Okay, this is with an 87 mustang GT, but I don't know of any more knowledgable group of people out there. The headlights are remarkably dim, even the highbeams barely light the road. They're not out of adjustment, they just, well... suck. The lenses are a little cloudy, but I don't think that's the main reason for lights being so dim. Also, the passenger side light blows rather frequently, and I mean explode, not just a broken filament. I would've thought that's got something to do with moisture getting inside the lens, but the driver's side has some moisture in it as well. No, I haven't been touching the bulb. Anyways, I hooked up my little digital multimeter to the wires and here's what I got. All numbers are in VDC, and the first number is with the highbeams off, the second is with the highbeams on.

Passenger side:

Green wire: 180mV / 14V
Red wire: 14.2V / ????
Black wire: 3mV / 3mV

Driver's side:

Green wire: 187mV / 14V
Red wire: 14.23V / ????
Black wire: 3mV / 3mV


Now, for a little more on the numbers. I plugged the positive lead of the multimeter into the connector that plugs into the headlight. Then I took the negative lead of the multimeter and connected that with a ground on the body of the car. As for the ????'s, well... the reading started out as 0, climbed to about 4 volts (in about 140mV steps), then said that there wasn't a connection at all, then started over. Now, for the battery, I got a reading of 13.35V-13.36V with the engine off, and 14.35V-14.36V with the car on.

Most of the numbers I understand: The red wire powers the lights normally, the green wire powers the highbeams, and the black wire is the ground. The 187mV is just some noise in the line, but I really don't understand the red wire's readings when the highbeams are on. I would expect some variations in the readings in any electrical system, but not like that. It seems like enough voltage is making it to the lights, too.

We're using high-intensity halogen bulbs in the car. I've heard that with some older cars, the alternator might not put out enough current to properly light these, so they may actually be dimmer than regular lights. Which brings me to another point, how many amps should be going through those wires? I forgot to measure that while I was outside a minute ago. I'll update this thread with amp readings in a few minutes.

 

Well with the voltage reaching the bulbs as close to alternator voltage as it is, there's no noticeable drop in the wires, so you don't have the symptoms of excessive current.

Check how they're aimed... it's amazing sometimes how much difference that can make.