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Brake basics Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Tue, 22 Jan 2002 14:17:42
In Reply to: newbie brake pump question, Dirk, Tue, 22 Jan 2002 13:45:03 Members do not see ads below this line. - Help Keep This Site Online - Signup |
The ABS system requires high pressure hydraulics to work. This pressurized hydralic fluid (brake fluid) is stored in an 'accumulator' - that sphere shape inside the engine compartment just forward of the firewall, over the brake master cylinder. The job of the ABS pump is to keep the pressure up in that accumulator.
Brake boost - the 'power' in your 'power assisted' brakes, also uses that same pressure. So yes, Brake Boost and the ABS use the same system, and the same pump.
The Brake Fluid AND Anti-Lock lights come on if the pressure in the accumulator is too low.
When the car has been sitting for a while, the pressure in the accumulator leaks down - that's normal, because no valve is perfect. The brake fluid in the accumulator slowly flows back into the brake fluid reservoir - the smoky plastic container in the engine compartment. If the car has been sitting overnight, look at the level of the brake fluid - it will be well above the "FULL WITH CHARGED ACCUMULATOR" line.
When you start the car, the ABS system sees that the pressure in the accumulator is low, so the ABS pump runs. But since that level was very low, you get the Brake Fluid and Anti-Lock lights. After a few seconds, the pressure goes high enough and the lights go out. But the ABS pump keeps running - the level that the lights come on is an emergency level, well below the normal range.
The ABS pump will run until the accumulator is fully charged. If you check the Brake Fluid level in the smoky plastic reservoir, you'll see the level DROP until it reaches the FULL level. That's the pump pumping the fluid from the plastic reservoir into the pressurized accumulator.
As you drive, the pressure in the accumulator will drop. This is due to a little pressure leakage (like letting the car sit overnight), or if you use the brakes - power assist requires some pressure, or if the ABS kicks in. Mostly due to using the brakes, unless you're driving in an ice storm. When the pressure in the accumulator drops to a certain point, the ABS pump starts running and pumps the pressure up again. This level is well above the 'emergency' level that causes the Brake Fluid and Anti-Lock lights to come on.
So - what does this all mean? First off, it's normal for the Brake Fluid and Anti-lock lights to come on when you first start the car, and take a while to go out. The longer the car has been sitting, the longer it will take for them to go out. As cars get old, it seems to take longer for them to go out. What counts is that they do go out, and stay out when you're driving.
If the lights didn't go out, it could definately be the ABS pump. But it could be a bunch of things. The pump is triggered by pressure sensors in the accumulator, and those sensor run through a relay to run the pump. So the problem could be anywhere in there - something as simple as a bad connection that got fixed when they were rooting around in there.
First off, have the brake fluid replaced if it's more than 2 years old. You should have the brakes flushed every two years. Brake fluid absorbs water, and that destroys the (as you've learned) very expensive brake components. It seems that after replacing the brake fluid, some cars get to pressure faster.
Next, keep an eye on those lights. NEVER drive the car if they don't go off. You may not have brake boost. You'll get to the end of the driveway and hit the brakes, and they'll feel like a rock and you won't stop. Very bad if a dump truck is coming along.
If you're driving and the lights both come on, get the car off the road and stop. When the lights come on, you have around 5 stops left with brake boost. Sometimes the lights will be on and the brakes will feel fine, so you feel like you can keep going. But you don't know if the next time you hit the brakes you won't have boost. What's the difference? Think about how you stop - you expect a certain amount of stopping for a certain pedal pressure. Now, think about what would happen if when you hit the brakes, the car didn't slow down. It would take a second to realize what was going on, then you'd need to press VERY HARD, and even then, stopping distances would be increase by a factor of two or three. Think about that the next time you come up to a red light. Chances are by the time you adapted, you'd be in the intersection, or hit the car in front of you.
Change the brake fluid, and keep and eye on the lights. It may have been a one-time thing. If so, be thankful. If the problem comes back, try to find a place that will actually troubleshoot the problem. It may be as simple as a sticky ABS relay or a bad pressure switch - much easier and a LOT cheaper than an ABS pump. Most shops will just swap out the expensive part. Find one that will invest the time to troubleshoot the problem.
Good luck!
posted by 140.157....
, Tue, 22 Jan 2002 14:17:42 <-- Viewing This Message
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