ABS light on

Tiny
PETEYT
  • MEMBER
  • 2002 HYUNDAI ELANTRA
  • 70,000 MILES
We have had some intermittent ABS and TCS dash lights on for the last 6 months.
Recently however, during the first km or so of driving out of the residential streets we live in, the ABS system has been modulating / pulsating along with the brake pedal. Once we get the car up to speed the problem goes away and does not return until the return trip home. The ABS and TCS lights again are off and on when this is happening.
I pulled the 2 ABS fuses (why are there 2?) And the problem does not happen, but now the ABS, TCS, Brake, and Engine lights are on.
Any Advice and Help would be great
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Monday, January 7th, 2013 AT 8:33 PM

5 Replies

Tiny
CARADIODOC
  • EXPERT
You're describing false activation, and the computer is detecting a problem and telling you about it.

Lets check out this video to show us how to run the ABS codes.

https://youtu.be/rTtAnsOlZU4

The most common causes for an intermittent problem to be detected while you are not applying the brake pedal is a cracked tone ring at one of the wheels, or a mechanical problem with one of the wheel speed sensors. In general, a mechanical problem causes no speed signal to be generated and will be detected after the car starts moving. An electrical problem such as a broken wire can be detected any time including when the engine is first started but before the car starts moving. Once the light has turned on there will be a diagnostic fault code stored in the computer. The place to start is by having those codes read. They never say to replace parts. They only indicate the circuit or system that needs further diagnosis.

The Traction Control system is an add-on to the ABS system and uses some of it's functions. It's common for that light to turn on also because the computer knows due to the ABS problem it can't run the traction control system properly.

There will always be two fuses for anti-lock brake and air bag systems. If there was only one feed circuit, there would be no power to turn on the warning light if that fuse blew.

One other important detail to be aware of, especially common for GM owners, is there is always a long list of conditions that must be met before a code will set. One of those conditions is there can't be certain other fault codes already set. The computer compares many things to figure out when there's a problem. For example, when driving straight ahead, all four wheel speed sensors had better be reporting the same speed. If one sensor has a problem and a fault code is set, the computer can't use that as a reference so it may not set another code if a different speed sensor develops a problem. That second code will not show up until after the first problem is fixed. That is the source of a lot of frustration among GM owners and mechanics. That problem rarely happens when a new problem is fixed right away. When it is ignored for a long time, as you've been doing, there is a lot of time for a second problem to develop, and there may be no fault code set for it yet. Even when your warning light turns off, any codes remain in memory and can prevent additional codes from setting. What this means is once the original problem is identified, diagnosed, and fixed, it is possible for the warning light to turn on again with a new code in memory.
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Tuesday, January 8th, 2013 AT 2:39 AM
Tiny
MDM
  • MEMBER
Thanks for this post I had this problem and had to replace the wheel speed sensor in the right front cost me $145.00 all fixed! I love this site.
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Wednesday, January 2nd, 2019 AT 10:06 AM
Tiny
EBERLMETALWORKS
  • MEMBER
  • 2002 HYUNDAI ELANTRA
  • 4 CYL
  • FWD
  • MANUAL
  • 110,000 MILES
This week while driving the ABS light and TCS light keep coming on and staying on for a few days, then going out, only to return in a few hours. What should we do. Thanks
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Wednesday, January 2nd, 2019 AT 10:06 AM (Merged)
Tiny
MERLIN2021
  • EXPERT
First step is scan it for "C" codes, this will pin point the bad part(s). Then test and replace.
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Wednesday, January 2nd, 2019 AT 10:06 AM (Merged)
Tiny
CARADIODOC
  • EXPERT
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.
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Wednesday, January 2nd, 2019 AT 3:41 PM

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