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Re: ABS & Brake Fluid Lights stay on for 30 Seconds Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Tue, 15 Jul 2003 06:56:56
In Reply to: ABS & Brake Fluid Lights stay on for 30 Seconds, Richard St George, Mon, 14 Jul 2003 16:28:43 Members do not see ads below this line. - Help Keep This Site Online - Signup |
You get both lights on when the ABS pressure is less than 100 bar. The ABS pump runs if the ABS pressure is less than 140 bar, and stops at 180 bar.
When the car is sitting, the pressurized fluid in the accumulator slowly leaks out and into the reservoir. So when you start the car, if the pressure is low in the accumulator, the ABS pump will run to pressurize the system. Until the pressure hits, 100 bar, the lights will be on.
It's normal for an older car to take 15 or 30 seconds, or sometimes even more, for the lights to go out. Old brake fluid can affect the time, but there are a host of other factors - engine voltage at idle (affects pump speed), pump efficiency, 'leakyness' of the various valves, underhood temperatures, etc.
If your brake fluid is over two years old, have the system flushed. This is more than just bleeding the rears - this is a flush of all the brake lines. Quite often this will improve brake feel, and may (or may not) improve the 'lights off' time.
When it comes to testing the accumulator, the 'fast brake push' test is good, but it has to be done right. The lights go out when the ABS pressure hits 100 bar, but the system isn't at full pressure until the pump shuts off at 180 bar. So the ABS pump runs for a while after both ABS and Brake Fluid lights go out. To do the accumulator test, start the car and wait until the ABS pump stops running. You can usually hear the buzz of the pump, and that it stops, probably 20 or 40 seconds after the lights go out. The other way is to start the car, and with the hood open, get out and look at the brake fluid reservoir. You'll see the fluid level dropping as the pump runs. Once the pump stops, the fluid level will remain steady (it should be at the MAX With Charged Accumulator line). Only after the ABS pump has stopped running should you do the accumulator test.
Typically you should get 12-15 hard, fast pushes before the lights come on. That's a good accumulator. A bad accumulator has the lights come on after 3-5 pushes. If it comes up 10 again, with a known fully charged accumulator, it's probably fine, but keep an eye on it.
A bad accumulator will affect the time it takes for the lights to go out, but not the way you'd think. With a truly bad accumulator, the lights go out SOONER. The accumulator is a sphere with a diaphram stretched through the middle, and one side is pressurized with nitrogen gas. As you pump fluid in, the diaphram stretches, pressurizing the gas. It's the compression of that gas that provides the 'push' - the stored energy is released as that ballon expands. It takes time and work to compress that gas. If the accumulator is blown, the gas has leaked out, and the accumulator is filled with fluid. It actually takes less work and time to pressurize that fluid, so the lights go out sooner. The reason it's bad is that the only stored energy is due to the flex in the brake lines and metal sphere - not much.
As to the squeal, that's probably OK. When you release the brake pedal, the pads don't jump back from the rotors - imperfections in the rotors push the pads back a little. So there is a little drag of the pads on the rotors. Brake squeal occurs when the pads vibrate as they rub on the rotors - just like a violin string vibrates when you pull the bow across it. There are springs on the brake pads to damp out this vibration, but some pads just squeal. You can pull the pads out and coat the BACKS with anti-squeal compound - just some sticky stuff that damps out the vibration. Or just wait, and see if the noise goes away as the pads bed in.
It is possible that the rear calipers are sticky. If the rear brake lines were corroded, I have to assume what was in those lines wasn't too pretty, either. If you notice sever or uneven rear pad wear, the rear calipers may need rebuilds.
But no matter what, have the brake fluid FLUSHED every two years or so. It's cheap (most shops can do it for about $50-$60), and much, much, much (did I say much?) cheaper than ABS brake hardware.
posted by 192.249....
, Tue, 15 Jul 2003 06:56:56 <-- Viewing This Message
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