Headlight harness

In the never-ending quest to improve my truck I decided to upgrade my factory headlight harness and bulbs. There have been several people do this on Ford-Diesel.com. Most have purchased aftermarket harnesses which run about $35-40. I decided to make my own.

Why upgrade the harness? In most automobiles the voltage for the headlights goes from the the fuse block, to the headlight switch, then to the high/low beam switch, then to the headlights. The combination of this long path, and usually small guage wire, creates a significant voltage loss at the headlight bulb. The second reason I'm upgrading the harness is that I'm installing higher wattage bulbs.

Here are the main components of my upgrade. I got the 80/100 watt bulbs from JC Whitney. I think they were $14-15 each. The 9007 plugs (P/N LS6493) were about $9 each at NAPA.

Here is the schematic. I used 14 guage wire for the harness. I got the power at my engine compartment juction point. I covered the wire with ziploom for a factory appearance.

I used the factory headlight plug as a source of the low and high beam signals to turn on the relays. This was a non-destructive way to get these signals. The green wire in this picture is the trigger wire for the low beam relay.

The various sizes of wire can be seen. The red, green, and blue wires are 14 guage. The black wire is from the NAPA plug-in. The puny little green/black wire is the factory Ford highbeam wire. The red and blue wires come from the passenger side headlight...blue is the highbeam and red is the lowbeam.

The new harness and bulbs are much nicer. The voltage at the headlights went from 12.6V to 14.1V, and the lights are noticeably brighter. The light is white now as opposed to being more yellowish before. I notice reflective signs are brighter now, especially at dusk. The whiter light makes it easier to see. The upgraded lights are not brighter to oncoming traffic.

I figure it cost about $30 in parts for me to build the harness, so buying a premade one for $35-40 isn't a bad deal.

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