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Background Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Wed, 25 Jun 2003 06:19:53
In Reply to: Re: anti-lock & Brake Fluid Lamps Came On, sjk, Tue, 24 Jun 2003 16:57:38 Members do not see ads below this line. - Help Keep This Site Online - Signup |
The ABS system and brake boost use stored hydraulic pressure to work. When you start the car, the ABS pump runs - this pumps fluid from the Brake Fluid reservoir into the pressure accumulator (black sphere JJ mentioned). That's why the FULL mark on the brake fluid reservoir is well below the top. When the pressure is below about 100 bar, the ABS and Brake Fluid lights are on, above that, they go out. The pump keeps running until the pressure hits 180 bar.
The accumulator is a pressure tank with a diaphram in the middle. One side has nitrogen gas, the other brake fluid. As fluid is pumped in, the diaphram moves, compressing the gas. It's actually this compression that provides the 'force'.
As you drive, every time you hit the brakes, some of that pressure is used to provide brake boost. Same if you go into ABS. Since no valve is perfect, the pressure leaks down a little. When the pressure hits 140 bar (well above 100 bar), the ABS pump runs again, bringing the pressure back up to 180 bar.
When the car sits overnight, the pressure in the accumulator leaks down because no valve is perfect. This fluid goes back into the brake fluid reservoir - open the hood first thing in the morning with the engine cold, and you'll see the fluid level is well above the FULL line. Start the car, and the pump should run, and you'll see the fluid level drop to the FULL line, as it gets pumped into the reservoir.
OK, great, what about your car? Getting both the ABS and Brake Fluid lights means that the ABS pressure was less than 100 bar. Why would this be? Most likely, the ABS pump wasn't running. As you drove, the pressure dropped. But when it went below 140 bar, the pump didn't run, so the pressure kept dropping.
At 100 bar in the accumulator, you still have 5-10 stops left in the system; it is after all, 100 bar of pressure. But if you run the system out of pressure, you'll lose brake boost - the brake pedal will get hard, and stopping distances will increase, even with much increased pedal pressure. Not a good thing if you're expecting normal braking coming up onthe back of that stopped truck (or worse, kiddie bus on the way to the Trial Lawyer's Association Day Care Center).
I have seen the ABS pump stop working in very hot weather, especially if the car has been stuck in traffic or idling a lot - underhood temps get very high. They usually start working again after cooling off.
Places to look - the ABS pump relay. Pitting on the relay contacts, or oxidation on the relay leads can cause high resistance, worse when it gets hot. Unplug and re-plug the ABS Pump relay a few times - this will clean the leads a little. If you're game, pop open the relay and clean the contacts with some fine sandpaper or Scotchbrite. If that doesn't fix the problem, you'll need the ABS schematic and a meter. Check to see that the 140 bar pressure switch is closed at low pressure (yes, closed - that's how it turns on the pump). It's easy to get low pressure - pump the pedal a bunch of times; since the ABS pump isn't running, the pressure will drop. Check to make sure power is getting through the switch, to the relay, and from the relay to the pump. Jumper out the relay and see if the pump runs.
If the pump is getting flaky, or the ABS pump relay contacts are getting dirty, you can get intermittent faults.
This isn't an uncommon problem, especially as cars age. The trick is that ABS components are astoundingly expensive. That's why regular (every 2-3 year) flushing of the brake fluid is a good, no make that Great, idea. But if you go to a dealer right now, they'll tell you to expect $2000-$3000 for ABS parts. That's why good troubleshooting is critical - you don't want to replace an ABS pump if the relay contacts are dirty.
As to failed accumulators - I don't think that's your problem. When the accumulator fails, the diaphram ruptures. Now when the pump runs, it fills the accumulator with fluid. So what? Fluid doesn't compress well. When the diaphram is working, it's like filling a balloon and letting it go; as the rubber contracts, it pushes the water out. That water stream provides the force. If the balloon where made of steel and rigid, the water would just dribble out. No force.
When the accumulator fails, the ABS and BF lights go out - you can pressurize the accumulator, you've just got no 'throw'. If you hit the brake pedal a few times, you 'use up' the little throw, and the lights come on.
If the lights never go out, the ABS pump isn't running for some reason. If the lights go out, and after the pump stops running (it keeps running after the lights go out - pressurizing from 100 to 180 bar), you should be able to hit the brakes hard and fast 12-15 times before the lights come on. If the come on after 2-5 hard, fast pedal pushes, the accumulator is bad. Not your symptoms.
posted by 192.249....
, Wed, 25 Jun 2003 06:19:53 <-- Viewing This Message
, Wed, 25 Jun 2003 14:31:42, Wed, 25 Jun 2003 20:58:34
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