Brake pedal issue

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  #1  
Old 11-13-10, 04:58 PM
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Brake pedal issue

Hello readers. I welcome all opinions and input regarding the following topic.
I'm assisting my sister in selling her 2003 Trailblazer that she would rather not involve herself in that process of negotiating a selling price. So I've taken it upon myself of selling her car. I'm not familiar with all the quirks and maintenance that have been performed on her car, but I know the car won't sell until the following problem is resolved.
When the brake pedal is depressed, it will travel about half way down with no problem. Brake application and stopping is ok. But when brake pedal is further depressed, pedal will fluctuate and a noise is emmited from braking system. Sometimes the ABS warning light comes on and sometimes it doesn't.
She recently replaced all discs, so I don't think they are warped.
Is there anything else I should look at? Thanks again for the feedback.
 
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  #2  
Old 11-13-10, 07:52 PM
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Highly recommend take it to a qualified shop and have them check the abs system

Good luck and have a great day
 
  #3  
Old 11-13-10, 08:12 PM
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hey, always go back to the last job done. if they had rotors replaced, it means they worked very close and around ABS sensors. rotors have to be removed, sensors then are exposed. careless work could have caused some physical damage to one or several sensors, causing ABS fluctuation. direct damage, dirt, unplug connector, etc. but you need to remove rotors and re-inspect all this.
 
  #4  
Old 11-14-10, 03:40 AM
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Real simple........

Go to the underhood fuse block, and remove the fuse marked ABS.....

The light will come on....Ignore it.....and drive the car....Are the symptoms rectified??????

If they are...replace the fuse , and look very carefully at the Tone rings in the front for cracked or missing teeth......

If there are no obvious signs of damage, or if the tonerings are hidden inside the wheel bearing, an oscilloscope pattern of the sensor output will be the next step.
 
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Old 11-14-10, 04:18 AM
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I would have said the same thing as diezel, but he beat me too it. Goes to show you how great minds think alike.

In taking this to the next level.

(Quote) She recently replaced all discs

Just a few curious questions that are born out of your above quote

Do discs mean disc brake pads or rotors?

If discs means the disc brake pads were changed....well then were the rotors resurfaced as well?

If discs mean rotors, why were just the rotors changed and not the disc brake pads as well?

What was the mileage when the discs were changed?

What is the mileage now?


diezel,

See where I'm taking this?
 
  #6  
Old 11-14-10, 05:06 AM
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See where I'm taking this?
Yep....But the "The pads were only 3 months old when I put The rotors on" argument is like beating a dead horse....I have that same discussion every morning, and it ALWAYS turns into a heated argument.

"Goes something like this here...."

Just for info......Once brake pads have taken a "SET" into out of round rotors.....THE PADS ARE JUNK!!!!... and if left in place with new rotors, will likely damage the new rotoors as well.......
SO.....Before you go thinking that your mechanic is a crook for changing healthy "Looking" brake pads...this is why.....

Now.....Again just for info....Pulling the fuse for ABS disables the system.....
If the vehicle still has a pulse, chatter or other abnormality with the fuse removed......THEN ABS CAN BE "ELIMINATED" AS THE CULPRIT....

I wont bash anyone, BUT......ABS sensors work on MAGNETISM.....And I have never seen , GREASE , or road dirt that was magnetic.....Mettalic particles would have to Pack themselves DEEP into a tone ring to cause an errattic signal......Not saying it Cant happen...but not really likely.
 
  #7  
Old 11-14-10, 06:14 AM
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windycity,

Just for laughs and giggles

Remove all the lug nuts
Re-tighten them
Using a torque wrench
To 100 ft-lbs
Using a criss cross pattern
 
  #8  
Old 11-14-10, 04:12 PM
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Thanks for all replies. I have good direction on trying to rectify the problem. I'll discuss the matter with my sister to determine what was done to the brakes and take it from there.But I think I'll start by pulling the ABS fuse, like it was suggested by some in this forum. Thanks again.
 
  #9  
Old 11-14-10, 05:39 PM
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well, no one says road dirt is magnetic.

but every time i had ABS sensors pulled out of GM hubs, their sensing surfaces where DIRTY. greasy dirty. with all kinds of black stuff in that mix. including brake pad material, which is quite conductive, isn't it?

also, i could refer to at least 3 GM truck forums, to members that had ABS problems, fixed with simply taking sensors out and cleaning them. GM ones where particularly prone to oxidation, and oxidation is oxidation - rust is non-conductive.

just my 2 cents. personally, Lord saved from having this issue. yet. but them sensors are getting dirty, and fast. i had 2 replaced when we had hub bearings replaced, they come with sensors. it is a HUGE difference between a new one and one that was there for 40-50K miles.

hmmm...

Cleaning Your ABS Sensor - ABS Light Avoidance by Cleaning - ABS Wheel Sensor Maintenance How To

or even better:

John's Repair Center ABS Speed Sensor Cleaning

Symptoms of a fouled or dirty Speed Sensor are that the brake will drop down in segments and seems like pressure is applied and something breaks and the pedal will drop down before engaging a second or third time. If these are your symptoms, odds are you have dirty sensors and they are not properly detecting the speed of the wheel believing it has stopped and reporting
you to the ECU that you have lost traction to that wheel.

let me see. his sister's vehicle symptoms are:

When the brake pedal is depressed, it will travel about half way down with no problem. Brake application and stopping is ok. But when brake pedal is further depressed, pedal will fluctuate and a noise is emmited from braking system.

A common problem with ABS is that the sensors and reluctor wheels, used to measure and compare tire rotational speeds, can become fouled by dirt and brake carbon. This typically occurs on the front axle where the sensors and reluctor wheels attract a considerable quantity of brake dust shed by the front calipers. As the wheel turns an analog signal is generated from each sensor and monitored by the control unit. If the amplitude of the signal decreases due to iron particles accumulating on the sensor the control unit cannot accurately compute the wheel speed and begins to modulate the brake pressure to the wheel whose signal has been lost. Since higher wheel speeds generate a higher voltage the problem does not occur at higher speeds since the differential value detected among the channels falls into an acceptable range. As accumulation of brake dust grows, heat fuses it into a carbon-like coating that attenuates the minute changes in reluctance measured by the sensors. This is interpreted by the car's ABS computer as a continuous difference in rotational speeds between one or more tires -- eventually exceeding a threshold value and triggering a "fault" light.

 
  #10  
Old 11-15-10, 02:40 PM
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(Quote) I'll start by pulling the ABS fuse

Good….Stay in touch….Also answering the questions that continue to go un-answered will help you to resolve the issue faster.
 
  #11  
Old 11-15-10, 06:39 PM
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Thanks for additional information. I love this website!
 
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