ABS randomly going off

Tiny
MR07BUNNY
  • MEMBER
  • 2002 BUICK CENTURY
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 120,000 MILES
I have the car listed above that I bought for my mom.

She is saying that the Driver Side Front ABS keeps going off while driving, turning and stopping. So I replaced the whole bearing assembly. (Other side was replaced about a year ago as well.)

It seemed to solve the problem for about a week and a half and now she says it is doing it again while driving but no ABS light or anything.

Any idea what the problem could be?
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Wednesday, April 17th, 2019 AT 10:25 AM

4 Replies

Tiny
CARADIODOC
  • EXPERT
First check for diagnostic fault codes. Next, if you have a scanner, watch the wheel speeds when the problem occurs. If this happens when the car is going rather slow, you're describing "false activation" where the ABS kicks in when it isn't needed. Given the one wheel bearing is a year old, that is the best suspect. This has been a huge problem on GM front-wheel-drive cars with bearings developing enough normal play in as little as 15,000 miles and causing repeat problems.

Speed is a factor in signal strength with magnetic sensors. These sensors develop real weak signals to start with, then, when some play develops in the bearing assembly, the tone ring can move away from the sensor ring just enough to cause the signal to be too weak to be read by the computer. It thinks that wheel locked up, so it activates the hydraulic modulator to try to get that wheel turning again. You'll see that on the scanner as the wheel speed drops to 0 mph when the car is still moving.

As a point of interest, the old wheel bearing will still work fine on another car that doesn't have ABS.
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Thursday, February 18th, 2021 AT 6:06 PM
Tiny
MR07BUNNY
  • MEMBER
Thanks for the information, but the year old bearing is fine. The one I just replaced two weeks ago is the one acting up again.

There is no ABS light on indicating a code and I don't have access to a scanner to watch the speeds.

Is it safe to say there is a bad sensor and start with that or could the new bearing be faulty?
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Thursday, February 18th, 2021 AT 6:06 PM
Tiny
CARADIODOC
  • EXPERT
If you don't have a scanner, how do you know which wheel speed is dropping out?

I'm not exaggerating when I said these bearing assemblies cause this problem in as little as 15,000 miles. Drive by any GM dealership and peek into their scrap metal bins behind their service areas, and you'll see many dozens of old bearings in there.

The absence of a warning light doesn't mean there's no fault codes in the computer. You are more likely to be correct in this case, but with Engine Computers, there's over 2,000 potential fault codes, but only half of them refer to things that could adversely affect emissions. Those are the only codes that turn on the Check Engine light. You can have any of the other half of the codes, but no Check Engine light.

Very often with safety systems such as air bags and anti-lock brakes, when a problem is detected, the computer sets a fault code, turns the system off, and turns on the warning light. If that is an intermittent problem that currently is not acting up, or is one that you corrected, the warning light may go off the next time the engine is started, but if that code wasn't erased, it will remain in memory for a predetermined amount of time. That gives you time to review stored codes and make a note of them in case the problem occurs again.

The point is don't assume there are no fault codes just because the warning light isn't currently on.

One way do-it-yourselfers and inexperienced mechanics often hasten the failure of a new wheel bearing is improper installation procedures. Specifically, the axle nut must be torqued to specs with a click-type torque wrench, and that must be done before any vehicle weight is placed on the bearing. Many people will install the wheel / tire, then set the tire on the ground to hold the half shaft from turning so they can tighten the nut. It's too late at that point. The damage has been done and the new bearing will be noisy. An easy way to hold the shaft is to drop a screwdriver or punch into a cooling slot in the rotor, then torque the nut.

Also, the torque value is real high on some GM models. 180 foot pounds is a common value for many cars, but it can be as high as 240 foot pounds on some GM models. If a person was to simply guess at how tight to make that nut, it is likely they won't get it tight enough. The play that develops in these bearings that allows the wheel speed signals to drop out is so small, that insufficient tightness of the axle nut can lead to that problem occurring sooner.

The sensor itself is nothing more than a coil of wire. There's no electronic circuitry in it. It isn't likely to short because the internal connections aren't close enough to each other. Since it is extremely fine wire, it would be more likely to go open circuit if one end of the coil would contract in cold weather and tear off its connection. The engineers accounted for that in the design, so that is not likely to occur. If it did, you'd have a fault code and a warning light, and the system would stop operating. That would be very rare, and your system is still working. It's just activating when it isn't supposed to.

When you do get a fault code related to an open circuit, it is going to say open circuit detected "in the circuit", not "in the sensor". By far the better suspect will be corrosion between two mating terminals in the sensor's connector, or its wire has been cut, or less commonly, a wire has cracked apart due to repeated flexing.
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Thursday, February 18th, 2021 AT 6:06 PM
Tiny
MR07BUNNY
  • MEMBER
I have a code reader and checked it. The only thing it showed was related to the charcoal canister. No inactive faults. So I know it's the drivers side front that has the issue because that's the one my mom says she can hear and feel while driving at speeds below 35mph.

I am a mechanic and have also been a mechanic in the military and federal government for several years. But in no way do I consider myself an expert especially when it comes to electrical which is why I am asking for help here.
My number one rule is to follow proceedure (repair manual) and torque specs. So yes, I installed it properly and torqued it according to the book.
The part has a 1 year warrantee so maybe I will exchange it and ask for advice elsewhere. Thanks
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Thursday, February 18th, 2021 AT 6:06 PM

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