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Changing the Bomb
I often get questions about changing the PA on Audis. The Hon. Bob
Kunz was nice enough to send me the following, which I have edited
and revised. I offer it to you for archivery.........
From: Bob Kunz <email@example.com>
Edits, revisions, etc: Al Powell
First and most important, do not confuse hydraulic oil (Pentosin) 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 accumulator to stick. This leads to power brake
loss unless you have a particularly good pump working for you.
There are reports that ATF is used in some systems. It seems to
depend on the car, so check your owner's manual AND read the label on
top of the hydraulic reservoir carefully!!!!!!!
The correct fluid (confirmed at both the dealer and the local German
auto parts store) 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 up to $25
per litre, but discounters may carry it as low as $12-$15. Check to
make sure you buy the correct fluid for your car!
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) is also called "The Bomb", as it looks
a bit like a German "potato masher" grenade. It has three hydraulic lines.
Two are high (> 2000 psi) pressure lines, the third 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 if it starts to
overfill; otherwise you can just let it drain out while changing the
bomb. Releasing the pressure is important, as you don't want to
crack open a line with 2000+ psi behind it!!!
2. The PA is (at least on mine) on the driver's side, inboard of the
LF tire. Find it by following the return line at the bottom of the
reservoir. Disconnect all three lines from the PA. A tubing wrench
may be helpful to disconnect the small HP line, but about a 17mm
wrench will be needed for the large one.
Fluid will come out of the low-pressure return line (and run up your
arm....) so be ready to catch it. This is an easy way to drain old fluid
if yours may have debris - just stick a pan underneath. If you're
saving it, stick a large (#3) Philips screwdriver up the hose and
tighten the hose clamp a bit.
3. The tip (facing front) of the PA has a threaded stud. Remove the nut.
Lift from the back at the return line connection and remove.
4. Some older boms have compression washers; most new ones have
O-rings. If you have the older system, install the new unit with new
compression (aluminum) washers if possible; two per banjo nut.
These washers deform to create the seal. At least turn them around.
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. Start the car and fill the reservior to about halfway between MAX
and MIN marks. The PA will now accummulate fluid and it will take
about a pint of additional fluid, or close to a liter if you drained it
When no more fluid is needed, do NOT fill the fluid reservoir to the
MAX line. If you do this, the reservior will not hold all the fluid that
the PA returns to it when you de-pressurize it.
7. With the front of the car off the ground, turn the steering lock to
lock ***5 times*** to bleed the circuit.
8. Check for leaks.
9. Have a good German beer!
The hydraulic pump is really two pumps in one. One runs a circuit for the
power steering, and the other circuit is 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.
You can test the PA and pump's performance by inserting a pressure
gauge into the brake master cylinder where the warning switch goes.
This tool can be made or purchased....but it has to be capable of
handling over 2000 psi safely! A hose and tool company made my gauge
with 3K psi gauge and 5k psi hose for $45. Check the Bentley for the test
Al Powell Voice: 409/845-2807
107 Reed McDonald Bldg. Fax: 409/862-1202
College Station, TX 77843 Email: firstname.lastname@example.org
W3 page - http://agcomwww.tamu.edu/agcom/satellit/alpage.htm
"Baseball is 90 percent mental. The other half is physical"...