Fluctuating headlights

Loose wires, bad ground.

Did the fluctuation change with the rpm?

I'd pull the headlight out - if you have the old fashioned sealed beams (single piece unit with non replaceable bulbs) trash them and buy new Hella H4 Vision Plus head lamps with replaceable bulbs - forget what they run maybe $35 each.  Then take apart and clean the 3 wire plug that goes to the bulbs - they are probably green with corrosion.  Apply some di-electric grease and reassemble.  The H4s are much brighter and use same wattage as old incandescent lights. Notice the euro spec ECE ones are less expensive - unless your inspection folks are really anal I'd go with the cheaper ECE ones.

I don't disagree with Greg that a headlight upgrade is a good idea.  However, I doubt that the headlights themselves are the problem, especially since both seem to suffer the same symptoms.  Think of the lights as a test meter.  They get bright, then dull, likely because they are responding to variable voltage input.  After checking the usual suspects (corrosion, fittings, cables, wires, etc) the regulator would be a likely place to check.  I would suggest that you cure the problem before replacing the lights.

Ron O...I'm not sure if it does or not; but sort of afraid at what you're going to tell me IF the intensity fluctuation changes with rpm!

Stand by...I'll check it out after dark (in about 3 1/2 hours)

Carl.....   Varying with engine speed is probably the easiest  to cure problem...  Most likely a loose belt slipping causing the alternator output to vary...  Another problem may be a weak battery (dead cell) causing the alternator out put to vary with increased load....  (PEP BOYS battery check should spot that.)

The first simple test is to measure alternator output voltage ( should be between 13.2 and 15 VDC. ) Then watch it as you turn on the head lights...  A drop of more than 1/2 a volt at moderate RPM, may indicate regulator damage. ( again PEP BOYS)...

Just my .02...

I agree with Jim - this is not a headlight problem.

Seen this a lot back in the school bus days where the lights (both external and internal) would flutter about 2 times per second.  It is usually RPM sensitive but you have to watch closely to see the fluctuation change.  

I would check these things in this order (from least likely to most likely):

1.  Check the fan belt tension to the alternator.  Also, take a good look at the current belt to see if, on the "V" slopes it has become burnished and shiny.  If it is and is really shiny, it may be slipping and correcting tension might not help much.  If the V slopes are nicely scuffed (flat, not glossy), then you're all set - just check the tension and move on.

2.  IF it has a separate voltage regulator it may be acting up.  This is best checked by a good automotive electrical shop (even Autozone, but there you're subject to whatever ability the tech that you get has).  It could be a corroded connection to the regulator, and the test tech should know whether to clean the contacts or not.

3.  If it has an integrated voltage regulator (inside of the alternator) then, again, it is best checked by a competent Auto Electric shop.  The biggest draw to an electrical shop is that they can apply a dynamic load (Think big, honkin' resistor) as part of the test to load down the system and make the alternator circuitry work hard and THAT will quickly point out the problem.

So, what do I think it is?  I think one of diodes inside of the alternator that convert AC to DC (and also provide the voltage regulation) is going soft.  At least that's what I was usually told when we had a bus with fluctuating lights.  Drop off the alternator in the morning (some shops will even remove/replace it there), pick it the next day and you're back in business 

BUT FIRST:  Get it checked out at that electrical service shop.

Oh, and if you don't do anything, nothing awful will happen.  The battery will still charge, the lights will still work, but you will always notice the fluctuating intensity when the headlights are on - this could go on for many months.  Irritating, but no damage done and if the diode(s) fully fail, the "ALT" light will come on full all the time and you'll need the service anyway.

Ron O, engine rpm doesn't effect the 'fluctuation'

But thanks to the knowledgeable/experienced responses (that's what I truly appreciate about this site) I'm now not in fear of a catastrophic electrical meltdown!!! In fact this fluctuation is sort of neat in that oncoming traffic will think I'm winking and flirting with it.

Greg, I like your idea of replacing sealed beam headlights. That's been added to my 'non-priority list'...What's an euro spec ECE?

Thanks everyone. I'll sleep better tonight. 

Hello Carl, get on the Pelican site and search for lighting issue on the board, you'll find info on headlight issues with the older cars.

I have a 77 S car and went through headlight/lighting problems with the headlight switch and combo switch, I ended up adding relays and upgraded lighting at the same time. A lot of times the previous owners install up graded lighting and run the car as is and when it comes time to sell they keep the upgraded stuff for another car or sell separate. But in the meantime the wiring has been taxed from the previous setup.

Its not a big deal to sort and to be expected after 38 years.

OK Carl.....   So its NOT rpm sensitive....   I've only experienced one problem of a similar nature...  Many years ago, I aquired a '67 Mustang that had more than a few miles on it....  It had a similar problem, although it's lights actually blinked off rhythmically....  Traced the problem to the previous owner's fog light installation being wired directly to the headlight circuit...  When they were turned on, it overloaded the circuit breaker in the headlight circuit which would drop out and almost immediately reconnect....   You might have someone check out the breaker, if used...

Carl, time to do some electrical work.