headlight blows

hi-beam & flasher switches blows headlight 74 Commando

Are you saying that selecting high-beam or using the high-beam flasher wipes out the high beam filament?

Too much voltage headed to the wrong connection, or a dead short in line.

Lineslinger said:

Too much voltage headed to the wrong connection, or a dead short in line.

Click to expand...

Agree on too high voltage.
I'm not a sparkie but I believe a short will blow a fuse, not a bulb.

Ken in Ohio said:

hi-beam & flasher switches blows headlight 74 Commando

Click to expand...

A little more (any!) background information would be useful such as whether this is a recent problem (or are you a new owner)?
Does it only seem to happen when the engine is running and the engine is revved or also when not running, or both?
Which type of headlamp bulb (BPF, H4, other, Hi/Lo wattage)?

You need to put a muti meter on the headlight connection to see what's the voltage, if it's high, when turned on main beam, it could be a fault perpetrating in a couple of places.

Hi voltage isn't a symptom we often see on Nortons. Engine running? What type of charging system?
Vibration is a bulb killer too.

System voltage should be the same at any test point unless your harness has some high resistance (corroded) connections which usually lowers voltage to the "appliance" that is drawing from that corroded connection. Ohm's law equation shows the E=IR, electromotive force (in volts) equals current flow (in amps) times resistance (in ohms). System voltage (E) above 3000 RPM should measure in the range of 14.1 to 14.3. If your measurement shows no more than 14.3 volts (E) then your charging system should be good.

If you have recently replaced the headlight bulb with a halogen replacement, and a 3 conductor plug you may have inadvertently got the connections in the wrong position(s) as halogen's use a common ground on one of the pins, hi & low on the other two. If you mix them up you can create at least 6 combinations only one of which is correct.

If you Google the pin layout of the bulb you have you'll see what should be connected to what pin. If you are using the standard bulb format make sure that you have a good ground (earth) path.

Still like to know if you are blowing one or both filaments??

Best.

Most of the suggested remedies are good for fixing causes that are not all that likely to suddenly blow a head bulb, but not cause any other symptoms.

What type of bulb? To blow a standard 12V incandescent headlight bulb you would have to have probably 16+ volts going to it- even more to blow it instantly. The battery cannot supply such voltage. Only thing that could is a defective regulator that is not correctly limiting the alternator output (after converting to DC).

As noted, check the voltage at the bulb connections as you do whatever caused the bulb to blow - same engine RPM/switch position.

Lineslinger said:

Too much voltage headed to the wrong connection, or a dead short in line.

Click to expand...

Lab touched on it, read what he said.

Without seeing any photos, wiring diagrams or connecting techniques and not knowing the voltage rating of the headlamp bulb in use or If there is even a fuse in play...or how that curcuit is wired much less the rest of the system....or if 12V is active, my original answer is based on my deductive speculation, just like the rest of the replies offered.

Cause' the only solid information available is "hi-beam & flasher switches blows headlight".

Assumptions when dealing with electricity can get very exciting and expensive when assuming specifics.
I once watched a couple of dummies put 440V to an HMI bulb (Hydrargyrum medium-arc iodide lamp) rated for 220V...the result was exciting and explosive. The bulb alone cost $2,300. The dummies on my crew assumed they had the voltage to the lamp correct.
I didn't fire them but they did have to buy a new bulb.


Now, with some visual reference and well founded input from the OP assumptions change to observations, or did I miss the posting of some photographs?

Nobody yet mentioned the Zener diode or the rectifier. If you still have one. Otherwise the rectifier/regulator.
With age, a functioning Zener is now quite rare.

A multimeter will easily tell if the charging system is producing excessive voltage...

I don't know if a rectifier failure mode includes passing AC from the alternator - I don't think so but it's easy enough to check with the multimeter set to measure AC. Obviously, there is plenty of AC voltage available from the alternator to blow a headlight if it ends up in the circuit.

Is there a chance a 6v bulb has been fitted to a 12v Norton?