Since we bought our vintage RV 4 years ago, we've suffered through the difficulties of dim headlights, flaming hot light switches, melted connectors, and headlights that decide to go out at the most inopportune times... Until now.
I'm sure lots of people have heard about the relay cure for dim headlights, but there doesn't seem to be many posts here on the forums regarding the fix... so I figured I'd throw this out there for anyone interested.
In short, on many old rigs with sealed beam headlights, the factory wiring harness just isn't heavy enough, and the voltage drop between the battery, through the headlight switch, and on to the lights is just simply too much. On our rig, I measured around 14.2 volts at the alternator... but with the headlights on, the voltage AT THE HEADLIGHT was only around 11 volts. This voltage drop is enough to effectively cut the light output of each bulb by 1/2.
The relay fix is pretty simple. Wire the headlights direct to a set of relays... and use the existing headlight wiring to control those relays. Voltage for the headlights is now supplied through the relays direct from the battery, so you bypass all of the thin 14 or 16 gage wiring and corroded connectors in the dash which simply cannot transfer the necessary current. That wiring now only has to carry the parking lights and the coils in the relays.
The result... is brighter lights, cool light switches, and reliable headlights. I won't go into a ton of detail on our install... Instead, I'll just post the link to the guy we purchased the relay kit from:
Daniel Stern Lighting
This guy loves talking about vehicle lights... he is very knowledgeable, and his website is loaded with helpful information. He doesn't have a "shopping cart" on his website, so he does all of his business kind of old fashioned through email.
Being familiar with relays and wiring, I could have easily laid this all out myself without ordering a kit from him, but he did offer some items that I really couldn't seem to source anywhere... namely the new ceramic sockets for the sealed beam bulbs with which to start building your wiring harness between the bulbs and the relays.
At any rate... the relay kit was a success, and we now have 14 volts at the headlights. According to the chart on Daniel Stern's website, 11 volts to 14 volts will more than double the lumen output of each bulb. I don't have a light meter with which to measure the actual output, but I can tell you the difference is astounding.
The picture below shows where I tied the relays into the chassis battery... it's a terminal on the firewall that is fed directly from the battery with a 4 gage wire with almost zero voltage drop. It's nothing fancy... but it certainly is effective. The wires to the headlights were replaced with all 12awg... and I soldered all of the terminals in the connectors. I know there is some debate as to whether a crimped connection is better than a soldered one, but because of the style crimps on these connectors I opted to crimp and solder both. The three relays are to control the 3 different filaments across the 1A1 and 2A1 headlight assemblies.
Overall, I'm very pleased with the outcome. It's a cheap upgrade, and now not only can I see at night, but I don't worry about the dash catching fire from the super heated light switch.