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Re: brake pedal pressure: ABS and pre-ABS (long)
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Sun, 26 Aug 2007 08:00:13 Share Post by Email
In Reply to: brake pedal pressure: ABS and pre-ABS (long), cultivate, Sat, 25 Aug 2007 22:09:30
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Quickie intro on how the ABS pressure works-
Brake boost and 'power' for the ABS is supplied by stored hydraluic pressure. There is a pump (ABS pump) that pressurizes the accumulator (black sphere) by pumping fluid into it. The accumulator has a diaphram in the middle, with nitrogen gas on one side, and brake fluid on the other. As the pump runs, fluid goes into the accumulator, which compresses the gas. This is the pressure that provides brake boost.

When the car sits, pressure leaks down from the accumulator - fluid goes FROM the accumulator INTO the reservoir. That's normal. When you start the car, the system sees the pressure in the accumulator is low. It runs the pump - fluid goes from the reservoir into the accumulator. when the pressure reaches max (180 bar), the pump shuts off. The pump will run again occasionaly to keep the pressure topped up if the pressure drops below 140 bar.

If the pressure is less than 100 bar, you get BOTH the ABS and Brake Fluid lights. It's normal to see both for a few seconds when a car has been sitting for a while. They shouldn't come on during driving.

Notice the comment about fluid going FROM the reservior TO the accumulator? The level in the reservoir changes. When the car has sat for a while, the brake fluid level in the reservoir should be ABOVE the FULL line. Yes, above. Start the car, and as the ABS pump runs, the level will drop. The level should drop to the FULL line when the ABS pump shuts off. If it is below that, add fluid. Folks with ABS cars sometimes get the levels screwed up - either they think the FULL line is OK after the car has been sitting (too little fluid when the pump runs), or they fill the reservoir, and it overflows.

So, step 1 is to make sure you've got the right fluid level. Start the car and wait for the ABS pump to stop running. Check the full line, and add fluid if necessary.

If the ABS and BF lights are coming on during hard braking, it may be a bad accumulator. The diaphram ruptures, and the entire accumulator fills with fluid. Since fluid doesn't compress, it can't 'store' energy.

The ABS accumulator test is to start the car and let the ABS pump stop running. Now do the "Spider Stop" - hit the brake pedal hard and fast, like you were stomping out a spider on the pedal. How many pumps before the ABS and Brake FLuid lights come on? I like to think 12 or more is a good accumulator; 8 to 12 is one to watch; anything less than 5 is a bad accumulator. They're relatively easy to replace, and check the board as some folks have found low-cost alternatives.

No ABS pressure means no brake boost, which is very bad. The ABS brakes do have a different pedal feel than the old vacuum assist style.

posted by 76.202.16...
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