's not much of a problem on smaller cars. On some trucks, (including two lemon law buybacks I was ultimately involved with), a difference in rotor thickness of.007", (the thickness of two sheets of paper), is enough to cause a wild brake pull after the brakes got hot. That's on rotors that are way over an inch thick. Years ago we never had such a problem. Bottom line is once the sticking brake cause is identified and corrected, a standard brake job with two new rotors will probably provide the most satisfactory braking performance. It doesn't sound like anyone did anything to your car to cause the problem. They just didn't find or search for the cause on previous inspections.
Caradiodoc
Monday, August 30th, 2010 AT 1:06 AM
