1985-1998 [Subscribe to Daily Digest] |
 | [Main 9000 Bulletin Board | BBFAQ |
Prev by Date | Next by Date | Post Followup ]
Member Login / Signup - Members see fewer ads. - Latest Member Gallery Photos
May I step in? Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Fri, 15 Nov 2002 09:22:48
In Reply to: ABS Question for Anders, Gary, Fri, 15 Nov 2002 08:55:24 Members do not see ads below this line. - Help Keep This Site Online - Signup |
I agree with Anders, the most common reason for the fault you're seeing is a failed accumulator. But it could be something else.
Lots of explaination. A recommended test at the end.
The accumulator has a diaphram across the middle. One side has nitrogen gas, the other has brake fluid. The brake fluid fills the accumulator, compressing the gas. This compression is what provides the 'reserve'. When the car sits, the pressure leaks down past seals in the master cylinder.
When you first start the car after it has sat a long time (overnight), the level of the brake fluid in the reservoir (plastic filler in the engine compartment) is high, above the FULL line. The pressure in the accumulator is low, say 50 or 60 bar. You start the car, and the ABS system sees the low pressure, so it starts the ABS pump. Since the pressure is below 100 bar, the ABS and Brake Fluid lights are on. As the ABS pump runs, it pumps fluid from the reservoir into the accumulator. As the accumulator fills, the diaphram moves compressing the nitrogen gas. The pressure increases. Once the pressure exceeds 100 bar, the Brake Fluid light goes out. The ABS goes out a little later after it finishes its self test.
The ABS pump keeps running until the pressure hits 180 bar. The brake fluid level should now be at the FULL line on the reservoir. As you use the brakes, the pressure drops. If it drops to 140 bar, the ABS pump starts, and brings the pressure up to 180 bar. Since 140 is well above 100, the lights should never come on during normal driving.
In an accumulator failure, that diaphram in the accumulator ruptures. Brake fluid fills the whole accumulator. You can pressurize it, but it has no 'throw'. Think of a rubber balloon and a glass bottle. You can pump air into the balloon and it expands. If you open the neck of the balloon, air rushed out as the balloon contracts. You could use this flow; if you let go of the balloon, it would fly off. If you put the same PRESSURE into a glass bottle, the air would rush out, but not very much. The balloon is a working accumulator, the bottle a failed one.
With a failed diahpram, the pressure drops very quickly, so the ABS and BF lights come on very soon. You also run out of brake boost quickly, because the accumulator really isn't holding any pressure - you're running on the ABS pump. That's why Ander's diagnosis is quite reasonable.
However, there could be other reasons. For example, the ABS pump could be shutting off too soon - instead of 180 bar, it shuts off at 110, so there is very little 'headroom' before the lights come on. I tend to doubt this one. Another possibility is that the boost servo in the Master Cylinder is very leaky. It uses up A LOT of fluid pressure when used, so it drains the accumulator very quickly. I'll bet this is the problem.
Try this. Let the car sit for a while, like overnight. With the car off, press the brake pedal hard a bunch of times. This will drain the pressure down even more. Note on the brake fluid reservoir where the fluid level is. It should be well above the FULL line. Start the car, and watch the level. You should hear the ABS pump run and the fluid level should DROP. At some point the ABS pump will stop running and the fluid level stabilize.
If the fluid level doesn't drop, or drops only a 1/2 inch or so, then the accumulator is bad. The pump transfers fluid from the reservoir to the accumulator, compressing the diaphram. If the diaphram is ruptured, there is no room left in the accumulator, so there is no place for the fluid to go.
If the fluid level drops an inch or so, then the accumulator is probably OK.
The ABS pump should be shutting off. If it doesn't, there is a pressure switch problem.
If the accumulator seems right, watch the brake fluid level while an assistant presses and holds the brake pedal. The level should rise just a little (1/8 inch at most?), and the ABS pump not come on, holding the pedal for 20 seconds or so. If the level rises a lot and the ABS pump comes on, then you have a leaky master cylinder. What's rising a lot? If two or three pushes of the brake pedal can get the fluid level up 1/2 way between the FULL line and the line where you started, and the ABS pump comes on a lot.
Good luck!
posted by 192.249....
, Fri, 15 Nov 2002 09:22:48 <-- Viewing This Message
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.
 |
 |
 |
 |
 |
