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Blowing out the lo beam

My friend’s classic PROJECT truck, 1954 GMC 1/2 ton, keeps blowing out the lo-beam. It has done this 3 times so far, both headlights at (nearly) the same instant. Hi-beams still work



The only time we witnessed it was when he was starting the engine after stalling. They flashed/flickered and poof, poof were gone. The other times… doing test laps around the block? restarting in the driveway/garage?



The engine and electrical are from a 1970 Nova (12V) It reads 12.25V engine off and 13.15 - 15.25v at idle - ~4500 RPM. The AMP do not read higher than 0.1 (maybe we need a new tester)

Unfortunately you can’t go by the readings unless you can happen to be watching the readings when the bulbs actually blow. You’re clearly getting a voltage spike. The culprit almost has to be the voltage regulator. In addition to a bridge rectifier (that turns AC into pulsating DC), they contain filters made with capacitors and resistors that make the output more of a continuous DC. Peaks are stored in the capacitors and released at a controlled rate through the resistors. The output becomes a smoothed out, lower average voltage rather than peaks and valleys.

I’m guessing the regulator. My guess is that an occasional voltage spike is not being filtered…

1954, weren’t most of the cars this era still on 6 volt electrical systems? Make sure the sealed beams you are using are for the 12 volt system you are using now.

Otherwise it seems you are getting a large power surge when you hit the starter. Some cars cut the power to the headlights when you crank the starter. Your old car apparantly isn’t wired that way and perhaps with old 6 volt systems it wasn’t necessary. Make sure to turn the headlights off, then crank the motor and turn the headlights back on when the motor is running.

There maybe a more permanent fix for the problem that someone else can recommend.

“1954, weren’t most of the cars this era still on 6 volt electrical systems? Make sure the sealed beams you are using are for the 12 volt system you are using now.”

You beat me to it, Uncle Turbo!
I am almost positive that this vehicle was manufactured with a 6-volt electrical system.
IIRC, 12-volt systems did not begin to become commonplace until 1957 or so.

So, if these are the original-spec sealed beams, the 12 volts being supplied to them would be the likely culprit.

Nice catch guys. I forgot about the old 6 volt systems.

The six volt issue sounds like the source of the problem, but if it turns out that is not it, then you should make sure the lamps are not getting more than usual vibration or heat.

It is NOW a 12V system

Sounds like the ground needs to be improved.

“It is NOW a 12V system” Yes, but are the lights 12V?

If the bulbs are also now 12 volt bulbs, then I go back to my original thought…my money is on the regulator.

Check to see if there is a AC voltage riding on the DC. If there is more than .1 volt AC while the engine is running then that is a problem. Also make sure the meter you use to measure the AC will block the DC voltage. Measure the battery volage while the engine is off using the AC mode of the meter. If the DC is blocked the meter will show a quick reading then drop to zero volts.

Update:
First off, thanks everyone for your ideas and suggestions
We checked the Volt Reg and yes, the contacts were dirty. Cleaned them and now it’s putting out a steady 10.25 - 10.30 volts regardless of the RPM (@ the headlight). It’s not 12, but it’s also not 15+V at higher revs. The Alternator [i]IS[/i] putting out 12V.

We have checked and can’t find any voltage leaks or shorts anywhere on the truck. We will check for AC voltage on the DC circuit on OUR next weekend

You still have not told us if those headlights are 6-volt or 12-volt sealed beams.

I’m 99.999999% sure that they are 12V. we had the high beams hooked up to the lo beam circuit when it was putting out 15+V. If they were 6V they would have blown the hi beam, right?

If you are burning out your low beams it seems improbable that you would not also burn out your tail and dashboard lamps as they were not mentioned.

If you have a relay type voltage regulator, the voltage regulating relay coil must have system power applied to it with ignition on or it will not partially pull in to regulate current to the alternator rotating field magnet to limit alternator voltage output. Make sure that your voltage regulator connections and wiring are sound.

Allow your system a little time to equalize its charge before deciding on a final regulating voltage. I don’t know how accurate your voltmeter is.

They wouldn’t necessarily burn out the high beam which may, by chance, be a little sturdier. But 6 volt bulbs on a 12 volt system will draw twice the current they would draw at 6 volts. I would think that might blow the headlight fuse … if there is one. Also, they would be pretty bright. Enough so you’d notice I’d think.

Since you need to replace the bulbs anyway, make sure the replacements are 12 volt.

If the multimeter shows ~26 V AC (engine off & on) does that mean the meter doesn’t block DC? Or does it indicate a more serious issue?

Also, how difficult is is to convert to an alternator with an internal voltage regulator?