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Old 01-25-2005, 09:24 PM   #1
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Brakes pulsating!?!

Driving home tonight I quick got on the brakes and got a weird pulsating vibration coming from the brake pedal that prevented the brakes from fully engaging.
This can't possibly be the ABS engaging?
It felt as though something was stuck in between the brake pad and the rotor.
Got out and did the visual check, didn't see anything but it was too dark.
Noticed this once before but I thought it was because I had my foot slightly on the clutch pedal at the same time. (Although "both feet in" as they after a spin should never be a problem) This time my foot was nowhere near the clutch but I was driving over some long patches of snow and sleet.
Wondering if I should have it towed to the dealer tommorow.

Any of this sound familiar to anyone?

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Last edited by Perfectlap; 01-25-2005 at 10:05 PM.
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Old 01-25-2005, 10:31 PM   #2
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Sure sounds like ABS to me.
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Old 01-25-2005, 10:54 PM   #3
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ok the ABS kicking over sand and snow makes sense since I have no grip to begin with. But does the ABS usually kick in during sudden dry surface braking? and is ABS unaffected by the clutch engagement?
I'm wondering how this is going to feel like when Autocrossing/track, I'm used to stopmping on the brake and locking up often. With ABS It feels like the car just doesn't want to stop quickly!
A few times recently the ABS kicked in and I wasn't really going that fast.
Is this often traction control initiated ?
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Old 01-26-2005, 02:47 AM   #4
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Red face Abs

Had the same issue with my 986 S ... when breaking with a road surface no perfect ( kind of small waves) the ABS was reacting in a strange way . With my new 987 it seems that thy corrected this issue .
JMG
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Old 01-26-2005, 02:21 PM   #5
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I've driven many cars and none ever felt this way (like a rock was stuck in the rotor). Does the PASM bypass ABS? Or is there any way to switch off the ABS and allow locking up? I'm only interested in doing this when driving competitively.

By the way does anyone know of a good link that explains the Porsche ABS?
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Old 01-26-2005, 02:25 PM   #6
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PASM? You talk on a 987 here or are you talking 986?
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Old 01-26-2005, 02:37 PM   #7
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PASM for 986
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Old 01-26-2005, 05:17 PM   #8
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I think what you're describing is the ABS. This year in the snow is the first time I"ve had it do that to me and really only when I KNOW the wheels are trying to lock up.

It does feel wierd though. Its an uneven pulsing as if the wheels are going to fall off. I have had other cars and this is the first one that seems to feel uneven when it pulses.

I don't think I'd worry about it. I definately think its the ABS
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Old 01-27-2005, 01:40 AM   #9
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Abs

I defintly think this is the ABS effect . On the 987 you could select the sport mode . it then push the limit of the PASM and ABS to react later ( really good when driving sportly ) You could also disconnect the PASM .... but it will automatically switch on if the computer detect the car is really in a bad situation ...
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Old 01-27-2005, 04:19 AM   #10
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Thumbs up

Simply go out on a deserted road and do a panic stop from say 30 MPH. The ABS will come on and you will know what the ABS feels like. It is unmistakable I believe and will let you know if what you experienced was the ABS.

Whether that is what SHOULD have happened is a judgement call on your part. You can chat with your local service tech also to support you own call on this.

Let us know how it all turns out.

Good luck!
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Old 01-27-2005, 04:22 AM   #11
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This might help!







Both Boxster models are equipped with Porsche Stability Management (PSM) as standard. While it cannot overcome the laws of physics, PSM can provide additional control in difficult driving scenarios.

Here's how it works: When the new Boxster is in motion, PSM monitors the car's direction, speed, yaw velocity (speed of rotation around the vertical axis) and lateral acceleration. Based on this information, it can then calculate the actual direction of travel. If your Boxster begins to oversteer or understeer, PSM applies selective braking on the individual wheels to help bring it back into line. If the wheel approaches the threshold of skidding under braking, PSM uses a modified ABS to shorten the braking distance.
If you're accelerating on wet or other low-grip surfaces, PSM uses the integrated ABD (automatic brake differential) and ASR (antislip regulation) functions to help maintain traction and stability.

When 'Sport' mode is selected on the optional Sport Chrono package, the PSM threshold is higher, allowing greater driver involvement. If you'd rather enjoy your Boxster unassisted, the system can be set to standby at any time.

To enhance your safety, PSM remains present in the background and will only intervene under heavy braking where both front wheels are being controlled by ABS. All PSM inputs are more restrained and more precise, thus preserving the natural agility of the car. Although driving in a safe and prudent manner is the driver's ultimate responsibility, PSM does provide an extra margin of safety whenever you need it most.
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Old 01-27-2005, 09:20 AM   #12
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sorry I should have clarified I hace a 2000 S which doesn't have PASM.

I'd like to see if there is a comprehensive analysis of the Porsche ABS system.

And tips for preventing its need. I was always told that with ABS you wanted to pump the brakes which is completely the opposite of what I have trained myslef to do. For instance in autocross I'm often "quick hard and quick off" the brakes before making a fast turn. It was a trick to do this without locking up the brakes with my old Miata. With ABS its going to be another challenge.
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Old 01-27-2005, 08:39 PM   #13
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Whoa, stop right there (no pun intended)!

You DO NOT want to pump the brakes on an ABS-equipped car. The ABS does the "pumping" for you. And about your technique of locking up the brakes...that is also a no-no. When you lock up the brakes, you are sliding. And if you are sliding you do not have control. Try locking up the brakes and then turning (actually, don't try it). You won't be able to turn. That's why ABS was invented. I would suggest finding an empty parking lot and doing some full stomps on the brake pedal to get used to the ABS activating.
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Old 01-28-2005, 05:54 AM   #14
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Talking

Don't pump the ABS. That is the cardinal rule. Stomp on them hard and keep the pedal down. The ABS does the rest.
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Old 01-28-2005, 06:22 AM   #15
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that's right pump the brakes for non-abs. Got it backwards.
Locking up the brakes isn't really a technique, more like an unexpected situation that is fairly common when really braking hard. Usually when you brake too late and stay on the brakes too long.
I had an instructor at a very challenging two day AutoX driving school braking so hard I thought he was going to put his foot through the floor board. yet he never locked up. A very violent feeling experience as a passenger.
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Old 01-28-2005, 11:06 PM   #16
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Hi,
I'm guessing you might have already considered this, but...
are any of your rotors possibly warped?
It can only take a few millimeters (and so isn't too visible by inspection), but that would be enough to cause a generous amount of pulsing.

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