ABS problems

Hi tacoma world,
I have a 2007 4wd tacoma with about 116,000 miles on it. Everything about the vehicle is great except for an issue with the ABS system.
The ABS system has been falsely engaging itself when braking under 20 mph. The tricky part is that it will only happen after driving for about 30 minutes. I will hear the click and buzzing sound of the abs engaging and I will feel the brake pedal get taken over by the system. There are no other results besides the sound and the sensation in the brake pedal.
I have taken it to numerous shops and even the dealership to try to fix this problem. All rotors, pads and other brake components are in 100% working condition. The issue the mechanics have been seeing is that the ABS senors will get dirty and accumulate metal shavings, which causes them to misread. A loose bearing was discovered to be causing friction with the wheel hub assembly and was believed to be the cause of the metal shavings that block the sensor. I have had the bearing replaced and have had both front sensors thoroughly cleaned. The problem still persists. During the life of this issue, the ABS light has only come on once (after the wheel bearing was replaced) and because that signals that the ABS system has been turned off, the problem went away. Upon taking it back into the shop, they were able to read the code from the light coming on (the first time a mechanic could get any code reads pertaining to the abs system). The code said that there was an obstruction on the abs sensors and they needed to be cleaned. The mechanic claimed that since the replaced bearing didn't solve the issue, that it was something in the wiring or the computer that tells the system to engage. He could not discover any electrical component that could be fixed.

If anyone has any advise or similar issues, please let me know.
Thanks!

 

What should be done is to monitor the wheel speed signal with a scan tool or techstream on a laptop. All 4 signal values need to be equal when driving straight. Any one(s) that the speed drops off are with still getting filings on the sensor tip or there is a problem with the sensor or tone ring. It could be as simple as some rust scale preventing the sensor from seating fully into the bore.

 

I am unsure about how they have been examining the systems. Would this be something that typical mechanics/ a dealership should do when the ABS system is malfunctioning?

 

It SHOULD be, but it depends on whether they now what they are doing or not - since you still have the issue, I'm going with the "not". ABS is pretty easy to diagnose with the right tools and knowledge, but the combination of the two can be hard to find.

Any wheel speed signal(s) that drop below the others at low speeds is where they need to look. Or if any are below speed at all times when traveling straight down the road- it could be as simple as a tire size or tire pressure mismatch.

 

I understand how different wheel speed reads will cause the ABS system to falsely engage, but can that explain the fact that this problem has little to no consistency of presenting itself? It tends to happen more frequently in hotter climates. Example: I will drive 30 minutes to work in the morning there will be no sound, but as my truck heats up in the texas sun and I drive it home in the afternoon, the sound will begin to happen almost 100% of the time I brake under 20mph.

 

Hmmn, the plot thickens a bit, doesn't it. Temperature changes certainly can affect electronic components though, I'm suspecting that a wheel speed sensor may be at fault, but I'd suggest having those signals monitored in hot temperatures while the problem can be duplicated and see if anything abnormal turns up.