Here is an article on replacing the hydraulic accumulator (bomb) on your Audi.

Date:          Fri, 29 Mar 1996 16:11:50 CDT
Subject:       Bomb Replacement Procedure
Reply-to:      a-powell1@tamu.edu (Al Powell)

Replacing the pressure accumulator ("da bomb"):

First and most important, do not confuse hydraulic oil and ATF. They are
not the same thing and ATF in the hydraulic system will ruin the seals.
This WILL lead to leaks in the steering and possibly cause the check
valves in the accummulator to stick. This leads to power brake loss unless
you have a particularly good pump working for you.

Some Audi models DO use ATF in specific systems.  Check before you 
assume this is safe for your hydraulic system!!!!  The reservoir cap 
normally specifies which fluid is correct.  Also check the owner's 
manual.

The correct fluid is Pentosin hydraulic fluid. This is specified by
both BMW and Audi (proabably others). There are two types. Both are
mineral based although one is called "synthetic." The primary difference is
the upper bound on temperature range. For the "7.1s" fluid this is 100
c. for the "11s" fluid it is 140 c. 1984 and older model years use the
7.1s fluid. 1985 and newer use the 11s fluid. This costs about $18 to 
$22 per liter.

If you have used ATF or hydraulic jack oil in the past, you must get this
out of the system. You can drain the reservior by disconnecting the return
line from the pressure accummulator. Gravity does the job. I used only
one litre of fluid to flush and fill mine. Don't forget this fluid also
circulates through the power steering. To move fluid here you need to 
turn the wheels lock to lock with both front wheels off the ground.

The pressure accummulator (PA) has three hydraulic lines. Two are high
(> 2000 psi) pressure lines, the third (largest) is a return line to the 
reservoir.  The PA is pre-charged with nitrogen. To change it...

1. Depress the brake pedal repeatedly until you lose assist. Could be 35x
   on a new PA. Remove fluid from the reservoir as it fills, if 
   necessary.  IMPORTANT to release pressure, as this system operates 
   at 2000 PSI!!  You can get hurt if you don't do this!!!

2. The PA is (at least on mine) on the driver's side. Find it by following
   the return line at the bottom of the reservoir. Disconnect all three 
   lines from the PA. Fluid will come out of the return line so be ready
   to catch it.  Have a large diameter Philips screwdriver ready to 
   stick up inside the return line if you don't want to drain it.  
   You may also have to put a clamp around the hose and tighten it 
   slightly - that Pentosin is slippery!!!  NOTE: If the fluid is old, why 
   NOT drain it now???

3. The tip (facing front) of the PA has a threaded stud. Remove the nut. 
   Lift PA from the back at the return line connection and remove.

4. Some hoses use compression washers; more reccent ones use O-rings 
   which can be re-used.  If washers, install the new one with new 
   (aluminum) washers if possible,  two per banjo nut. These washers 
   deform to create the seal.  If you don't have new ones, at least  
   turn them over.

  Getting leverage to tighten the banjo bolts can be difficult.  A 
   large pair of (what I call) water-pump or slip-joint pliers can be 
   handy to hold the PA while getting them tight.  You don't have to 
   kill them.

5. Fill (and/or flush) see above the reservoir. The reservoir has a center
   fine mesh screen that pulls out and can be cleaned. 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. Mine had some very fine oily dust in it that wiped
   up very easily.

6. Fill the reservior to minimum and start the car. The PA will now 
    accumulate fluid and the level will drop as the car idles; it will 
   take about a pint of additional fluid to top it off.  Don't fill 
   it to the top full mark, as some fluid returns to the reservoir when 
   the car is turned off.

7. With the front of the car off the ground, turn the steering lock to
   lock five times (as per the manual) to fully bleed the circuit.

8. Check for leaks. 

The hydraulic pump supplies two systems:  it runs a circuit for the
power steering, and also runs the circuit for the brakes. There is a way
to test a PA by examining the pressure at which the power assist goes
away. Usually on a broken one, this is easy to do since with the engine
off there will be less than one stroke of the brake pedal before assist
is lost. On a weak PA (i.e. one that has lost some nitrogen), it could
be several strokes before loss of assist occurs. The normal minimum
number of strokes is 20. A new one could be 35. That's what mine is now.
You can decide when it becomes important to replace by measuring the
number of strokes.

Al Powell    

Top of Audi page
Mail to Charlie

10915 visits since July 20, 2002