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OverEd

ABS brake chatter ... when cold? {mystery solved}

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Okay folks, I think this may be a fun one...

In the morning (every morning) shortly after my '96 OBW begins to roll (with the wheels slightly turned and I'm lightly applying the brake) I get a short pulsing from what feels like the brakes. I feel the pulse in the brake pedal, but it does not seem to effect rolling speed. I also can hear a slight pulse/chatter from my front wheel areas (left, right or both, I can't tell.) The sound and the feel are identical to proper ABS performance during heavy braking and sliding. However the duration is short (less than 1 second), and I am not applying heavy brake pressure.

 

This happens nearly every time (90+%) the car has sat for a while (overnight or more than 12 hours). Usually it occurs in the morning, but occationally after the car has sat all day.

 

It happens if the car moves immediately or if it idles for a while. My brake pads have been changed. The ABS works fine. The problem has been around since 2003. It doesn't seem to be a problem, just darn annoying.

 

What could it be? The ABS computer booting up? (If there is such a thing.) failing ABS sensor?

 

Waddaya tink?

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Well, the ABS pump does perform a self-diagnostic procedure, which will give a whirring since the pump motor is checked.

 

So, it could be normal. If you've had this for two years with no increase faults, then maybe you're okay.

 

 

However, for peace of mind you should hoist the car up and remove the wheels. Check for a "ballooned" brake hose or dirt around the wheel speed sensors. These are comprised of a ring that has notches all the way round and a "Hall effect" sensor that's screwn into the wheel hub.

 

You might be able to see these things with the car on the ground, but on lift you have much better access.

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i agree inspect everything .. it could be a sensor getting tired, but if it has been like that for a while, i wouldnt worry about it.

 

nipper

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I replaced the font struts and coils recently and while I had *most* everything apart I too a look behind (or would that be "in front of") the speed sensor. Nothing seemed to be "out of place." No hose problems, fluid level's fine, I bled the brakes a bit "just to make sure"...

 

Maybe it's okay? I'm still hoping. But it would be nice to stop the noise... I can't imagine it's "supposed" to do this.

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It might, ill have to pay attention to mine and see if it does it.

 

 

nipper

ok nipper are you out driving around the neighborhood to see if yours will do it too?

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It happened again this morning. I even let it idle for 20 seconds before backing out. (problem occurs forward or reverse, steering or straight) But I think it is on the driver's side. I guess I'll take the wheel of tonight and re-inspect.

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the only thing i can thinkof.. the chatter is the ABS unit releiving pressure. The sensors themselves are hall evffect sensors.. sensing a magentic pulse. Now the only thing i could think of is that maybe some humidity and dirt from sitting over night is affecting one channel of the unit, making the unit think it has a locked up wheel, until the unit does its boot-up and realize that it really isnt moving anywhere, then corrects itself. Could also be a hairline crack in a sensor.

I dont know the logic on the ABS unit, but thats the most reasonable thing i can think of. ABS Units are pass through designs, meaning that if the have a real failure, they will not work at all, but wont affect normal braking either.

HAve you tried starting the car wwith your foot on the brake pedal?

 

nipper

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im just curious if the ABS is trying to releive some sort of internal pressure, or it just has an off but functioning sensor

 

nipper

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Source found, normal operation:

 

"You may feel a slight shock in the brake pedal and hear the operating sound of ABS from the engine compartment just after starting the engine. These are caused by an automatic functional test of the ABS system being carried out and does not indicate any abnormal condition."

 

- 1996 Legacy Owners Manual

- Page 7-32

 

I always thought you could find good stuuf in those manuals...

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