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Likely not the sensor
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Sat, 5 Jul 2008 08:46:37 Share Post by Email
In Reply to: brake fluid and anti lock light, Saab900man, Sat, 5 Jul 2008 05:36:52
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The ABS light comes on if it senses a problem in the ABS system, typically a sensor. The Brake Fluid light comes on if the brake fluid level is low. The BOTH come on if the ABS pressure is low.

Quick ABS primer - The ABS pump runs, pumping fluid into the ABS Accumulator. The accumulator is that black sphere sitting on top of the MC. It stores the pressure, so the ABS pump doesn't have to run all the time. The system uses the stored pressure for both ABS AND Brake Boost. The ABS pump runs if the pressure is below 140 bar; it stops at 180 Bar. The ABS AND Brake Fluid lights come on if the pressure is below 100 bar.

Do this test. After the car has been sitting overnight or at least a few hours, check the brake fluid level. It should be ABOVE the FULL line. That's normal. Now, start the car. You'll hear the ABS pump run, and the level will DROP. When the pump stops (usually about 40 seconds to a minute), the lines should be at the FULL line. If it's well below that line, add fluid. But only add fluid when the pump has finished running - fill it after the car has been sitting, and you'll overfill.

Now, with the car running, stomp the pedal hard and fast, like you were trying to kill a particularly nasty spider on the brake pedal. Bam bam bam. Count how many pedal stomps it takes before the ABS and Brake Fluid lights come on. Over 12, and the Accumulator is OK. 6 to 12, the accumulator is weak. 6 or less, time for a new accumulator.

The accumulator has a diaphram stretched across the center, with compressed nitrogen gas on the top side. As the pump pumps fluid in, the gas compresses and the diaphram stretches. This is what stores the pressure. Since fluid doesn't compress, the gas does, and it's the stored pressurized gas that provides the force. As you stomp the pedal, it uses up some of that force, and the pressure drops.

If the accumulator fails, the diaphram ruptures, and fluid gets in behind it. Now there is less gas to compress. You can't compress fluid - you can pressurize it, but it gets 'used up' quickly. So with a bad accumulator, you'll get the ABS and Brake Fluid lights come on during hard braking, and the ABS pump will be running a lot, which is bad for the pump.

If your car fails the accumulator Stomp test, get a new accumulator. Do a search; folks have found an GM (?) accumulator works and is cheaper than the Saab one.






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