[urq] Foglight relay is not switching if engine runs

[urq]

... sorry I don't have BTDT to be able to give the full answer, but perhaps
I can provide a clue ... or perhaps not ...

I have heard that on some cars they use a "pseudo" ground to ensure that the
fog lights go on only with the headlights out.  The control relay for the
fog lights get grounded through the filaments of the lights that are
supposed to be off for the fog lights to be enabled.  I've never had to deal
with this sort of a situation on the fog lights myself, but the same
technique is used as a fail safe for the cruise control ... the cruise
control module gets grounded via the brake light filaments, so if all of
your brake light filaments go bad the cruise control will stop working.  I
use the same technique to get the side marker lights on my ury to flash with
the turn signals and still operate as marker lights with the headlamps ...

I can't see where the tie in to the engine on vs. off operation you observe
... except possibly a bad engine ground braid ...

Hope this helps somehow!
Steve B.
San Jose, Kaleefohnia (USA)

> Somehow the foglights never worked on my '85 CGT, and after investigation,
> the foglight relay terminal 85 does receive 12V from the main light
switch,
> but fusebox terminal 10 (86 on the relay) is not connected to earth and
> hence does not switch terminal 11and 13 via 30 and 87 in the relay to
power
> the bulbs.
>
> A. There are two spare M5 earths points in the box. I jumped relay
terminal
> 86 with the M5 closest to the front of the car and yes, the foglights work
> when the engine is off, but strangely enough, when the engine is running,
> they don't.
>
> B. When I bypass the relay (J5) by jumping 11 and 13 on the fusebox with a
> piece of wire, the foglights turn on again even when the engine is
running.
>
>
> My best guess would be that the relay 'looses' this earth on terminal 86
> when the engine is running and opens the relay via 11/30 and 13/87 again,
> turning the foglights off...
>
> Any idea how to fix this or why this is happening?