Piston Slap: Droppin' Knowledge on Headlight Wiring Voltage Drop

Randy writes:

A few weeks ago I installed a pair of 9003 Philips Vision Plus 30 bulbs in an attempt to improve my wife’s Tucson headlight output. The light down the road is pathetic yet the headlight lenses are clear and the reflectors are still mirror like, at least to the extent I can see from the bulb mounting opening. And the light patterns are well defined, both low & high beam, which also suggests the reflectors are okay. But no joy whatsoever from the new bulbs.

I then did some voltage tracing down the headlight schematic. There is ~14.4 V coming from the alternator & battery into the SJB (smart junction box) that the Tucson uses to switch power to the bulbs. But only 12.5 V on the pins out to the bulbs. And then another couple of 1/10th V loss down the 0.5 mm2 (~20 AWG) wire to the headlamp connector netting 12.2 or 12.3 V at the bulb. The lead electrical tech at a local Hyundai service department says this is 2V headlight wiring voltage drop normal, even as much as 2.6V.

BUT, by my calculations, this means that although DOT specs for 9003 bulbs indicate 910 lumens (+/- 10%) at 12.8V, my wife’s car will be generating a little less than 800 lumens at the voltage actually supplied to the bulb. No wonder night driving is such a frightening experience.

I’ve toyed with the idea of trying some off road 90/100 9003 bulbs under the notion that at this lower voltage the output would be only 30 percent or so above the DOT standard and not likely to be blinding to oncoming traffic. And one hopes not so hot as to melt anything in the headlight assy. But I’d still wind up putting ~7 amps down a pretty thin wire which at 14 A/mm2 is double the recommended maximum current density I’ve seen for automotive wiring. I don’t wish to trade better light for burnt wiring or perhaps a fried SJB @ several hundred $.

My best idea at the moment is to wire the headlights via some fused relays using the car’s high/low circuits to switch the relays. I’d happily trade shorter bulb life for safe night vision.

Can anyone suggest an alternative solution? And yes, I’m aware of HID and LED “conversions” but am not willing to go that route, trading more lumens for rogue beam patterns.

BTW, is it common nowadays for headlight bulb voltage to be so low? Or is this unique to Hyundai? I realize that this improves bulb life … but at such a cost to safe night vision.

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