RE:74 GTV brake switch query

["John Fielding" <johnf@domain.elided>]

In AD V8#674  bubonic@domain.elided  aka Kelly M  enquired:

Can someone please explain the operation of the brake light switch for a
>74 GTV.  I am having intermittent brake light problems.
>
>I have 12 + volts on one side of the switch and Aero volts on the other,
>when I push on the pedal, the 12+ volt side drops to zero and the lights
>do not work.  Is this how it should work?

To which Dana Loomis replied:

Yes, exactly.  Pushing the pedal down closes the circuit, so there is no
voltage differential across the switch.

Dana Loomis
71 GT Veloce
71 Giulia Super
66 Giulia Super

To which I responded:

Hi Dana,

I don't think you are answering the question correctly!  You both seem to be
talking at cross purposes.

With the 12V on one side of the switch with the brake pedal not pressed you
should see zero volts on the other side - the side which connects to the
lights.  However, when the brake is pressed the 12V on the battery side
should remain 12V and the other side should now also show 12V, and the lamps
should be on.

From the description it sounds to me that there is a short on the lamp side
of the switch to ground and the battery side has a high resistance in series
with it.  If the lamp side of the switch had a short to ground the fuse
feeding the switch should blow.

My guess it that there are two problems here.

1.    The lamp side of the switch has a trapped wire somewhere causing a
short to ground.
2.    The fuse side of the switch has a poor connection causing the voltage
at the switch to drop to zero when the switch is closed.  Possibly a bad
connection in the fusebox?

John
Durban
South Africa
Alfetta 1.8L turbo - still headless following head gasket failure.
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