Headlight relays with new wiring

[Canonman67]

I'm planning to upgrade the headlights in the F100 when I replace the wiring harness. The new harness is a heavier gauge than the stock one do I still need to put relays under the hood for the headlights?

[Nitekruizer]

If you're going to use brighter, higher wattage bulbs, then, yes, you should use Headlight Relays. The Headlight Switch itself will be "the weak link in the chain", so to speak. The new high wattage lamps will overload the Headlight Switch, which contains a Circuit Breaker. This will cause the lights to flash on and off repeatedly, until the switch eventually burns out completely. The Dimmer Switch on the floor may last longer with Headlight Relays too. With relays, there will be less load on all of the truck's original headlight circuit.

In my other project, the connector at the Dimmer Switch melted when I converted to Halogen Sealed Beams without installing relays and , of course, the Headlight Switch burned out too. I decided on installing Headlight Relays in both, my truck and my other project. My truck is getting Halogens too.


I hope this helps

[Canonman67]

It does, thank you. I'm doing high wattage halogens on the rebuild and may go to hids in the future. I was kinda thinking the switch would be the weak link but wanted to be sure before spending any more money. I know, I know, they're not THAT much but the budget is getting stretched tight at this point and I need to watch my pennies now.

[69Ford]

Head light relay harnesses are cheap and some don't require any modifications to your original harness. LMC has them but I would upgrade them to a good over the counter relays.

[HIO Silver]

The LMC headlight relay harness is about $30. It's pretty much a wash with fabbing your own (relays, relay plugs, headlight plugs, fuse/breaker holder, wire, terminals, +time).... It'd be better than getting stranded at night due to a tempermental headlight switch/breaker cycling on and off at random. Contrary to the pic in their catalog, the harness sheathing is black rather than yellow.

[Canonman67]

Thanks guys, I wil definitely put them on the to do list. Being stranded in the boonies isn't on my top ten activities list.

[Chucks1]

For what it's worth, the headlights on my '71 F250 were intermittently blinking on and off. Wiring is all factory stock and looked to be in good condition. I noticed one of my headlights was a halogen, so thought maybe too much current draw. I bought the headlight relay assembly from LMC truck, installed it and problem solved. I think I was into the wiring harness by $40 with shipping, but well worth the cost.

[Canonman67]

Thanks Chuck, I've bought a wiring harness for the truck and will be adding heavier gauge wire and relays when I install it. I found some cheap 7" Volkswagon housings and will be running 100/55w H4s. I'm looking forward to having the truck back on the road.

[kmcypress]

Might be a dumb question but i currently have a painless kit and am planning on putting a 3g alternator in. That being said, will a wiring kit necessary if I plan on putting H4 bulbs in? Wouldn't the higher amperage output from the newer alternator allow them to run just fine?

I'm currently getting that intermittently blinking on an off w/ stock lights and stock alternator.


not sure :/

[bluef250]

No dumb questions, a few answers maybe. Yes, I see a relay in your future. The weak link is the switch which is the cause of much consternation. Secondly a heavier gauge wire will have less resistance so you will benefit with more power for the lights.

[cep62]

Is there any reason the relay can't be in front of the dimmer switch so one relay can work high and low beam?

[HIO Silver]

To use a single relay, it would need to be a "cross-over relay" that switches between terminal 87 and 87a. Standard relays energize 87 and 87a at the same time.

Here are illustrative schematics of a cross-over relay: