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Re: ABS and brake light on when pumping pedal Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Fri, 31 May 2002 12:00:40
In Reply to: ABS and brake light on when pumping pedal, Harry, Fri, 31 May 2002 08:11:22 Members do not see ads below this line. - Help Keep This Site Online - Signup |
I need a little more information before a diagnosis.
The ABS AND Brake Fluid lights both come on when the ABS pressure is low. The ABS pressure is stored in a pressure accumulator, just like Anders says. When the car has been sitting, the pressure in the accumulator leaks down. When you first start the car, if the pressure has leaked down far enough, BOTH the lights will be on for a few seconds. When the pressure is low, the ABS pump runs to bring the pressure back up to normal.
So it's perfectly normal for the ABS and Brake Fluid lights to come on and stay on for a few seconds when the car has been sitting for a while. The longer it's been sitting, the longer the lights will be on, because the pressure has had more time to leak down, so it takes longer for the pump to get it up above minimum. On older cars, it's reasonable for the lights to be on as long as 30 - 45 seconds after sitting overnight.
The question comes from 'pumping the brakes when stationary'. Is this right after the lights have gone off, or is it after you've been driving a while, and just happen to do this at a light?
The ABS pressure is used for the ABS AND to provide brake boost. So every time you hit the brakes, you bleed down some of the ABS pressure in the accumulator. The ABS pump will run and bring it back up, and normally this pressure drop and rise is well above the minimum pressure that turns on the two lights.
Under normal driving conditions, after the pump has brought the pressure up to full, you shouldn't see the ABS and Brake Fluid lights on after hitting the brakes a few times. If you do, then Anders is quite right - it's probably a bad pressure accumulator.
HOWEVER, if you start the car, watch the ABS and Brake Fluid lights go out, and then immediately pump the brakes a few times, yes, the lights will come back on. Normally the ABS pressure is right up at the maximum - that's where the ABS pump keeps it. This is well above the minimum that turns on the lights. When you first start the car, the ABS pump runs to bring up the pressure, but the pump will continue to run for a while AFTER the lights go off - it has to get from that minimum up to max. If you pump the brakes right after the lights go out, the pressure is still pretty low, and by pumping the brakes, you bleed down the pressure faster than the ABS pump can make it up. It's the difference between letting a boxer get up from a knockdown and let him shake it off for a minute or two, or nailing him the second he gets up off the mat.
So - when you can make the ABS and Brake Fluid lights come on by pumping, is it after the car has been running a few minutes, or right after the lights go out?
posted by 12.90.16...
, Fri, 31 May 2002 12:00:40 <-- Viewing This Message, Sat, 1 Jun 2002 00:00:06
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