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Re: '73 B Brake Light Switch

To:, mgs@Autox.Team.Net
Subject: Re: '73 B Brake Light Switch
From: DANMAS <>
Date: Thu, 23 Apr 1998 21:50:19 EDT
In a message dated 4/23/98 9:19:12 PM Eastern Daylight Time, writes:

> Logic tells me that the when the plunger to the brake light switch is
>  DEPRESSED, the lights should go ON.
>  My brake lights were staying on, so I removed the switch from the housing
>  and fiddled with it, pulling the little plunger out and pushing it in. At
>  first, the lights just stayed on, then they started going off when I
>  RELEASED the plunger...
>  Anyone seen this before? Or Is this a Lucasism where the switch needs to be
>  aligned so that the pedal, when depressed, releases the plunger?


No, this is not a "Lucasism," every car works that way, unless it has a
pressure switch in the brake lines.  It has to be this way, nothing else would

You want the brake lights to come on as soon as you depress the pedal, not
just when the pedal is depressed all the way.  Most of the time, you never
press the pedal all the way anyhow.  If the switch worked the other way, the
lights would only come on at the bottom of the pedal motion, meaning they
would never be on except in a panic stop.  If the switch were arranged to come
on at an intermediate point, either the switch would break when the pedal was
fully depressed or the pedal would be limited in its travel (I'm sure there is
some Rube Goldberg arrangment to allow this, but no simple setup).

The brake pedal is pivoted at the top.  When you press the bottom forward, the
top moves towards the back of the car, releasing the switch and lighting the
brake lights.  The switch contacts close as soon as the pedal is depressed
just a little bit.  The switch needs to be adjusted very carefully, or it will
preven the pedal from retracting fully, dragging the brakes.  This happens
fairly often when people replace the switch, or do other work on the pedal

Evidently, your switch was adjusted too far out.  When the pedal was released,
it still did not make contact with the switch.

If a car has the switch in the brake lines, operated from hydraulic pressure,
then it needs to turn on when the pedal is depressed.  As long as the switch
is properly mde to withstand the pressure, it will not be damaged by a panic
stop, yet will operate in a gentle stop.

I had a problem with the brake switch in my TR6 failing frequently, so I
replaced it with one from GM and have had no troubles since.  As I said, all
brake switches work this way, so if you can find one that will physically fit,
there is no reason you can't use it.  I walked into the local auto parts store
with the Lucas switch in hand and asked the counterman to find me one like it.
he brought out about a dozen or so, and I picked one that would work.  I had
to file a flat spot on the threads, but other than that, it was a simple one
for one swap.

Dan Masters,
Alcoa, TN

'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74

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