Hi deweymc. Welcome to the forum. This is the "ever since" syndrome that every mechanic dreads. &Quot;Ever since you changed my oil, I got a flat tire a month later so it must be your fault". &Quot;Ever since you aligned my car, the power antenna stopped working a week later. It has to be your fault". &Quot;Ever since you changed my oil, my anti-lock brakes worked fine for two days, but whatever broke two days later must be your fault". Does the flaw in the logic make sense? There is a natural tendency to blame someone else when things happen.
I have to agree a loose hose on an air box that was just opened could have been caused by the mechanic, but why did it fall off so easily? And if it took two days to fall off, it makes sense it at least had the appearance of being in place after he checked the air filter, so why would he monkey with it further?
You found a real big potential clue with the anti-lock brake warning light. That is the light doesn't turn on until the car is moved. As soon as you turn on the ignition switch or start the engine, the ABS Computer performs a self-check of the electrical system. When that passes, the light turns off, typically after just a few seconds. There's your proof the wheel speed sensors are working and any pressure sensors or switches in the hydraulic system are working properly. The only thing that hasn't occurred yet is signals being developed by the wheel speed sensors because the wheels aren't turning. While driving, the computer expects to see the same speed reported from each wheel speed sensor. Turning a corner causes all four wheels to turn at different speeds, but that is programmed into the software. What the computer does not expect to see is a different speed from one wheel when the other three are exactly the same. There are two common causes for this. One is a tire that has a different circumference than the other three, but that is pretty obvious as the light would come on as soon as the tire was installed. Even a few pounds low on air pressure won't cause enough change in wheel speed to be a problem. The other cause is a cracked tone ring, (toothed ring) under the sensor. The gap can be detected as an extra pulse once per wheel revolution or as a momentary wheel lockup. This usually happens to a tone ring on the front wheels because they are exposed to salt and water. Water can seep under them, freeze and expand causing a rusty ring to crack. This happened to both rings on my minivan, about two years apart. They are not available except by buying the entire remanufactured half shaft, but I found some at a "pick-your-own-parts" type of salvage yard. They are normally press-fit, but I glued mine on with silicone gasket sealer to prevent water from getting in there again. In my case, it could take up to a few hundred yards before the crack in the ring was detected and the light turned on. It's a "latching" system meaning the system will not reset and become active until the ignition switch is cycled off and the engine is restarted.
It's real easy to inspect the tone rings on the front of some vehicles, Chrysler products in particular, but some are buried. Some are even sealed inside the wheel bearings and cause a real lot of trouble. GM is a good example of that. If you can't see yours, an easier test is to connect a scanner that can access the ABS Computer and view the individual wheel speeds under "live data". It will also read out the stored diagnostic fault code(s) which will lead to the circuit with the problem, not necessarily the defective part.
Be aware too that some systems don't perform their self-check until the vehicle starts to move or is moved from "park" to "drive". When that is the case, you will have to rely on the fault codes to know where to start looking. You still might be able to see the tone rings, but also look for a cut wire from a wheel speed sensor and low brake fluid level.
Cut wires aren't real common anymore because manufacturers are always improving the way they are anchored at the ends and routed to prevent catching on debris, and to keep flexing while driving over bumps and turning to a minimum. Low fluid level isn't too common either because the reservoirs hold more than enough for most conditions. It is also not common knowledge that it is proper for the mechanic to not top off brake fluid during other routine service such as an oil change. If the fluid is low, there is either a leak that must be addressed, or the disc brake pads are worn and should be replaced or inspected. When new thicker pads are installed, the pistons must be pushed back into their housings. (Moving out over the life of the pads is the self-adjusting feature characteristic of all disc brakes. When the pistons are pushed back in, all the fluid behind them goes back up into the reservoir and the level will be full again. Any previously-added fluid will spill out onto painted surfaces and cause a big mess. Bottom line is if your brake fluid level is real low, you might try adding a little to see if the warning light turns off, but you're ready for a brake inspection. Low fluid normally makes the red "Brake" warning light turn on which also deactivates the ABS system and turns the yellow warning light on because the system can't guarantee proper ABS operation.
The red warning is light related to low fluid level, a leak in one half of the system, or the parking brake isn't fully released. If just the yellow ABS light is on, those three things can usually be assumed to not be the problem.
Monday, July 5th, 2010 AT 11:33 AM