Low speed abs engaging

[Psych0matt]

Lately I've been having issues in the 04 where at low speeds (pulling into the driveway, or a parking spot) the abs kicks in for no apparent reason. In doing some google research it seems like it might be an abs sensor or an entire hub assembly, but I don't know where to start as far as pinpointing or diagnosing. Any one have any input?

[runt]

Any ABS codes? Might consider getting it scanned regardless in the event someone pulled a bulb.

[Cutlass350]

You'll likely need real-time data to be sure.

It could be a dirty/chipped/rusty ABS sensor. I'm not sure if they are sealed units on you car.

If a sensor or ring is rusty/broken, then there will be a weak/poor signal going back to the ABS module.

Fwiw, the amount of voltage/current induced into the sensor is proportional to the speed.

NOTE: It's the pulses (rate) that are used for each wheel speed, not the signal strength/value. But, that helps to explain why dirty/rusty sensors often "misbehave" more at low speeds.

 

Check for broken wires, where they attach to the suspension and body. And, that the connectors are on tight. It's very unlikely. But, it's free and easy to do.

 

Good Luck!

[Psych0matt]

can the ABS fuse be pulled to bypass this for now?

[Schurkey]

Perhaps a moderator will move this to "Brakes/Suspension/Steering/Wheels/Tires".

[Cutlass350]

can the ABS fuse be pulled to bypass this for now?

When the ABS light is on, the ABS is disabled. If you pull the fuse, the ABS light will still likely be on.

In general for Bosch units, the ABS light also indicates if the ABS "main controller" has any power. The light is connected to a relay that supplies power to the "main controller". (There are actually a number of controllers/micro-processors in modern ABS units.)

When the ABS-error light is off, the "main controller" has power.

When the ABS-error light is on, the "main controller" does not have power.

 

Also, for an '04, many GM Bosch ABS units, like for other auto manufactures, started having their Bosch ABS units do "double duty" by also being used as an electronic brake proportioning valve. So, you do not want to yank the ABS fuse.

Note, I'm not sure what the electronic brake proportioning valve function does, or how it works, when there is an ABS error, or the fuse is pulled for the ABS module. I haven't found any good very detailed technical information on those features.

 

Btw, from: http://auto.howstuffworks.com/auto-parts/brakes/brake-types/master-brake2.htm

Proportioning Valve

The proportioning valve reduces the pressure to the rear brakes. Regardless of what type of brakes a car has, the rear brakes require less force than the front brakes.

 

The amount of brake force that can be applied to a wheel without locking it depends on the amount of weight on the wheel. More weight means more brake force can be applied. If you have ever slammed on your brakes, you know that an abrupt stop makes your car lean forward. The front gets lower and the back gets higher. This is because a lot of weight is transferred to the front of the car when you stop. Also, most cars have more weight over the front wheels to start with because that is where the engine is located.

 

If equal braking force were applied at all four wheels during a stop, the rear wheels would lock up before the front wheels. The proportioning valve only lets a certain portion of the pressure through to the rear wheels so that the front wheels apply more braking force. If the proportioning valve were set to 70 percent and the brake pressure were 1,000 pounds per square inch (psi) for the front brakes, the rear brakes would get 700 psi.

 

Good Luck!

Edited August 14, 2014 by Cutlass350

[Addicted To Boost]

My 04 did something similar when I first got it. I sucked the brake fluid out of the reservoir and thoroughly flushed the whole system out with fresh brake fluid and that seemed to help immensely. For what it's worth, you are supposed to replace your brake fluid every 2 years, but everyone neglects it. Brake fluid is hygroscopic (absorbs water) and water can not be compressed.

[Psych0matt]

I'll try that first when I get a chance in a week or so! I don't seem to be getting any codes, so hopefully this helps