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 Post subject: Odd brake pulsing issue, NOT warped rotors
PostPosted: Fri Dec 13, 2013 10:30 am 
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Searched here and the interwebs for an answer. Found one other post on another forum describing the issue I'm having but the topic had no resolution. 2006 Liberty CRD

Description:
I have an intermittant brake pulse at highway speeds. It is a thumping and sometimes gets loud enough to be heard. It starts out barely noticeable and builds until the steering wheel is vibrating and the noise is loud enough to be heard throughout the cabin. At this time the steering is very sensitive, wheels are hot as fire, fuel mileage goes down, and boost shoots up to 12psi+ from a normal cruise of 5 or so. It has done this on two sets of rotors. It is NOT warped rotors because I can put the Jeep in neutral cut it off, leave it off for several seconds, start it back up, and put it back into gear. It takes just a few seconds to do this and I go from 75 to 60 mph quickly. When I perform this little procedure, the problem instantly goes away. Sometimes it comes back after a few miles, sometimes not. ESP on or off makes no difference, no warning lights lit, and speedo/odometer/cruise work fine. I can not feel the pulsing through the pedal when applying brakes but it is obviously still there as the thumping/vibration intensifies greatly, at it's worst, almost losing control of the vehicle. It has done it a couple of times when I first bought it causing me to change out brakes/rotors, even though the ones that were on it looked fine. It has been gradually getting worse/more common over the past few months.

What I have done related to this issue:
New brakes and rotors all the way around
Lubed calipers and bled the system
New (not reman) axles
Rebuilt front diff
New timken front wheel bearings
New front ABS sensors

My Jeep sees 100% highway driving (no offroad) and I live in salt free southeastern TN so corrosion is not an issue. I've wrenched on my own vehicles all my life and have worked in the auto parts industry for several years before my current career. This issue absolutely baffles me. I have no clue as to what it could be other than a weird ABS malfunction that the PCM is not picking up or causing. I've spent a few thousand keeping this Jeep running, upgrading the bean counter BS, and am growing sick of working on the darn thing. Any insight would be appreciated.

On a side note, this is the last Chrysler product I will ever own. A great engine surrounded by absolute crap, VM built powerplant so go figure. If I weren't so deep into it moneywise, I'd get rid of it.

Thanks
Kane

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 Post subject: Re: Odd brake pulsing issue, NOT warped rotors
PostPosted: Fri Dec 13, 2013 10:41 am 
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Is it all 4 wheels that get hot or just the fronts/backs?

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 Post subject: Re: Odd brake pulsing issue, NOT warped rotors
PostPosted: Fri Dec 13, 2013 11:13 am 
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If you've opened up the lines to bleed the brakes, its possible you have air in the ABS module. Not sure if the symptoms you describe could be caused by that.

You need the starscan tool to bleed the ABS module. I found it was the same cost to have the brake fluid changed at the dealer vs having them just bleed the ABS unit. Same procedure, but you get fresh fluid, about $100 bucks.

Good time to install speed bleeders with check valves.

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 Post subject: Re: Odd brake pulsing issue, NOT warped rotors
PostPosted: Fri Dec 13, 2013 12:07 pm 
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mass-hole wrote:
Is it all 4 wheels that get hot or just the fronts/backs?


Just the fronts. Rear has typical parking brake problem but is not affected by this.

I thought air, but why would it stop doing it when I take the switch off?

Kane

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 Post subject: Re: Odd brake pulsing issue, NOT warped rotors
PostPosted: Fri Dec 13, 2013 2:20 pm 
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Vibration, thumping, steering wheel issues, wheels hot, boost going up as engine "fights" brake drag, etc - been there done that on a Dodge Ram, a Plymouth minivan, and a Mercedes. You have a "grabbing" disc brake issue and the vibration is the disc grabbing essentially imperceptible run out on the rotor aggravated by slight warping as they get hot which goes away as they cool.

Common cause is associated with the use of caliper pistons that are not stainless steel but rather a synthetic and it is not uncommon for the problem to show up after a brake job that bleeds the system while retaining the old caliper assembly. The synthetic piston(s) absorb brake fluid and tend to stick in the bore so they don't allow the pad to retract off the rotor which can result in random light pulsing or in extreme cases, I know, violent wheel hop. Symptom sometimes goes away if you really stomp on the brakes coming to a complete stop and then drive off.

Other causes could be:
1. dirty brake fluid in caliper;
2. bad brake lines - the flexible sections not the hard pipe. Those sections have been know to get soft and produce strange caliper piston pressure issues.
3. problem with the master cylinder and/or power brake boost system.
4. I'll go out on a limb a bit and speculate that the 06 stability control system might could go "nuts" from time to time and do this too.

There is nothing "special" about the CRD brake system; it's the same as the gasser one. I'd suggest you get with a good local brake shop for a chat.

Your key off comment suggests the cause may be vacuum related which suggests power brake boost system as keying off would shut the vacuum pump in engine off and drop vacuum which I'm guessing would cause the brake boost system to "back off" if it's doing something it shouldn't be doing on its own so to speak. I don't know the keying off would turn off/reboot anything in the ABS system that would do this but that's also something to consider.

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