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Pulsating Brakes


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#1 skippy7024

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Posted 19 August 2009 - 08:24 AM

I have a 2003 GMC Sierra 2500HD. The brakes were pulsating when I purchased this truck. I replaced all the brakes and new rotors. It still pulsated. I can pull the emergency brake and it comes to a stop smoothly. I went aheah and replaced the front rotors and brake pads a second time. Still pulsating. I have never seen when you replaced rotors and pads a pulsating brakes ocur. Any help would be greatly appreciated.

#2 bill190

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Posted 19 August 2009 - 12:23 PM

The ABS - Anti-lock brakes will do this.

This problem can be caused by the wrong size tires.

Basically the ABS system needs to "know" the size of the tires you have installed. This would be a GM dealer service thing. (They use a $3000 Tech II scanner for this.)

#3 DofD

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Posted 19 August 2009 - 12:26 PM

Can you describe the pulsing a bit? Is it a really fast hard hitting pulse or is it a slower softer pulse?
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#4 2COR517

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Posted 19 August 2009 - 06:55 PM

Do you hear a buzzing type noise, and the pedal feels like it is pushing back against you?
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#5 skippy7024

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Posted 20 August 2009 - 06:45 AM

Yes it does have a buzzing sound. Is this the anti lock brakes doing this. Tire size are the same size as what came from factory. It does not have any codes from the abs.

#6 2COR517

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Posted 20 August 2009 - 09:46 AM

This is an ABS problem for sure. Sounds like you have a bad wheel speed sensor, at least. What is happening is when you apply the brakes, the computer thinks one wheel (or more) is travelling at a different speed than the others and modulates braking pressure accordingly. This is exactly what it is designed to do. Start buy pulling that ABS fuse under the hood, the problem should stop, and your ABS light will come on. You need to take the truck to a shop with a good scanner. They need to be able to read each wheel sensor as you drive. Hopefully just one sensor reads differently than the others and the truck VSS. You could even have a bad ABS computer. That's what my problem was. All the wheels were VERY different than the truck. I had doing 10mph, one doing 1000mph. Never threw a code, but it was frigged up.
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#7 bill190

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Posted 20 August 2009 - 09:46 AM

You can eliminate the ABS as being the problem by removing the ABS fuse and seeing if this makes the problem go away or not.

This would be the ABS fuse (Antilock Brake System).

I would recommend doing this in a vacant parking lot or wide open space for safety.

If that is not the problem, you may also have a "Supplemental Brake Assist" vacuum pump which supplies additional vacuum to the power brakes when the engine can't.

Might try removing that fuse next. This would be the SBA fuse (Supplemental Brake Assist). Again vacant parking lot or wide open space for safety.

Information on fuses and their locations would be in your owner's manual.

Then you may have power brakes with a vacuum booster and this is what the above pump would be connected to. I suppose you could connect a vacuum gauge with a "T" fitting to this and see what is going on.

#8 bill190

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Posted 20 August 2009 - 09:50 AM

Well there ya go! Two posts at the same time saying the same thing (remove fuse to test).

#9 skippy7024

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Posted 23 August 2009 - 04:06 PM

I have 2 seperate problems. I did pull the fuse and that did fix one of the problems. The buzzing noise amd the pedal pushing back. It still pulsated like bad rotors. The harder you press the more it pulsates. I took the frront speed sensors off and cleaned them. The right front jad a pieces of rust on it. Cleaned them up with some emery cloth and my abs problem is now fixed. I am going to put some lugs on the rotor and use a dial indicator to check if warped. Any other suggestions would greatly be appreciated.

#10 bill190

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Posted 23 August 2009 - 04:43 PM

WHICH fuse did you pull?

#11 skippy7024

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Posted 23 August 2009 - 06:23 PM

I pulled the abs fuse. The problem with buzzing noise and pedal pushing back went away. I took the speed sensors out and cleaned them re-installed the fuse and that problem is fixed. Still pulsates as per my previous post. The harder you push the more it pulses. I will try the other fuse. and will let you know soon.

#12 2COR517

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Posted 23 August 2009 - 06:40 PM

Glad to hear you fixed one of the problems. Pull the rotors and see if there might be any debris preventing the rotors from sitting squarely on the hub. Also, do the calipers/pads slide freely?
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#13 skippy7024

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Posted 23 August 2009 - 07:30 PM

I have been down these avenues. Yes I have checked all that you have asked. I don't see anything obvious. This is the 2nd set of rotors that I have installed.My hubs feel fine, but I am wondering if they could cause this problem. This is a 2wd truck. That is why I was going to set up a dial indicator and check to see if there is something wrong. I checked the rotors on a lathe and they have a run out of .12mm. I don't think this is a problem.

#14 nathan60

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Posted 28 August 2009 - 09:34 PM

four wheel disc?




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