[s-cars] Brake Light and Thrumming Pedal

[Postupack, Jeff]

Doug:

I've had a similar brake light and thrumming pedal experience, so I can
empathize with you.
It's a bear to diagnose the root cause.
Hopefully you have a spare car during all this as diagnosis takes
considerable time.

Further to what Fred wrote. definitely test and rule out the Brake servo
first as that's the most difficult part to change and most expensive.
Fred's method earlier in this post is much better than that prescribed
in Bentley.

Next  I recommend your test the hydraulic pressure and hyd fluid
delivery capacity either via qualified shop or yourself.
All depends how much independence you subscribe to!

I rigged up a 2000 PSI gauge with steel lines and brass fittings to
measure the actual pressure.
I tapped into the line feeding the Bomb after removing the hose from
crossover to intercooler for access.
Bought a pressure gauge for about $30 USD, plus $10 in line and
fittings.

My pump was modulating between 200-300 psi, as opposed to Bentley
specified 1800-2000 psi.

Measuring the hydraulic pump capacity is a matter of opening the high
pressure line {TO the Bomb} and routing it with plastic tube into a
clear bottle.
Spec is 0.2 liters/min  so I marked off a 1 liter wine bottle and timed
the period to fill 3/5 ths of the bottle. (0.6 liters in 3 minutes.)
 
Crude, but that told me my pump was toast, and I replaced it.

Hang in there man, you're almost done.
Jeff Posto 



Message: 1
Date: Fri, 30 Jun 2006 20:19:08 -0400
From: "Fred Munro" <munrof at sympatico.ca>
Subject: Re: [s-cars] Brake Light and Thrumming Pedal
To: "Douglas Fifield" <douglas.fifield at gmail.com>
Cc: s-car-list at audifans.com
Message-ID: <AFECLHABEEILGMHBNDFEOEOEDHAA.munrof at sympatico.ca>
Content-Type: text/plain;	charset="UTF-8"

Douglas;

If the level doesn't fall, the bomb is not being charged with fluid.
This could be a really bad servo or a bad pump. At this point I'd do the
servo test to rule that out. A totally dead bomb would also not draw
fluid out of the reservoir, but it should allow the system to pressurize
and the light would go out.

I replaced the servo in a couple hours. You don't need to do a brake
bleed - you remove the two bolts securing the master cylinder to the
servo, release the brake lines to the MC from the multi-line clamp, and
slide the MC off the servo without removing the brake lines. When the
new servo is installed, you slide the MC back onto the servo and bolt it
up. You don't have to open up the lines or bleed the brakes.

As Brian mentioned, the servo is secured by four 13mm nuts that also
secure the pedal cluster. The top left nut is a bit tricky as Brian
points out, but I got it using 18" of 1/4" drive extensions and a flex
end. You have to use 1/4" drive stuff or you can't see past the
extensions to the nut. You have to use multiple extensions and assemble
them as you insert them through the hole in the pedal cluster. Remember
to get a new sponge gasket to seal the servo to the firewall.
Unfortunately I didn't do a write-up on this.

Hopefully you don't need a servo, but the alternative is probably the
pump. If the servo tests good, you'll have to make up a pressure testing
rig or take the car to the dealer to test the pump output pressure.

HTH

Fred