1979-1993 & 94 Conv [Subscribe to Daily Digest] |
 | [Main C900 Bulletin Board | BBFAQ |
Prev by Date | Next by Date | Post Followup ]
Member Login / Signup - Members see fewer ads. - Latest Member Gallery Photos
Re: Braking problems HELP!!! Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Fri, 15 Nov 2002 12:45:06
In Reply to: Braking problems HELP!!!, Jochen900S, Fri, 15 Nov 2002 12:06:41 Members do not see ads below this line. - Help Keep This Site Online - Signup |
The ABS requires a high pressure (140-180 bar) to operate. This pressure is also used to provide brake boost. This pressure is stored in an accumulator (the sphere above the master brake cylinder).
The ABS provides the pressure. When the car has been sitting for a while, the pressure leaks down. This is normal - the pressurized fluid in the accumulator leaks back into the brake reservoir. That's why if you open the hood first thing in the morning before starting the car, you'll see the brake fluid level is ABOVE the FULL line. Start the car and the pump runs, dropping the fluid level in the reservoir as it pumps up the accumulator.
The ABS and Brake Fluid lights will be on if the ABS pressure is below 100 bar. Once the ABS pump gets the pressure up to 100 bar, the Brake Fluid light goes off. The ABS light stays on a little longer as the ABS system finishes up some self-tests. The pump will run until the pressure is 180 bar- 20-40 seconds AFTER the lights go out. Using the brakes uses up some of the pressure - if it drops to 140 bar, the ABS pump runs again, bringing it up to 180 bar.
When you first start the car, the ABS pump runs to bring the pressure up, so it's normal for the ABS and Brake Fluid lights to be on. As cars get older, just like people, it takes longer to get the pressure up. The ABS pump is electrical, and on cold mornings system voltages tend to be lower, and the brake fluid maybe a little thicker.
It's normal for the ABS and Brake Fluid lights to be on for a few tens of seconds when you first start the car. Usually they will stay on longer, the longer the car has been off. The pump will usually run for another 20-40 seconds after the lights go off. As long as the lights stay off when driving the car and using the brakes, all is fine.
If the ABS and Brake Fluid lights are on, the ABS pressure is low. You shouldn't drive the car until the lights go off. There are a few reasons for this. First, if the car is cold, it's nice to let the oil at least get to the lifters before you put a load on it. Letting the car idle for a minute when it's cold is good for engine longevity.
Second, let's assume that the ABS pump has died. You start the car, and it doesn't pressurize the system. You have some residual pressure in the accumulator, enough to give you adequate brake boost for your first two stops. That gets you out of the driveway, and down the street. The lights are still on, but so what? You come to the stop light at the end of the street, and you have NO brake boost at all. Your stopping distances are up by a factor of three, assuming you recognize the problem and stand on the pedal. Between the recognition time and the longer braking distances, you're in the middle of the intersection. Hope you had a green light.
Yes, this is meant to scare. It's not a good idea to drive off until the lights go out.
There is an easy test for the accumulator. Start the car and wait until the lights go out, and the ABS pump shuts off. Pump the pedal HARD very quickly, until the lights come on.
It's normal to get 12-15 hard, fast pumps before the lights come on. If the lights come on after 5 or 6 pumps, you have a failed pressure accumulator.
But it is normal for the lights to be on when you first start the car.
posted by 192.249....
, Fri, 15 Nov 2002 12:45:06 <-- Viewing This Message, Fri, 15 Nov 2002 12:46:08
No Site Registration is Required to Post - Site Membership is optional (Member Features List), but helps to keep the site online
for all Saabers. If the site helps you, please consider helping the site by becoming a member.
 |
 |
 |
 |
 |
