The hydraulic pressure accumulator holds pressure in the system for emergency assist if the hydraulic pump fails or the engine quits altogether. It's a nitrogen vessel that holds several thousand PSI. It is believed that over time the nitrogen leaks out of the system (diffused through the diaphragm), because almost all accumulators start to go bad after about seven years of service. The unit has been given the nickname of "bomb" because of the shape of older versions -- rather like a cartoon bomb seen on the Rocky and Bullwinkle Show. The standard tests:
- If the brake light stays on for several seconds after starting the car, suspect the bomb.
- After driving, shut down engine and pump brake pedal until it looses power assist (gets stiff). If it takes less than 10 presses of the pedal, you could have a bad bomb. Beware that when doing this test, the Pentosin reservoir may overflow (spew out the cap). This is because you are discharging the fluid from the bomb, and often the reservoir doesn't have enough space to hold the extra fluid. Consider removing some fluid before this test.
- Press brake pedal as hard as possible while engine is running and car is stationary or moving slowly. If brake light comes on and the pedal initially feels like it has no assist, the bomb may not be holding pressure.
- Engine running at idle, steering to right or left full lock 20 seconds, at the same time pump brakes hard 10 to 15 consecutive times. Does brake light come on? Yes = bad belt, pump, or bomb (check valve). Try a pentosin bleed before replacing the parts!
- If light doesn't come on: stop engine, pump brake 10 to 15 times (short quick strokes). Brake light on? Yes = bad bomb (pressure accumulator).
- If light doesn't come on: Drive the car in a very safe place at 65 mph then apply full stop pressure to brakes (emergency stop pressure!). Does ABS actuate several times? Yes = good bomb! Brake light on? Yes = probably bad ABS sensor.
Replacement:
(Pablo Carselle, Peter Schulz)
Before replacing the bomb, do a complete pentosin bleed/refill with fresh fluid. If your fluid is old and/or dirty, it could be causing one of the valves to stick. If you must replace it, try Linda at Carlsen Audi (800-523-2408), GPR (800-321-5432), or Blaufergnugen (800-683-AUDI). The P/N is 893 612 061 C for older cars (CoupeQ up to #6900) and P/N 4A0 612 061 D for newer '90 cars and all '91s. You should probably try to look at the one on your car to make sure you're getting the right one, especially if your car was built around the time of the switch (10/89). Bentley says that the switch was "gradual," which is a nice way of saying that you can't be sure just by looking at your VIN. If you can look at the bomb (from underneath LF wheel, just above subframe), check the large banjo connection near the top. If it has copper washers it's the old-style unit, if it has rubber o-rings it's the newer one.
Make sure you replace the 2 copper washers or rubber o-rings when you replace the bomb. I recommend you use this opportunity to replace hydraulic fluid (Pentosin) for brake booster and steering rack. During the bomb replacement, you will have to disconnect hose from reservoir to pressure accumulator (bomb). At this time you can drain the Pentosin, assuming it has not been replaced recently.
- You may want to spray the connections that you will be removing with Liquid Wrench or WD-40 for a few days before beginning this job.
- Decide first wether you will reuse "pentosin" (hydraulic fluid) or replace. If you want to reuse, have available a very clean 1 quart container, or one not as clean if you intend to replace. Have some old rags ready for the inevitable stream of Pentosin that runs up your arm.
- After car is in position, pump brake pedal (with engine off) 30 to 40 times to relieve all pressure from the "bomb".
- Put car on jackstands and remove left front tire.
- Siphon out all the Pentosin from the reservoir (turkey baster works well). Disconnect hose from hydraulic reservoir at the top of the "bomb" (hose clamp), remove hose by rotating (twisting by hand) and quickly insert into recovery reservoir. Allow for full drain.
- Loosen the 17mm banjo bolt from high pressure hose (from hydraulic pump) and remove by hand retrieving two copper or rubber washers (both need to be replaced) located above and under the hose connection. There should be minimal fluid presence at this time.
- Loosen 11mm connector of line from "bomb" to booster cylinder at "bomb" side.
- Loosen nut at the front side of the "bomb" and carefully lift the bomb up and diagonally towards the passenger side. This will allow the front screw to move free from the short slot in the retaining bracket.
- Allow the bomb to drop a little and twist it (rotate) in both directions while pulling towards the front of the vehicle, until the back rubber bushing becomes free from the back hanging bracket. The bomb is now free....
- Install new "bomb", tighten forward retaining nut, attach line to booster cylinder and high pressure hose (2 new copper or rubber washers).
- Reattach hose to hydraulic fluid container. The reservoir has a center fine mesh screen that pulls out and can be cleaned or replaced. Since filtered fluid is in the center (and drawn out through the bottom), if you take the filter out, you might as well clean the canister and get the dirt out of there.
- Start the car and fill the reservoir to about halfway between MAX and MIN marks. The PA will now accumulate fluid and it will take about a pint of additional fluid, or close to a liter if you drained it completely. It will take some time for the pump to refill the bomb to proper pressure, usually a good 5 minutes or so.
- When no more fluid is needed, do NOT fill the fluid reservoir to the MAX line. If you do this, the reservoir will not hold all the fluid that the PA returns to it when you de-pressurize it.
- With the front of the car off the ground, turn the steering lock to lock 5 times to bleed the circuit.
- Check for leaks.
