Back in the day, most automobiles fed full power, drawn directly from the charging system, through a headlight switch, a mechanical dimmer switch, then back through the firewall, and finally to the headlights.
With this setup there was a significant voltage loss by the time power finally made it to the headlights — which is why your classic car’s headlights can leave a lot to be desired at night.
Thankfully, there is a simple solution to get full system voltage to the headlights — All it takes is a pair of 30-amp relays, one for the low beams, and another for the high beams.
This works because a relay, mounted near the headlights, will pull power directly without losing voltage through the headlight switch.
If you want to see the difference before you dive in, simply measure the voltage at the headlights and compare that to the voltage at the alternator. You’ll likely be surprised how little voltage makes it to the headlights in a stock wiring system.
How To Wire Headlight Relays
This video demonstrates how to wire headlight relays…
Relay Upgrade Bonus: Your headlight switch’s life will be greatly extended because the heavy current is now channeled through the relay — the headlight switch will now only trigger the relay with a very low current load.