[s-cars] Brake light stays on.

[Fred Munro]

Hi Jack;

The light is telling you the hydraulic system pressure is low, as validated
by the loss of brake boost after two applications.

This means that:
1) the bomb is toast and is not storing pressure or/and
2) system pressure is low and cannot charge the bomb.

Item 1) is usually caused by loss of the nitrogen pad over time until there
is nothing left to compress in the bomb. This results in loss of boost after
a few (or one) brake applications. When the stored pressure is gone, you can
feel the pump pressure pulses in the pedal.

Item 2) can be caused by:
a) failed brake servo bypassing pressure from the bomb to the reservoir
(tested by removing the servo RETURN line with the system pressurized. The
servo should only be leaking a few drops per minute)
b) pump air-locked or defective. Hard to test without proper pressure
measuring equipment. Cracking the high pressure line doesn't tell you much.
I don't recommend doing this - it is dangerous to release high pressure oil,
makes a mess, and usually cuts the o-rings when the oil sprays out.
c) stuck pressure relief valve in bomb (I'm assuming there is one there -
I've never looked at the valving in one of these).

I'd suggest flushing the Pentosin to see if that clears up the problem. It
fixed my 30 second light problem for 2 years and is a cheap fix if it works.
Try the servo test to rule the servo out. You can't do the "bomb test" if
you can't pressurize it.

If the servo checks out OK it leaves the pump and the bomb as potential
problems. You can feel the pressure pulses from the pump so it is putting
something out. Given the age of your car, it would most likely be the bomb -
that's where I'd start as it is a more likely cause and is cheaper than the
pump. Better to replace the bomb and find you need a pump than replace the
pump and find you need a bomb.

HTH

Fred Munro
'94 S4

-----Original Message-----
From: s-car-list-bounces at audifans.com
[mailto:s-car-list-bounces at audifans.com]On Behalf Of Jack Gagnon
Sent: January 16, 2005 11:01 PM
To: S-Car-List
Subject: [s-cars] Brake light stays on.


Hello,

I am having trouble with my 92 S4.  The brake light does not go off no mater
how long I run the car.  It used to stay on for 10-30 seconds in the morning
and then stay off for the rest of the day.  It behaved like this for about
two-three years of use.

Just yesterday, it decided to stay on for good and I don't have power brakes
after the first or second application of the brakes.  I suspect the
hydraulic pump, but the steering feels fine and I cracked the high pressure
line and the fluid squirts out just fine.

Could it be a stuck valve pressure relief valve in the bomb?  Is it the
servo in the master cylinder? or is it the pump.  I can feel the brake pedal
pulsate a little when the assist is used up.


I have a bad feeling that this on could cost me a lot of money if it is not
diagnosed properly.

Any help is appreciated.

Thanks

Jack
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