Both Headlights Blown At the Same Time - Why??

Irish Reiver

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This afternoon I was helping a mate fit a replacement exhaust to his '96 80-Series. We did zero electrical work other than disconnect the O2's and fit a hall sensor to the output of the alternator. The battery was removed to give it a top up charge - he experiences laboured starting's in the mornings but wasn't sure if he had battery issues or a charging issue. The truck was idling for a good 20 minutes as we checked the exhaust fitment. I turned on the headlights to show him the difference in alternator output with a load on the system. The lights came on and then both went out.
I ploughed through the wiring diagram looking for a common point of failure. After a few minutes I tested the bulbs and both were failed. New bulbs fitted and all is good.

That said I am no believer in coincidences. What could possibly cause both bulbs to blow? The 15 amp fuses were fine. What am I missing?
 

Irish Reiver

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I thought about that but for both go out at the same time is a stretch, that said, its the only practical explanation so far.
 

Irish Reiver

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A hall sensor is passive so it won't (and hasn't since the new bulbs were installed) have any active impact on the system. The sensor was installed to show how much current was being outputted by the alternator- he was suspecting a faulty alternator as the cause of his sluggish starts.
 
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Just an idea, but maybe a voltage spike caused by a flakey fusible link? You would have disturbed that when you removed the battery. A compromised fusible link would also explain the charging problem.

In fact, as far as using a hall sensor goes, the FL-MAIN link is where you want to monitor. Monitoring at the alternator's output doesn't exactly tell you if the battery is charging, and it certainly doesn't tell you if it is discharging. Sure it's cool to have an ammeter, but measuring the voltage at the battery will tell you what you need to know.
 
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Most likely voltage output issue from the alternator. You charged the battery off vehicle. Then hooked things up again, started the vehicle, alternator had minimal load since the battery is charged, something hiccupped and the alternator/regulator increased voltage. And you did touch a lot of key wiring stuff as in the post above this one...

Most bulbs in a vehicle are relatively low wattage and aren't pushed hard since they are just indicators. They would less likely fail with the spike. The electronics is designed to handle at least 40-60V momentary spikes.

The headlights (low & high beams) are pushed hard and voltage much above their operating spec will be bad.

Incandescent bulbs have relatively low resistance when cold, they draw lots of current and go to incandescence and as they reach that level (filaments stinking hot) their resistance increases to limit the current to 'steady state' for a given voltage. Exceed their operating voltage and the current will increase higher and filament will get hotter (till it melts). Given 2 bulbs failed at the same time, has to be a voltage issue (too high).

This is also why incandescent bulbs fail when you first turn them on, common to happen in your home lights (prior to CFL and LEDs). Bulb works great, you turn off the lights, come back at some point, flip them on and you get a quick flash of light as the filament burns out. Filaments wear out over time and eventually that turn on surge pops them.

With halogen bulbs there's also an issue if you run them too cold (too low voltage). Halogen bulbs differ from their run of the mill incandescent brothers because the halogen gas works with the filament to create a cycle where the off-gassed filament material cycles back and is deposited on the filament versus sticking to the glass (bulb slowly turning blackish on the glass - that black is your filament material - filament gets thinner, filament pops). The halogen bulbs run very hot (hence you don't touch, skin oils etc, the halogen bulb glass when handling them). They run hot so that the filament metal burns off the glass and redeposits onto the filament - this can only happen if the bulb is run at the rated current (gets hot enough). If you run them cold, then they will fail sooner like their lesser incandescent brothers.

Yes, I must be bored right now :)


cheers,
george.
 

Irish Reiver

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Thanks @george_tlc This is a very comprehensive explanation. I agree on a voltage spike being the cause of the issue although figuring out what caused the spike might be harder. That said, this afternoon my buddy took his truck to Autozone and had them test the battery - it failed - voltage was ok @12.8v but something in their test triggered a "Bad Battery" output. The autozone guy wasn't able to add anything else to the diagnostics.
 

PIP

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I had both headlight bulbs go out same time on my 94. It was so strange I thought it was the switch, no way both could burn out same time. Troubleshooting quickly showed I had power to the bulbs. Swapped new bulbs and all it fine. Never had that happen on any other vehicle before.
 

smritte

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I've seen this a few times in the shop. What normally happens is they don't notice the passenger side go out, then when the driver side burns out, they notice it. On the alternators with the voltage regulator in the PCM, I've seen the voltage spike over 20 volts. If the lights are on, they get real bright before they go. Loose battery terminal on the ground side. Common on the Jeep TJ.
 
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I've seen this a few times in the shop. What normally happens is they don't notice the passenger side go out, then when the driver side burns out, they notice it. On the alternators with the voltage regulator in the PCM, I've seen the voltage spike over 20 volts. If the lights are on, they get real bright before they go. Loose battery terminal on the ground side. Common on the Jeep TJ.
Yes, Chrysler products have the voltage regulator in the PCM and yes, when they go out or there is a bad wiring harness it can and will blow both headlights. In my opinion, this is a poor design.

The charging system on the LC has the regulator inside the alternator. Any place you have a loose and or corroded connection, you can induce a voltage spike in the system.

Check all wiring connections touched. Consider replacing the alternator. A bad plate in the battery could also cause a spike.
 

smritte

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The charging system on the LC has the regulator inside the alternator. Any place you have a loose and or corroded connection, you can induce a voltage spike in the system.
Yea, was just referring to one of the systems I've seen that on. Unless the OP is driving around, having popping sounds from his stereo and lights going bright. I wouldn't worry. I would put the odds in favor of a coincidence. Ive spent too many years in a shop to say they don't happen.

I did quite a few alternator welding systems on TJ's. Having the regulator in the PCM made it easy. The guys not tightening the battery terminals is what I saw cause the issue. Loud popping from stereo ending with headlights doing super nova. (ask me how I know this)
In my opinion, the spikes you would get in the Cruiser 'system should be fast enough the lights wont care. I would be more concerned with the PCM but, Toyota did a good job of spike protecting the PCM so, that has a lesser chance of being damaged. If this were an 80's vehicle, that would be a diffrent story.
 

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