One word of warning. Diagnostic fault codes never say to replace a part or that one is bad. They only indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition. When a sensor or other part is referenced in a fault code, that part is only the cause of that code about half of the time. First we have to rule out wiring and connector terminal problems, and mechanical problems that are causing the defective condition to occur. Most electrical parts can not be tested directly, so we have to rule everything else out first before spending money on a replacement part.
In addition, many fault codes get real specific and knowing their definitions and what it takes to set one will help in finding the cause. For example, with an ABS wheel speed sensor, you can get a fault code for "left front wheel speed signal missing", or "left front wheel speed sensor circuit open". Both of these involve no signal showing up at the computer for a left front wheel speed signal, but the causes are very different, and so are the clues. In the case of the "wheel speed sensor circuit open", that is commonly caused by a torn wire going to the sensor, and that will be detected during the initial six-second system self-test when you turn on the ignition switch. The yellow ABS warning light always stays on during that six seconds, then, when that defect is detected, the light might blink off for an instant, but it will turn right back on and stay on. The clue is the car isn't moving yet.
That sensor circuit can still be okay when the self-test takes place, but still fail to generate a usable signal. The wheel speed sensor circuit is not open, so that test passes and the fault code for that is not set. However, once the car starts to move and good wheel speed signals show up from the other three wheels, the computer sees the fourth one is missing, and that is when it sets the code "signal missing". In the first example the warning light stayed on all the time the ignition switch was on. With this one, the light turns on like normal during the six-second self-test, then it turns off, indicating no problem has been detected. It isn't until the car starts to move, and often it takes a few hundred yards before the one missing signal is observed. The fault code for that doesn't set until the car is moving.
This type of defect is real common on GM front-wheel-drive cars. Their front wheel speed sensors develop very weak signals to start with, then, with as little as 15,000 miles of wear on the wheel bearings, the normal looseness that develops can let the sensor move away from the toothed wheel just enough for a signal to get even weaker, to the point the computer can't read it. That problem may not be bad enough to be detected until the car is driven many miles. This is a common cause of the warning light turning on once or twice per day. Once that type of defect is detected, the warning light stays on as long as the ignition switch is on. When you stop the engine, then restart it, the light will turn off after the self-test, and not turn on again until the problem is detected again. The fault code remains in memory, and should not be erased until it can be read on a scanner or code reader. It is that fault code that tells the mechanic where to start looking for the cause of the problem, even though the warning light is not on when you get to the shop.
Another common GM problem occurs on four-wheel-drive trucks. Rust builds up under the wheel speed sensors and that pushes the sensors away from the toothed tone ring. The greater air gap results in a weakened signal. All of these wheel speed signals naturally get weaker at lower speeds, but when one gets too low, the computer sees that as that wheel is locking up. It will try to reduce braking power to that one wheel during stopping. That is felt as "false activation", meaning the anti-lock function is kicking in when it isn't actually needed. That may not be detected as a problem or set a fault code when the weak signal occurs while you're braking. It's when the loss of signal is detected when the brake pedal is not being pressed that the computer knows there's a problem with that circuit.
Those two problems are examples of mechanical problems related to the sensor. One is solved by replacing the wheel bearing assembly that includes the built-in sensor, and the second one is solved by cleaning away the rust under the sensor. No parts need to be replaced even though the wheel speed sensor is referenced in the fault code.
Thursday, July 16th, 2020 AT 1:25 PM
(Merged)
