Lighting the way forward

The graph shown below was constructed by Keith Morris (aka CumbrianKeith) and represents Hella’s published light intensity output against supply voltage for a typical headlamp bulb. It indicates why a small drop of applied voltage (due to direct switching contacts, bad earths, too thin wire for instance) can result in very poor headlight brightness. Read on to be amazed how poor your light can be! He used a relay and heavy duty wiring to transform his headlights – there are tailor-made relay and wiring kits available from the States (ask on vanagon or syncro forums) but these parts can be acquired in the UK too of course.

From the Tech 2004 message boards, 31-03-2004

Hella Headlight  Output Vs Voltage (small)

Fitted H4 optics and even with 100/80 w bulbs the lighting level is not great. So I’ve been reading this article ( and do the voltage drop measurements as described to ascertain how the old wiring, terminals and switches have affected the juice flow. I get a total voltage drop in the main beam circuit of 3.4 volts! This equates to allowing through only 75% of the supposed 13.5ish which should be happening. So I plot a graph of the Hella research figures and extrapolate a figure of around 30% light output compared to what it would be if voltage was 100% of the alternator output (which is actually only producing 12.9 with the lights on)
Maybe this is why the lights are poor…
So I get some nice thick (27amp) cable and some relays and wire it all up (and get some 130/90s just for the hell of it – cos I can!) and lo and behold the voltage drop now is only 0.05v and boy are those lights bright!
Only slight problem and this is where my question comes in: the mainbeam tell-tail on the instrument panel now doesn’t work – it flashes extremely momentarily when you put the lights up but doesn’t stay on; and it doesn’t come on at all when you flash the heads without the other lights on. Solution – pick the relay signal feed up off the left-hand headlamp, not the right. The latter doesn’t trigger the tell-tale!

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