Burned Out Headlight Bulb

[Duckwing]

I took a short spin late yesterday. Upon return, my wife said one headlight wasn't burning. High beam OK but nothing on DIM. I disconnected the plug and verified power present. Checked resistance looking into the head lamp socket. One element was open circuit. Removed the bulb and verified by measurement directly on the bulb that one element was open.

I had two spare bulbs. I checked each and found that one was OK. The other was open circuit on one element so I installed the good bulb. Worked fine on DIM. Switched to ON. Worked fine. Switched back to DIM. Now nothing. Now the "good" bulb is shot. The other headlight works fine in both switch positions.

So what do you think is going on? Bad luck with bulbs or some undetected issue?

[JohnH]

If the bulbs are burning out because of excess voltage, the usual cause of that is a bad connection somewhere between the generator cut out and the battery positive terminal, or a bad connection between the chassis and negative battery terminal.
What happens is that if there's no battery to load down the generator, the generator voltage increases to a destructive level, and anything switched on while the engine is running will be severely overloaded.
If the car has a starter motor and this is working reliably, that would eliminate the battery ground strap, so I'd be looking at the wiring between the cut-out and the battery positive. Bad ammeter connections are a likely candidate.

[varmint]

Get a reading of the voltage with a VOM.

[TXGOAT2]

Check system voltage as previously advised with engine at 1,000 rpm or so. It should not exceed 7-8 volts. If voltage is OK, I'd suspect that your good bulb was not so very good. If your car has normal system voltage and keeps burning out a particular bulb, check for excess mechanical vibration at that bulb's location and also look for any poor connection that could cause the bulb to flicker or flash off and on while driving. An intermittent connection will shorten a bulb's life dramatically.

[Duckwing]

The engine was not running when the replacement bulb burned out - just sitting in the garage. I measured the voltage at the headlight plug contact to be 5.90 volts. The other headlight and two tail lights were loading the battery at the time. I started the engine this morning but my digital VOM goes nuts. I will run up to WalMart and get an analog meter later today. The starter works fine so believe the ground connection to be OK. When I went for the spin last night, it was not dark enough for me to notice any headlight flicker. I would not have known the one headlight was out without my wife’s input. Just thought it strange that one of my new spare bulbs had an open filament and the second new bulb checked ok but blew the first time I applied power. The two replacements had different filament windings. In one, the two filaments are “bent” and look the same. The other had one straight filament and one bent one. I have not read any reports of faulty bulbs. I will bust the glass on the bulbs today and see where the fault is located. I can see no opens in the filaments. The last bulb did have a “flash over” spot on the glass after it blew. High beam works ok on all three bulbs.

[Duckwing]

Well, WalMart has been taken over by technology. They no longer sell the analog VOM I purchased a few years ago. I have two at the office so if I don’t find one at the auto parts store I will have to drive over to the office.

[TXGOAT2]

I have had new bulbs fail from being dropped. While the glass did not break, apparently the filament did. Bulbs often fail when switched on. They get a surge of current due to the low resistance of the cold filament, and they get a thermal shock at the same time, as the filament temperature jumps from ambient to white hot in a second or less. The lo beam filament is also more delicate than the hi beam, assuming they are different wattage. The flashover spot suggests overvoltage, but it could also indicate a gassy bulb. A 6 volt battery is incapable of supplying over about 6.5 volts, as far as I know, and most "6 volt" automotive bulbs are supposed to be good for 7 to 8 volts. Modern 12 volt systems typically run at 13 to 14.5 volts with the eggine running, and I'd expect a 6 volt system to run at 7 volts or a little more with the engine running above idle speed under most conditions. Model Ts lack an automatic voltage/current regulator, so they might typically see a wider voltage range and a higher peak voltage than newer vehicles with a 2 element or 3 element voltage/current regulator.

[Duckwing]

I still plan to check the voltage with the engine running but wasn't when the last bulb blew. The bulbs were probably at least 10 years old so maybe they just got tired. Thanks for the inputs.

[George Mills]

Often, old bulbs had the glass sealed to the base with shellac…twisting them in decades later, shellac breaks, air seeps in, filament lasts about as long as a match.

I always trickle a new seam of super-glue around the base on old bulbs.

New bulbs should stay sealed.

[Duckwing]

Could be the cause. Makes sense.

[Art M]

I have had similar trouble with old headlight bulbs burning out prematurely. Never knew why. Now I will seal the base.
Art Mirtes

[Duckwing]

Update - replacement bulbs arrived today. After installed, no light at all. Bulb checks ok with ohm meter. Oops, forgot to reconnect the cable. Plugged up cable now all is good.