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Re: Brake test switch and Logic

To: danray@bluegrass.net, gofastmg@juno.com, mgs@Autox.Team.Net
Subject: Re: Brake test switch and Logic
From: DANMAS <DANMAS@aol.com>
Date: Tue, 20 Jan 1998 03:05:38 EST
In a message dated 1/19/98 11:09:57 PM Eastern Standard Time,
danray@bluegrass.net writes:

> Gee....what a lovely "warning" system! Let's see, if I lose half, and still 
> have braking, then the light warns me, if ALL the fluid goes bye bye, then I
> get no warning. Interesting....

Dan,

Actually, that is a very interesting warning circuit, and very logical as
well, if you think about it.  Let's assume for a moment that it were set to
warn you of zero pressure in the overall braking system -- how would the
sensor know the difference between zero pressure because of a loss of fluid,
and zero pressure because you are cruising the interstate with your foot on
the gas pedal and off the brake?  It could be done, but it would require an
additional sensor on the brake pedal to tell it to look for zero pressure only
when the pedal is depressed.

That would be difficult to do, because how would you know where to set the
pedal sensor?  On car A, with poor maintenance, the pedal may be half way to
the floor before any pressure is applied, while on well maintained car B
pressure may be applied with just a slight movement of the pedal.  Even so,
what benefit would it be? 

Lets say you are ripping along a narrow country lane and you have a
catastrophic failure, and loss of all fluid.  Until you try to use your
brakes, you still wouldn't know it.  Suppose, then, you crest a hill to find
two farmers sitting in the middle of the road on their tractors, discussing
the weather.  When you hit your brakes and nothing happens, having a little
light come on would be rather redundant, I would think. 

The way the sensor is set up to operate, as Rick pointed out, is to monitor
the difference in pressure between the front and back of a split system.
Suppose you drive to the grocery store, just up the street a bit, and you
drive rather sedately, using your brakes only lightly.  A complete loss of
fluid in one half of the system may or may not be noticed, as half of the
system would be more then adequate to stop you with no trouble.  If you are
sensitive to changes in your car's behavior, you might notice the reduction in
braking power, but would not necessarily think it was due to a complete
failure of half your braking system.  The differential pressure switch will
detect the difference though, and give you a warning.  On the other hand, a
complete loss of brakes would be most definitely noticeable, even on the
grocery run, but again, the light would be redundant.

The test light on the dash doesn't monitor the warning switch operation, it
only tests the electrical continuity of the warning light circuitry.  It would
be of no benefit to have the switch actuate if the lamp is burned out, or the
wires have come loose.

For a real good description of the workings of the PDWA (pressure differential
warning actuator), see:

http://www.vtr.org/maintain/pdwa.html

Dan Masters,
Alcoa, TN

'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
                    http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition, slated for a V8 soon!
'68 MGBGT---organ donor for the '74

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