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When both lights come on-
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Tue, 25 Jun 2002 15:27:58 Share Post by Email
In Reply to: ABS and Brake light went on a 1991 900s SAAB 120k miles, gerald g., Mon, 24 Jun 2002 15:41:16
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First, a story:
The ABS and Brake Boost are provided by pressurized brake fluid. This pressure is stored in a pressure accumulator (the sphere sitting near the brake master cylinder).
If the pressure in the accumulator is less than 140 bar, the ABS pump runs, increasing the pressure in the accumulator. It shuts off at 180 bar. Using the brakes brings the pressure down.

If the pressure drops below 104 bar, BOTH the ABS AND Brake Fluid lights come on. That's an indication of low accumulator pressure. At 104 bar, you have between 5 and 10 stops left with full brake boost. After that, the accumulator pressure is too low to give brake boost, and your pedal gets very hard, and braking distances get VERY long. That's the dangerous part.

So if you have both the ABS and Brake Fluid lights on, that means the 104 bar pressure switch has come on. This can be for two reasons - either your pressure is low, or the switch is flaky.

A lot depends on how much braking you did when both the lights were ON. Since 104 bar in the accumulator is enough for 5-10 full stops, if you did very little braking in that hour, then the pump may have quit. I've seen this happen due to overheating - happens with lots of braking in very hot weather. Like getting stuck in stop and go traffic on a hot day.

If you were able to brake many times (more than 10 or 20) with the lights on, then it's probably the 104 bar pressure switch.

It's very important to know. Just because you may have been able to do lots of stops the last time the lights were on doesn't mean you will the next time. Sudden loss of brake boost can very easily cause an accident.

If the problem has gone away, well, leave it where it is. If you haven't flushed the brake fluid in two years (three at most), then do so. Brake fluid gets old (absorbs water and gunk) and should be flushed. This sometimes helps.

If the problem occurs again, you can test for it pretty easily. First off, pull over to a parking lot or someplace safe. Open the hood and look at the brake fluid reservoir. There is a line on it that says "full with charged accumulator" and if you notice, it's well BELOW the top. When the accumulator is charged (pressurized), fluid goes out of the reservoir into the accumulator, so the level DROPS to the FULL line. If the accumulator has no pressure, the fluid has to go somewhere, and the reservoir level will be ABOVE the full line. Check your car some morning before you start it - you'll see the fluid level is above FULL.

So if the lights are ON and the level is at FULL, chances are it's a bad pressure switch. If the lights are ON and the level is ABOVE full, then chances are your ABS pump isn't running, and the fluid is in the reservoir, not the accumulator.

Pump the brake pedal a few times, and listen for the ABS pump to come on - it makes a buzzing sound. Pumping should make the level inthe reservoir rise, and the ABS pump will drive it down.

There is another situation, one that Anders alludes to. The accumulator has a diaphram in it - fluid on one side, nitrogen gas on the other. As it fills with brake fluid, the gas side compressing, providing pressure. If the diaphram ruptures, you still pressurize the accumulator, but you have no flow.
If the accumulator diaphram ruptures, the ABS pump has to run a lot to keep up with your braking. If it runs enough, it'll overheat and stop.

As a test, start the car and let the lights go out. Wait until you hear the ABS pump stop running. Hit the brakes hard repeatedly and quickly, until the ABS and Brake Fluid lights come on. If they come on after 12-15 quick, hard applications, that's normal. If the lights come on after 4 or 5 applications, the accumulator is probably bad. Thanks to Anders for this test.

Good luck!


posted by 140.157.4...
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