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Old 01-22-2010, 07:13 PM   #1
MA Shaw
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Default Brake slipping feel through pedal and ABS/warning lights on

My car has suddenly developed an odd brake pedal feel. It almost feels as if the brakes are sliding/activating on and off. However, this only happens are very slow speeds, a couple of feet before coming to a complete stop. Heavy braking, or higher speed braking is unaffected, as far as I can tell.

On start up, there are no warning lights. However, within the first couple of feet, the green traction control (c4) light turns on/off as I accelerate. This happens for about 20-30 yards, then the ABS/warning lights turns on. When this happens, the speedometer completely dies on me, and stops working.

It's been raining hard (for socal) for the past couple of days. I checked the brake fluid reservoir with the car running, and it's right at the max mark. Also checked to see if there was any wet areas where water may have been getting in, and I couldn't find anything. However, this was a very rough search, as I am not sure where I am suppose to be specifically looking at/for.

Any ideas?

thanks.

-Don
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Old 01-22-2010, 10:56 PM   #2
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Hi Don,

1. How old is the brake fluid? May have absorbed water and the ABS make be cutting in.
2. Make sure all 4 sensors are still attached. Not sure what the system does when one or two are not connected.

I have all 4 sensors taped back and now and then I get an ABS lisght especially when it is wet. I hope this helps.

Tom
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Old 01-22-2010, 11:47 PM   #3
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un plug the abs computer .
Does it feel the same same ?
If it does feel the same same its not the computer .
If it does not feel the same same , its computer related .
If its the computer , check the accelerometers , are they clean inside and looking good ?
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Old 01-23-2010, 05:21 PM   #4
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I will test it out. Thanks for the suggestions.

Last night, it didn't turn on until 20 miles into a drive.
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Old 01-23-2010, 05:45 PM   #5
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Don,

They symptoms you describe can be caused by tires which are not the same diameter front and rear. Have you changed tires recently? If the rolling diameter is not close enough front to rear or side to side ABS gets confused. This results in ABS gong active and actually preventing the car from slowing as much as we'd like.

Another way to get an ABS warning is to have one wheel brake while another is rolling. I used to get this leaving a service station in my GT3. Had to do diagonally and if I had to brake one wheel in the air would stop turning while the other ones on the ground merely slowed. That would give me ABS failure. Aquaplaning on one wheel could also cause this.

Hope this helps!
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Old 01-23-2010, 08:13 PM   #6
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I did change the wheels, but that was about a year ago.

-Don
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Old 01-25-2010, 08:11 AM   #7
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I have the same problem on my 964C4 after replacing front & rear calipers (965&993) and front & rear rotors (965-964RS). No control on green traction control (no light on Euro cars) The issue appear only at very very low speed like MA Shaw said. I have very stiff suspension (H&R Cup) and Michelin Sport Cup. Maybe a non omogeneous road surface & cold tires can magnify this issue.
BTW: I have 205/50/17 & 255/40/17
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Old 01-26-2010, 02:49 PM   #8
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No news?
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Old 01-26-2010, 02:52 PM   #9
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I've searched, and it seems like there are many possible related causes/problems to this. First thing I am going to do is possibly add a bit more brake fluid to the reservoir (already at the max mark) and then check the distributor wires for possible corrosion/water.

-Don
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Old 01-26-2010, 10:35 PM   #10
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Dear Don aka MA Shaw

The abs trips when it senses one wheel is turning faster than the others. via. the wheel speed sensors. The wheel speed sensors send a frequency modulated signal directly proportional to wheel speed. If the amplitude isn't strong enough the computer will interpret it as a slower rotating wheel and in turn activate the abs. The fact that it is OK at hi speed when the amplitude of the signal is at its' maximum suggests the computer is OK. I'd remove each sensor and clean the surface, they tend to get gummed up and become magnetized attracting dirt and if you have metal brake pads, the metal dust tends to accumalate on their surface degrading the signal strength being induced by the rotating index wheel. If that doesn't help and/or while you're at it and you're handy with a voltmeter, you can measure the signal coming out of each sensor to determine if a sensor is suspect. Simply tap into the 2 wires coming off the sensor at the connector with a voltmeter set on AC volts and with the wheel off the ground spin it, noting the voltage. Perform the same for each wheel trying to be consistant in wheel speed both fast and slow. Each wheel sensor should produce relatively the same voltage for the same wheel speed.

Bill
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Old 01-27-2010, 03:36 AM   #11
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Is there a good DIY on the cleaning of the speed sensors, or where/how to locate them? Trying to search for it, but can't seem to be able to figure out their exactly location inside the wheel area.

-Don
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Old 01-27-2010, 06:40 AM   #12
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Dear Don

The #3 wire indicated is the wheel speed sensor for both the front and back wheel. The connector is inside the barrel shaped container which also contains the brake wear wire. Screw #2 secures them to the hub. You may want to just inspect the connector first, before removing the sensor. Also check the ground wire leading from the barrel shaped housing.

Bill
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Old 01-28-2010, 01:23 PM   #13
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Thanks for the diagram, it made it really easy to find. The rears were a bit dirty, and I cleaned them up, but it seems like the problem is still unfortunately there.

So far I have added some extra brake fluid and cleaned the speed sensors.

-Don
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Old 01-28-2010, 04:21 PM   #14
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my sensors are cleaned twice but no improvement.
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Old 01-28-2010, 08:40 PM   #15
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Did you try my tests ?
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Old 01-28-2010, 08:40 PM
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2004, abs, brake, brakes, control, feel, front, lights, pedal, slippage, slipping, tranction, turning, warning, yukon

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