round logo
Nat'l POC logo


Getting a full 6 volts to your 6-volt headlights

by Arn Landvoigt

Earlier I wrote that 6-volt headlights would not shine brightly enough if powered by less than 6 volts.

I mentioned the importance of having battery cables of adequate size, #1 gauge at a minimum. However, the next step in getting those headlights working as brightly as possible is to assure that none (or at least very little) of the 6 volts originating at the battery is lost on the way to the headlight bulbs, or in the ground return path.

If the earlier test of measuring the voltage at the high beam filaments of your headlight showed much less than the full 6 volts, then you need to go through the entire electrical path between the battery and headlight to clean up the losses. If there is less than an ideal connection at any point, than part of that voltage will be lost. So, this drill will be to check EVERY connection between the battery and the headlight bulb and clean them to minimize these losses.

There are multiple connections on the 6 volt’s path from the battery and back through the ground path. In addition to connections between wires, I have seen the ground path run from a fender, where the light is mounted, back to the body where it is bolted together. Small losses at each connection can add up to a significant loss when added up. You must go after and minimize each loss individually.

Step one is to get a digital multimeter if you do not own one already. Harbor Freight lists a basic unit for only $6.99. Turn it on and set it to the 2-volt DC scale. With your car running, set at a fast idle and with the headlights on high beam, start looking for voltage losses starting at the battery terminal. (Note this is the set up for all measurements taken throughout this procedure.)Dig one of the probes into the center of the battery terminal and place the other onto the shell of the cable clamp on the end of the cable at that terminal. Are you measuring a small voltage (several tenths of a volt) between the battery post and the cable end? If so, turn off the car, disconnect the cable end from the battery and clean both the battery post and the cable end clamp. Harbor Freight has a battery terminal cleaner for $2.99.

After getting both terminals/cable-clamps on the battery cleaned up and dropping NO or VERY LITTLE voltage, next dig one lead from the digital meter into the center of the battery ground post and the other into the block or frame where the other end of the ground cable connects. Again, if there is more than a tenth of a volt of so drop, disconnect the chassis/block end of the ground cable and really clean up the connection. Clean both the cable end and the surface where it bolts to. You might even place a star washer between the two when you reassemble them.

From here on you need to follow the path from the headlight switch through all the cable connections to the headlight bulb filament. Measure the loss across each connection and clean, tighten or whatever is needed. And do not forget to check the ground path back from the headlight. In the end you should be able to get all but a few tenths of a volt to the high beam filaments of your headlight bulbs. And because you are cleaning up the supply of 6 volts to the car’s wiring, other things on your vintage Plymouth (or whatever) should work better.