Headlight burns out!

[Jon37]

I replaced a burned out headlight bulb two days ago, only to have the "new" bulb burn out within a day. It could be a problem with the old-stock replacement bulb of course, but I wonder if anyone can suggest other possibilities that I might look for, when investigating? Bad socket? Grounded wire? Not sure what "external" things might cause a bulb to go bad.

I have a 6-volt positive ground system with the old prefocus (3-pin flange) bulbs. I installed a dual relay years ago, even grounded each socket to the frame by soldering a separate wire to it.

Both filaments were burned out on the previous bulb; only one is burned out (so far) on the "new" bulb. The inside of the glass (in both new and old bulbs) has a grey fog on it.

A couple years ago I'd installed two new 6-volt halogen bulbs and they worked fine. But in June of 2007, when I turned the light switch on, both bulbs (high and low beams) blew out simultaneously. At that time I suspected some sort of high-voltage surge so replaced the voltage regulator, and put the old style tungsten lamps back in the lights. They worked just fine for the rest of the season, but this spring the tungsten bulb blew, followed (a day later) by its replacement! If this were some sort of voltage surge I'd have expected both tungsten bulbs to blow, just as the halogen bulbs did, so I'm thinking the problem might be in the wiring.

Any suggestions would be appreciated!

[ply33]

Not sure what kind of generator and regulation you have. If your car is from the 1930s or earlier then it could have third brush regulation of current. And if it is from the early 1930s or earlier then the only regulation probably has is third brush current regulation.

With a third brush system the generator attempts to put out relatively constant current. An external voltage regulation stage could bring that down some but basically you still have the generator attempting to be a constant current device. Unless the connection between the generator and the battery is good (the battery acts as a reservoir to store charge) this means that the system voltage will run high. (Voltage = current * resistance, so high resistance to the battery will mean higher voltage out of the generator assuming constant current.)

So if you are running a third brush generator, verify that your connections and wiring between the generator and battery are good.

If you have external regulation you should also verify that the field wire between the generator and the regulator is good (not shorting to ground, etc.).

Edit: If you have a battery that has developed high internal resistance then this may also be a problem with a third brush regulated system.

[Guest imported_JPIndusi]

John:

ply33 gave some good advice. If the battery ever becomes disconnected from the system while the engine is running, the voltage put out by the generator will go up. The only thing I would add is to take several voltage readings at the headlight socket undere varying engine speeds. Most 6 volt systems should regulate well enough so that the voltage in the system never goes above about 7.4 volts.

Good Luck.

Joe, BCA 33493

[Jon37]

Thanks for both of those thoughts. I believe my generator is in fact a 3-brush system. It's an Autolite GDF4802 (for what it's worth); sorry I forgot to mention that.

[Guest Ken G]

As a postscript, my Rover has a three-terminal generator ("dynamo") and indeed the voltage goes sky-high if the battery is disconnected. I remember one journey in the 1950s when for some reason we had no battery, and my father briefly pressed the horn button; the Klaxon produced an extraordinarily high pitch! Thus any high resistance in the leads to/from the battery or in the battery itself will mean that the lights and anything else electrical will receive a higher voltage than intended (assuming the charging is on; on my car you have a choice of charge or no charge). Bulbs will not stand that.

Ken G, 1925 Rover 16/50 (San Francisco)

[Guest elmo39]

jon 37 before i started checking to see if the voltage was going up or down 3 brush gen or not i would check first to see if there was a short in the system, it is usualy the most common cause of bulb failures.

[Jon37]

A new twist:

A friend who's more "electrical" than I, suggests that a headlight relay actually shortens the life of headlights. That the contacts "chatter" or "bounce" slightly each time it's engaged. He suggested a solid state relay might be better than my old-style mechanical one (evidently there are some 6-volt electronic relays manufactured though they're not cheap).

I'm curious: has anyone been using a relay on their 6-volt headlights for a number of years and if so, have you notice your headlights seemed to be burning out rather rapidly?

[Owen_Dyneto]

When I was young and even more dumb than I sometimes am now, I decided to kill my 34 Packard, with the headlights on, buy revving the engine and then hitting the battery disconnect switch. As you guys have pointed out, the voltage when sky-high without the resistance of the battery in the circuit, and the lights got VERY bright for a second or two before every single one of them burned out, including a pair of the Mazda #3003 triple filament headlamp bulbs which, until they were reproduced recently, often went for several hundred dollars each.

[zacknolden]

My observation would be you probably damaged them when you are installing the bulbs. If you did, they probably wouldn't work at all. I'm pretty sure you have either too much juice, which could be caused by the wrong kind of fuse in the car, or a surge problem.

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[56Roadmaster]

Hi,

One of the things you have to do with the 3rd brush system is set it for the type of driving you do, if you use the head lights alot then you should set the generator to be putting out for that condition and drive with the lights on during the day if needs be to prevent overcharging the battery.

You mentioned though, that you replaced the regulator, are you sure it isn't a cut out only? The cutout would have only 1 wire in (from the generator) and one wire out. It is just a soleniod switch that opens when the battery gets to a certain level of charge and has no effect on the generator output.