I just replaced the head lights not that long ago on my 1966 F100, after losing the low beams. I replaced what PO had in there with stock replacements from Auto Zone Auto Parts. Lost them again tonight. Low beams came on briefly then they went out. High beams still work. Seems like they just don't last that long. Any thoughts on what might be causing this? Thanks.

 

Did you check them to see if they were actually burned out? I have never seen both headlights burn out at the same time. Sounds to me like you have a wiring or dimmer switch problem.

 

You can test the bulbs using your multimeter on the ohms setting. Ground is the horizonal connector so black meter lead goes to it - although not critical for testing ohms. The vertical connectors are low and high beam. Touching your red meter lead to one and then the other should show a low ohm reading of between about 1.0 - 3.0 ohms. The current from the meter battery going through the filament will heat up the filament slightly and change the ohms value for a few seconds. As long as they are low ohms and not infinite ohms they should be good. The values might vary slightly between one connector and the other. That resistance combined with the current from the truck battery when turned on causes the filament to glow which is the light that shines.





Likewise, the floor dimmer switch can be tested. The horizontal connector is 'power in' in this case and not ground but it's the common connector so black can go to that one. Then red to one or the other. It should show very low ohms on this like 0.1 to 0.3 on whichever side is active. Once you see the low reading then push the button on top of the dimmer and it should go from very low ohms to very high ohms - as in infinite ohms. Do that for each side and you may detect a problem, like if one side doesn't go low ever or if one side fluctuates constantly that is a bad sign I've seen as well.

 

I checked the wiring at the headlight connectors and the dimmer switch with a test light. Had power where I was supposed to have power. Power at low beam terminal when low beams on, no power when they are off. Same with the high beams. Checked the sealed beams with ohm meter, no reading, on either terminal, meter stays on OL and does not emit tone. Checked dimmer switch with ohm meter, low reading of .0 on both terminals. Replaced the sealed beams with new beams from O'reilly Auto Parts. Head lights working correctly again.

 

A bad connection may show proper voltage with no load. Then you turn on the lights and as the connection heats up from resistance it can fail after a while. Went thru this on my 99 f800. Bad wire connector on the dimmer switch. It would test good, but 10 miles down the road and high beams would go out.

 

It could be loose rivets inside of the dimmer switch, corrosion inside, cheap components used by the manufacturer, dielectric grease breaking down. It could be corroded wiring or loose connections. If the low beams are actually burning out fairly quickly it might be that they are not getting enough voltage when under load. This is where checking with an ohm meter can possibly show a defect in the system.

 

Just for fun, a local repair shop guy famous for being the electrical wizard, told me when testing circuits he used a sealed beam headlight bulb with test probes attached instead of the normal test light. reason being, the headlight bulb draws more amps which may show a problem the normal test light would miss

 

Finally found some time to get back to looking at the headlight issue. Found 14.5 volts at battery with engine off, 17 volts at battery at idle. Replaced alternator and voltage regulator. Now have 14.4 volts at battery engine off and 15 volts at battery while idle. 12.9 volts at low beam connector plug and 12.3 volts at high beam connector plug. Have not put new sealed beam lights in it yet. What do you think? Thank You.

 

17 volts at idle, certainly more driving down the road would certainly cook headlight bulbs. You should check the acid level in the battery, it may have cooked low too. Usually at severe overcharge it will smell like rotten eggs, or sulfur when a battery is cooking its juice out

 

How do you check the electrolyte level in a modern sealed battery?

I also smell,like rotten eggs occasionally, but it’s not from overcharging. It’s my age. YMMV.

 

Batteries still have caps, you need to peel off the label in most cases to see the " no user serviceable parts" caps

 

I should add, that yes, the modern glass mat batteries are sealed, no caps. The plain old lead acid batteries still have caps, they just hide them so you buy a new battery.

I have a great sense of smell for over charging batteries from owning a 1989 diesel dodge. I've had it for about 15 years, put about 150k on it, 325k showing on the clock now. Love the cummins...the dodge not so much. In fact, i could say dodge should have marched all their electrical engineers out the back door and shot them, but I won't let my personnel feelings be known.

The old dodge used to burn out a voltage regulator every 6 months. It would run at 15.5 volts all the time, then you'd smell the rotten eggs...that means full charge, maybe 17volts or so, buy another regulator. I soon got wise and bought the life time guarantee units, and kept a spare in the glove box. I couldn't nail down the exact problem, and since the amp gauge never did move, I put in a 24 SI Delco one wire alternator. My choice since these are common on cummins applications for medium trucks and tractors and all sorts of equipment, so I should be able to buy one anywhere on the planet when traveling. At this time, I also installed a voltmeter. While testing the new system I finally learned the cause of it all...years of problems...was main power going into the fuse panel showed battery voltage, but all circuits going out showed 1 volt less. So....the regulator always thought the battery was 1 volt low and kept trying to push it up to 15.5 instead of 14.5 where most systems should be at full charge. Problem solved? Well, sorta. That was 5 years ago and now For the last year or so I've had switches going bad. heater, dimmer, climate control, etc, all have burnt connections from the years of over charging. So, someday when I grow up I will have a 64 F350 with a cummins motor and the dodge can go away!

 

Take the battle to your local FLAPS they can test it there.

 

What’s a FLAPS?

I’ve never peeled open a sealed battery. Usually, I get about 6-7 years use out of one, and I replace them before they cause me grief, but every once in awhile, one fails before I think of changing it. Rusty, you must pick up a scent like a good hunter, never have smelled what you’re talking about.

 

Favorite Local Auto Parts Store.... Duh... How long you been a member here?