Re: mystery noise

["Kevin M. Kobielusz" <kmk8258@xxxxxxxxxxxxxxx>]

On Fri, 23 Apr 1999, Johnathan Schwartz wrote:

> This week's challenge is to figure out exactly what is malfunctioning under=
>  the hood that I believe is tied to the anti-lock braking system.  Nearly=
>  every time I start the car and back up out of a parking space, driveway,=
>  etc., I hear a short burst of sound from under the hood, together with a=
>  vibration.  Perhaps it is coming from the front driver side.  The closest I=
>  can come to describe it is the sound of someone pulling the trigger once on=
>  an airgun, but a much lower octave.  (Brrrrrr!)  It usually occurs I take=
>  my foot OFF the brake pedal (we've just backing up at 1mph).
> 
> I theorize that it is the antilock braking system malfunctioning in some=
>  way.  Perhaps there's a hydraulic pump associated with the system that=
>  kicks in to activate the brake cylinders. If there was such a pump that=
>  turns on instantaenously...that would probably be it.  The brakes don't=
>  actually lock up or get applied by the system when this happens.  I'm=
>  thinking that somehow the system is starting up, then quit immediately...pe=
> rhaps a bad sensor.
> 
> I've asked both the dealer plus my trusted mechanic..who both says thare=
>  there  is no such pump.  I would get a code if the system malfunctioned. =
>  They just scratched their head. So much for their help.  After a year of=
>  this, I'm tackling the problem myself.
> 
> Unfortunately, it's really hard to tell what is making the noise while=
>  backing up.

Hi Johnathan,
	You don't say what kinda car you have.  That may help someone with
more experience in diagnosis of the noise.  I am familiar with ABS systems
on GM cars, so perhaps I can help.  From my experience there are only
minor differences in ABS systems.  Here's what I know applies to all ABS
systems - if the system does detect a malfunction, the ABS light will
illuminate and the system will not operate.  If there is no light than
it's safe to say your ABS is okay.  The operation of the ABS requires
pressure of course, and a pump is needed.  This pump will kick in
momentarily when the vehicle is first started to build up pressure, and
shut off when the pressure is reached.  It will not apply the brakes, ABS
won't kick in unless you have your foot on the brake pedal.
	The sound could be your cruise control module malfunctioning.  It
would explain the sound only occuring when you take your foot off the
brake pedal.  Do you have a auto or manual tranny?  Does the sound stop
when you push in the clutch?
	Hope I've helped.
- -Kevin
    	

------------------------------