My truck seems to burn out headlights quick

I have a 1996 Toyota Tacoma 2WD truck ( basically the no-frills pizza delivery model) It has 230,000 miles and so far has run great. But it started doing something weird lately. About a year ago one of the headlights went out. It had lasted 12 years so I figured its time had come. I replaced both of them ( because you’re supposed to do that) and thought nothing of it.

two weeks ago, one of them burned out. I thought that was weird seeing as how it was only a year old. But I replaced it thinking it was a fluke. A week later the OTHER one burned out. I replaced that one too.

So either I just have some incredibly bad luck with headlights or they are getting too much power. They don’t seem any brighter than usual and I took the alternator out and tested it and it was showing factory spec output voltage. About 14 volts.

Nothing else is burning out either. I’m kind of scratching my head. Should I test the output voltage going to the headlights themselves? If so, what’s an acceptable voltage for headlights? I assume 12?

You didn’t by chance handle the new replacement lamps with your bare hands/fingers?


No because these are basically " old school" sealed beam lights , pre plastic cover era.

When you changed them both, but one was still working, did you save the good one? Try re-installing the good one again.

Who told you that you are supposed to change out a good headlamp just because the other burned out? Yes, I would buy two so one was on hand, but I would never change out a good bulb when it wasn’t necessary. Besides, last time I changed a headlight, they were expensive enough that I didn’t want to spend the money on two when I only needed one.

Regarding your problem, I think it is possible you replaced high quality sealed beam headlights that came with your truck with el cheapo not-so-sealed beam headlights, and water is getting in.

Headlights tend to lose brightness over time so when you replace headlights, you should replace both because otherwise one will be dim, the other bright. Doesn’t bother me on my other car, a 55’ Mercury, but I drive my truck a lot at night. The lights I replaced were your typical Sylvania lights. As mentioned, these are totally sealed beams with a sealed bulb inside. One of the old lights actually had a small hole in it from a rock and it worked for years like this.

Its sort of a mystery.

Replacement Sealed Beams are (sadly) mostly made in China, regardless of brand name. Increased vibration as your old truck loosens up will shorten bulb life…

Well… despite the age she still runs and rides pretty good. What’s funny about Chinese parts is that I use a lot of aftermarket parts and the water pump, clutch, fan clutch, cap and rotor, and flywheel on my truck are all Chinese. I replaced these parts at one time or another. The flywheel was $60 versus paying more to have the original resurfaced. So far none of the Chinese parts have given me problems. I figure by the time this truck bites the dust 50% of it will be Chinese made.

It may be revealing that your other lamps are not burning out. Partially defective lamps can be quietly released into the supply system and used up with few or no consequences. Try another brand.

The old working headlight was dimmer than the new one? I suppose that might make it worth it. Still, I would have kept the old working one as a spare. I bet you now wish you had done just that. If you had, you could put it in just to confirm the problem isn’t the replacement headlights instead of speculating about the quality of the replacements.

Honestly, you seem to be ruling out people’s suggestions based on assumptions rather than real diagnostic tools. I suggest you try putting in the old working headlight if you kept it. If you didn’t keep it, try buying an OEM headlight at a Toyota dealership and see if it does any better.

You dont live on a real rough stretch of road do you?-Kevin

Just for fun, check for ac voltage from the alternator. AC voltage, plus dc voltage, can equal excessive voltage.

There are often three things that could be shortening your sealed beam bulb life:

1: Excessive vibration
2: Defective bulbs
3: High voltage - including spikes

I tend to doubt it’s due to vibration.

It could be due to defective or poorly manufactured bulbs. Have you tried another brand?

It’s often due to high voltage. Checking the steady state voltage is a necessary-but-not-sufficient test. You need to know if random voltage spikes are ever occurring. Your average DC voltmeter won’t show you any spikes you might be getting. The car battery acts as a huge sync and should absorb normal alternator spikes. Maybe you have some weak grounds causing spikes the battery can’t level out. Others may have ideas for how to test for this.

I’m curious. I don’t understand this reply. While the alternator does produce AC, it’s rectified to DC before it ever gets delivered to the output terminal of the alternator.

Help me understand how the initial “behind the scenes” AC voltage plays a role here?

Ideally, all the ac voltage output from the alternator is changed to dc; but, it’s known that, when defective, an alternator can output ac with the dc. Taken together, the ac and dc voltages can add up to excessive voltage.----- I did say that this check was, “just for fun”. If there is no ac voltage riding the dc voltage, it won’t be of further consideration, will it?

but, it’s known that, when defective, an alternator can output ac with the dc.

Good point. I never thought of that.
So it’s the checking for the presence of any AC voltage that’s key. I don’t believe any should exist. If any does, the alternator (diode circuitry) is suspect.


My vote is for buying a different brand of light. If you measure your voltage at the light plug (right behind the headlight) there is no reason you should see any higher voltage than you’re getting from your alternator output. 14.7 VDC is as high as you should see, course as soon as someone makes a blanket statement such as “you’ll never see higher than 14.7 volts”, then someone will come alone and rebute that claim. As far as the AC voltage getting added to the DC voltage, I’m not sure I buy that. If your alternator was to put out AC voltage, (say we had a leaking diode) I would think it might be in parallal to the DC voltage, so it still would not be higher than the DC voltage. I know diodes can let AC pass when they fail, but my experience is that when a diode fails, it simply cuts down on the current output of the alternator. I did quite a few experiments in school using bad diodes in the diode block. Truthfully I don’t know if the new alternators still use the old “diode blocks” as in the past with 22R alternators, or if they’ve come out with some new, high tech gizmo that is digital and all sealed up that does the same job as the old school diodes. I don’t see vibration as an issue. After all, if the sealed beam cannot stand vibration, it should never be sold to be installed on a vehicle. What vehicle is never going to be driven on bumpy roads? That leaves us to a suspecting an intermittently bad voltage regulator. I would tend to think if your voltage regulator (which should be inside your alternator) was to fail only off and on, you might see other bulbs burn out. But, if it fails where it only exceeds the maximum voltage by just a few volts (say it allows 16 volts to pass occasionally) maybe the headlights are more sensitive to excessive voltage than your parking, signal, dash, etc lights. At this point we’re getting out there, that would be a long shot type of thing to diagnose.

Vehicle alternators produce 3 phase AC power that is full wave rectified producing not smooth but pulsating DC power with peaks and valleys that are higher and lower than the normal regulated voltage. The peaks and valleys are damped by the battery. If you would disconnect your alternator and run it, you would normally get both DC and AC voltage readings. Yes, I can run an alternator without a battery; I have one driven by an electric motor that I use as a battery charger.