View Full Version : Headlight Trouble

11-15-2016, 04:02 PM
Friends, I have a strange problem - one of my headlights burns brighter than the other; so far I have done the following:

Changed the headlights around to make sure the problem is not the bulb itself.
Replaced the original, corroded three prong headlight plugs
checked the grounds on both sides - they are clean, solid and tight.
checked the terminal junction where the wires from the dimmer and the head light wires join - all clean, all good.

Driver's side is the dim side.

Any advice my friends....

11-16-2016, 07:48 AM
Chasing a problem like this is sometimes called a "bug hunt". You're going to poke here and there until you find the lil' critter!

I am assuming that you have the three post terminal strip for the headlight junction. You probably should have a Harbor Freight cheapie voltmeter to chase this around.

One of the terminals is ground. That wire eventually gets connected to the chassis and frame somewhere. Since only one headlight is dim - it's probably not a ground problem on the chassis side since that would dim both lights.

Now, is the light dimmer on both high and low beam? If so, you are looking for something common to both states. The ground wire from the three-prong headlight plug (which you replaced) is the most likely culprit. With the headlights ON, take your meter and check the voltage from the screw terminal of the junction strip (not the nut) to the ground screw and to chassis. This should give you basic battery voltage.

Now, look at the voltage at the three terminals of the headlight plug (at the base of the light) with the lamp in and ON. The voltage at the HIGH beam and LOW beam terminal, measured to chassis or the ground post on the terminal strip should be about the same as the battery voltage.

Now, look at the voltage on the ground terminal of the lamp, referenced to chassis or the ground post on the terminal strip. That voltage should be ZERO, or very close. If not, you've found the problem. Another approach, measure the resistance from point A to point B for all three wires. It should be the reading you get with the meter leads shorted together on the Rx1 scale.

Highly likely - the crimp on terminal that sends ground from the terminal strip to the driver's side headlight is loose or corroded.

Cut off the old terminal and crimp on a new one. If that doesn't fix it - check the wire very carefully for a cut or nick that let it get wet and corrode internally. Replace it if needed.

On the '48 truck, someone forgot to put the little fiber insulator under the power post for the horn. The horn is the first thing that gets power from the battery. When you tried to put the battery terminal on, you got a lot of sparks instantly. This isn't something you leave connected and see where it goes. It took a while to figure out that the horn was making a very good connection to ground. Only the resistance of the wire (a #12 if I recall) was limiting current - and it did get hot.

Let us know what you find.

11-16-2016, 02:55 PM
Thanks Larry, I'll give it a shot. I do know that all the insulation is worn off the three post terminal, I'm going to order a new one while I'm doing this.

11-18-2016, 10:47 AM
Well, the problem was definitely the ground on the dim light. It is the only can I had taken off to treat some rust underneath - in the process of taking the can out, I had to grind off a couple of screws and did not replace the screws when I put it back on - this probably caused the problem. I am going to re-attach the can properly when I have time, but for now I'm just going to run a wire to the other headlight can to fix the problem.

I'm also going to replace the other wiring as you suggested - purchased it all this week.

Thanks LarrBeard

11-18-2016, 04:00 PM
Even blind squirrels find acorns on occasion!
We won this one... now if we can just figure out the brakes...

01-11-2018, 03:51 AM
First of all to check whether the lamp sleeve depreciation, and then see the type of the big bulb whether is the same. At last,confirm all is right,that the route is caused by iron.