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Old 01-21-2019, 01:30 PM   #1
tomj76
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ABS triggers when almost stopped

This is an FYI post.

My ABS was "firing" when the van was nearly stopped, almost every time.

At first I considered that it was due to my flickering lights problem because the flickering was not present at higher speed, only when the car came to a stop. However that was not the cause. I eliminated that possibility by testing the ABS with the alternator output disconnected from the battery. Since the car was running only on battery, there was no flickering of the lights at any speed, but the ABS still triggered when I came to a stop. Voltage ripple was not the cause.

I turns out that the cause was the rear sensors, which were the original ones with the full 380k miles of "wear". The rear sensors have an iron tab that protrudes next to the speed sensor ring, but the iron tabs on both sides was worn/corroded down to a "point", making the pickup area next to the ring a lot smaller. I could see it on the scan tool by monitoring the speed of each wheel, which drops to zero after the speed dropped below ~3 mph. When the speed decreases, the amplitude of the signal from the sensor also decreases, and it must have been dropping below the threshold of the amplifier circuit in the ABS module.

I replaced the sensors, and the ABS is working well again.
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Old 01-25-2019, 12:27 PM   #2
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Re: ABS triggers when almost stopped

Good info, thanks.

I also get the random, seldom, ABS notice. Now I will go to all wheels and replace those sensors ... I've always been a bit suspicious of them, as I have never have replaced them (that I remember) in almost 500K miles.
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Old 01-28-2019, 12:06 PM   #3
tomj76
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Re: ABS triggers when almost stopped

Glad to be of help.

FYI, the front sensors are different from the back, with no iron tab to corrode on the front ones. However, I replaced both of those several years ago when I was getting strange ABS behavior. I didn't have an ABS compatible scan tool back then, so I asked my transmission shop to read the code while it was there for other service and they pointed to one of the front sensors.

The front ones are mounted through a hole in the steering knuckle and I had to drive them out of those hole (destroying them in the process) to get them off. I believe the mounting screw was also a challenge to remove. I think the magnetic nature of these keeps rust from flaking of normally, and so you get some strange results when metal around these things corrodes.

I noticed similar stuff on the rear ones, but the holes they protrude thorough have a larger clearance, so it wasn't as hard to get them off.
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Old 02-11-2019, 11:33 AM   #4
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Re: ABS triggers when almost stopped

The front ones definitely fall apart when you try to remove them. My driver's side one just stopped working a couple of years ago (symptom = speedometer reports around half the actual speed since that's how it's calculated.)
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