As I start to work the wiring harness, I am wondering if I want to direct wire the headlights, or use relays to power them.
anyone made that decision? I am using the EZwire harness, and the Ron Francis ignition switch, rated at 40amps. I want to use the H4 halogen bulbs. On my sons Miata and his 66 Chevy pickup before that the ignition switch couldn't take the power load, so we used relays instead.
On Bonus Built trucks at least, and most cars, the lights aren't powered thru the ignition switch. You couldn't use the parking lights unless the ignition were on (like if you're parked). They are usually on a circuit breaker direct off the battery.
Assuming you're running 12v, lights should be less than 15 amps total. I just don't see a lot of need for relays on our trucks, and it would add a lot of wire, since they should be right up near the headlights for max benefit. The halogens like full voltage, so just use generous wire size to them.
Mark another vote for "relays!" I've used them on several vehicles, with and without headlight upgrades. The advantage is eliminating as many connectors and as much small gauge wiring as possible. Elimination of connectors and small wire reduces the resistance in the circuit and ultimately, the voltage drop. This has always resulted in brighter headlights on these vehicles. Lots of good information and a more technical description at MadElectrical.com - Mad Enterprises
Headlight relay article here: Catalog
The last time I did this, I ran the wire from the battery instead of the alternator. It was still an improvement, but picking up power at the alternator "sensing" point provides the most improvement.
The wire from the battery was 10 awg with a 30 amp self resetting circuit breaker near the battery. This provided the power to both the "high beam" and "low beam" relays. The existing wire harness was used to turn the relays "on" and "off." A 12 awg wire was run from each relay to each headlight. The relays I purchased had two "output" terminals so each headlight had it's own connection with no splices required. I think I picked these up at Auto Zone for ~$8 each.
Hiding the relays behind the grill works well. I'm not sure where you are talking about "under the gravel guard." I would protect the relays from dirt and crud from under the truck, if possible. You might also want to get "relay bases" that allow the relay to be snapped into the base. These are a little harder to find, but if the auto parts store doesn't have them, check with your local car stereo installer. They used them frequently to power amps and such.
Unless you want them exposed to water I would mount them as many car manufactures do, on the firewill, in the engine bay. Buy all three relays the same so you can switch them to troubeshoot problems. The idea is to keep high amp use items away from your switches/fuse panel/under dash area and provide direct batt to component through relay connected wiring.Fires and fuse box melt downs have been around a long time. Unless you are using a old or under size battery, a couple feet of wire is not going to make that much of a differance, remember the relay for the elect fuel pump in the tank has several feet of wire and it works ok. The average three relays: elect fan, fuel pump and headlights. Some may put high amp music systems, etc. All high amp relays contacts arc and slowly wear out so don't buy Chinese made. Relays should have amp rating written on them. Have a great day,chuck
Sam, I lived in Houston for almost 20 years. My son had a HO mustang in the eightys having a alternator down low in the steering pump area. When it rained, the under passes flooded and shorted out the alt ($350.00). The relays we are talking about have one side direct to the battery. If it shorts out, the truck dies and flooding/loss begins. I understand the clean look. I just wanted you to be aware of potential problems. Have a great day,Chuck