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Re: Is a relay needed in a fog light installation

To: Bullwinkle <>, mgs <>
Subject: Re: Is a relay needed in a fog light installation
From: Barrie Robinson <>
Date: Sun, 29 Dec 2002 11:44:56 -0800
Folks at world,

Christmas cheer has dulled my senses - but surely a relay will not 
compensate for wiring intended for only one lamp - and allow two current 
sucking lamps to be put on a wire intended for one only one.  Doubling 
amperage on the wire may just run it a bit hot.

While mumbling on....I have to confess that I am puzzled over the use of a 
relay.  It does not change the voltage, it does not change the draw of the 
electrical device, it does not make wires carry more current 
(devices).  What it does do is allow a power circuit drawing big amps to be 
controlled by a low amp circuit (switch).  Obviously this saves on thick 
wire and heavy duty switches but this is negated by the cost of a relay. As 
to the drop in voltage, the use of slightly thicker wire would help the PD 
which surely does not drop all that much - or does it?  Never worked it out 
or measured it, but I suspect the resistance of connecting wire is very 
small.  I am missing something as manufacturers would not use relays 
especially as they are comparatively expensive.  So can anyone explain in 
engineering terms what their advantage(s) is(are) please?

At 02:26 PM 12/22/2002 -0600, Bullwinkle wrote:
>On the MGA and earlier, the original wiring for the fog lamp
>was intended for only one fog lamp.  If two lamps are fitted
>then a relay is needed as the wiring is not heavy enough for
>two.  The MGA parts book lists the relay as for "USA only."
>My guess is that at this time it was permissable in England,
>and maybe Europe, to run with only one "head" lamp.  Thus in
>fog conditions the heads were shut off and only the fog
>used.  In the US, probably most states had laws requiring
>two "heads."  Thus if one were to shut of the low beams, you
>needed two fogs to comply with state regulations.
>There is and added advantage to using a relay in that the
>wiring may be shorter depending on the battery location.
>Thus there may be less "line loss."
>The wiring of the relay may be done in such a manner that
>the fogs only operate when the heads are on low beam only,
>work only with no heads, etc.
>My driving lamp is wired into a two position switch.  In one
>position the lamp is aways on.  The other position the lamp
>is only on when the high beams are up.  Thus putting the
>heads on low beam causes the driving lamp to go off.


Barrie Robinson -

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