H4 Headlight Upgrade Wiring Questions

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Jul 7, 2014
Hi all

I posted this in the lighting section, but I don't think too many people venture over there.
Hopefully, I'll have more luck here.

I have a 1987 4runner. Let me just say the factory light output is terrible. I have done a great deal of reading on this upgrade and am planning to make my own replay harness. Attached is a picture of the schematic I drew up (I tried to scan it but the scanner wasn't working, will post scanned version later). Daniel stern's lighting website was very helpful, as I basically just copied his diagram. I did tweak his diagram, after learning about negatively switched headlights on Toyotas.

My question for the electronic pros out there, does this look correct?
My second question is what gauge wire should I run out of the relays to the headlights? I'm planning to run 12 gauge from the battery and 14 gauge for the ground wires. However after looking at H4 connectors, most use 18 gauge wire. That just seems too small in my opinion. I think I would rather run 14 gauge all around except for the 12 gauge power supply wires to the relays.

Find the H4 relay harness thread in the 60's section. There is a schematic posted there. The one for an FJ60 will work well for a Mini.

Can not look at the wire sizing purely from a pure ampacity perspective. Need to look at it from a voltage drop perspective. You want very little voltage drop for best light performance. If you're going to wire the lows to continue to be on with the highs then you need to consider the ground wires to be operating at the combined amps.
I made my own headlight harness, way back in 2001, it was a lot of work because I'm a fussy SOB but it was worth it. Pretty sure I ran #12 wires to each headlight, and the feeds from the battery were #10's.

But I think you could solve the dim light issue these days with a good quality E-code headlight housing (Cibie or Hella), then a good quality H4 LED bulb. They draw so much less current, that the voltage drop is minimal. I upgraded the H11 headlights in my 200-series with IPF LED bulbs, and wow it made a huge difference. They do make them in an H4 format, but just be advised they are not cheap. Good stuff isn't.


You could do the headlights and bulbs, see how they are, and decide if you need the harness.
Thanks for the replies. Led is a possibility but I think I am going to stick with halogens as I don't want my headlights to be too bright that they annoy other drivers.
I found some good threads for 60s that go in depth with lighting upgrades.

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