[xj40] Better headlight wiring?

For my 92 Sovereign, I am running the brightest and highest wattage
bulbs I could find that would fit the stock rectangular headlights
and foglights. For bulb colour temperature, I have stayed at around
4000 K, again to maximize light output, which is quite white.
Anything above about 5000 K looks to weird to me, never mind the
silly looking purple beams at about 10,000 K and limited light

I use the same bulbs in my various vintage motorcycles. In my old
motorcycles, the light output is noticeably brighter from the same
bulb, compared to the light output of the same bulb installed in
the Jag.

The fundamental difference would appear to be that in my various
Italian vintage motorcycles with their decidedly feeble stock
wiring systems, I have modified them so that the bikes wiring and
switchgear is used only to switch a non stock relay that I have
added. The actual power to the headlights then comes direct from
the battery/charging system on 4 guage wire through the switched
relay to the headlight. Little or no voltage drop occurs, as the
power just goes direct on really heavy guage non stock wiring, and
the bulbs give maximum output even with the pathetic feeble
charging systems, some of which don’t even use a
regulator/rectifier as the charging output is so low that they
barely have enough power to charge the battery and not run flat
just trying to run a taillight and a headlight…

A bulb puts out 100 per cent of its light at 13.2 volts, I think. A
very minor drop of voltage to the bulb results in a substantial
decrease of light output. I read somewhere that at 12 volts a bulb
only puts out about 80 per cent of its potential output. So, even a
minor voltage drop in a now 15 year + old Jag wiring harness to the
headlights will decrease bulb light output substantially.
Similarily, a higher draw on the wiring harness from a higher
output bulb than stock, more resistance, etc, may cancel out any
advantages of the bulb that should be way brighter than stock, but
is disapointing when the money has been spent on it and it doesn’t
really give all that much more light. Most likely a problem with
the wiring harness, not the bulb…

So, I spent some time wading through the archives here after a
search on relays. Problem is there are so many stock relays in the
car that the archive search returned a huge number of unrelated
posts. I did come across an exchange from December 2005 where the
topic of running power directly to the headlights from the battery
through a relay was being considered, but the exchange didn’t seem
to reach any definitive conclusions.

HAs anyone done this? Did it result in a noticeably brighter light
output? Is there any reason hidden within the Jag wiring harness
not to go direct to the headlights through two relays, one for the
low beam and another for high beam? Maybe even a third relay to run
power direct to the fog lamps?

Any comments appreciated. Thanks.

Paul LeClair–
1992 Jaguar XJ 4.0 Sovereign
Calgary, Alberta, Canada
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Hi Paul. I know this is an ancient link but i wonder if you ever worked out the relay issue? i’ve been thinking of ways to achieve the same thing, and if not possible then to replace the original wire from the factory loom with a fresh, slightly heavier gauge to feed better voltage to the bulb monitoring unit relays