Jay
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.
Blake
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