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Re: ABS/Brake Fluid light
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Thu, 18 Jul 2002 10:39:24 Share Post by Email
In Reply to: ABS/Brake Fluid light, Mark Kleinschmit, Wed, 17 Jul 2002 14:25:05
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I'm not sure where you read the 20 second number, but I don't agree with it.

When the ABS pressure is below 104 bar, the lights are on. Once the ABS pump runs and brings the pressure up above 104 bar, the Brake Fluid light goes out; the ABS takes a little longer because of a self-test it's doing.

The amount of time it takes for the lights to go out is variable. When you shut off the car, the pressure starts to leak down - that's normal. The leak rate is a function of the valve, temperature, age of the car, etc. In general, the longer you let the car sit, the lower the pressure, so the longer it'll take to get up to 104 bar. Sitting overnight, the time for 'lights off' will be longer than for a restart after 10 minutes.

The condition of the brake fluid has an effect. Old, water and air-filled fluid takes longer to build up pressure. Sometimes a fluid change will speed up the time. If it does, that's usually an indication that the condition of the brake fluid was pretty poor. Brake fluid should be changed every 2-3 years.

Of course, the speed of the ABS pump comes in. Faster = lights off faster. Older pumps don't run as fast, but there's a lot more to it than that. Pump speed is a function of voltage - more = faster. Alternators have a harder time providing lots of current at engine idle. So if you're sitting there at idle, and there are a bunch of other electrical loads on (A/C, headlights/DRLs, brake lights (foot on the brake?), defroster, etc), the pump will run slower. If your alternator is tired, the pump will run slower and the pressure rise slower.

In my experience, the time to get the lights off gets longer the older the car. After sitting overnight, I routinely see 30-40 seconds. No big deal, as long as the lights go out. Never drive off with the lights on. Just suppose the ABS pump died the night before, and you drive off. You'll be running on whatever residual pressure there was in the accumulator, and when that's gone, you have NO brake boost. You'll expect normal braking distances, and instead, you'll get 3-4 times that. Whamo at the stop sign at the end of the street.
Since the lights take the longest to go out when the car has been sitting, I use them as a timer. It's good for your engine to let the oil circulate a little before you drive off, longer after a cold start. So when the lights go out, I know the oil has had a chance to get to most of the engine, prolonging its life.

If you haven't changed the brake fluid in the past 3 years, do it. The brakes will feel better for it, and it'll extend the life of some VERY expensive parts.

If you want to check the system, start the car, and wait for the lights to go off. Then keep waiting. The lights go off at 104 bar, but the ABS pump runs until 180 bar. You'll know because that 'hummm' from behind the dashboard will stop. It'll take another 30 seconds after the lights go out. With the system fully charged, hit the brake pedal Hard and Fast. The lights should come back on after about 12-15 pedal pushes. That's normal. If it takes more than that, good - you've got lots of reserve (although you probably weren't hitting the pedal hard or fast enough). If the lights come on after only about 3-5 pushes, the accumulator is probably bad.

If the lights come on after the car has been left sitting only about 5 minutes, and take 30 seconds to go out, then the valving is leaking too much. It's OK if they flicker on for a few seconds after the car has been sitting a few minutes.

If it takes over a minute for the lights to go out, or if the lights ever come on while driving and braking, then the system has trouble.

Good luck!

posted by 140.157.4...
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