put on a model-A tail light and now no headlight?

[750bobber]

sooooo, i'm working on a 1979 gs750. i installed a '28 model-a tail light (only two wires) to replace the stock number (three wires). i figured out i had to attach the ground wire to the housing of the new light and everything was working fine... running light, brake light, headlight...everything. then i put the lens on the tailight and BOOM the fuse for the headlight and rear running light does nothing but instantly blow... which totally blows. i don't know if this is related or not, but the front brake lever has never worked for operating the brake light. i "read" the blown fuses and there is no black mess on the inside, the filaments inside just get bright red and melt apart...

i dunno, i'm kinda lost here, any help would be greatly appreciated

thanks in advance!

[Macguyver]

It sounds like maybe your running light wire may be pinched between the grounded casing and the cover, creating a short circuit. Do you have the switch to turn off the running lights on the left handle control? If so, does it blow the fuses if the running lights are turned off?

I would open up the light and make sure no wiring is piched and shorting, then go from there. Let us know what you find out.

Best of luck,

[Steve]

Do you know for sure what the connections are supposed to be on the "new" tail light?

Is it really supposed to be one wire for tail, one wire for brake and ground through the case?

Are you sure you have the correct wires connected from the bike?

Expiring minds want to know.

.

[Ghostgs1]

6 volt versus 12 volt?

[Redman]

Fuse glowing red and melting is not from a short circuit to ground. If it was a power wire shorted to ground the fuse would blow in a flash as the current would be lotsa lotsa amps like a hundred times the fuse rating. THe fuse glowing and melting is more of an overload, more like only twice the fuse rating.

Which does not make sence relative to this happening when put the cover back on. Maybe the fuse survived the first few times, but then melted after a while, and that just happened to be when you were putting the cover back on.

I dont know enough about the wiring on 70s 750 to know about if the headlight and tail light on same fuse. But being a custom, maybe that has all been custom rewired anyway.

[spchips]

The problem is the 28 vehicle is meant for a 6 volt system. At 12 volts, you are drawing twice as much current through the taillight. I imagine that even at 6 volts, that light used more than the original one.

Measure the resistance of the original bulb circuit and compare to the resistance to the new one. If the new one is much lower you are in trouble.

You will need to find a bulb for that socket that is a 12 volt bulb with the right current draw.

[Nessism]

Dude, 6 volt, 12 volt, blah blah blah, who cares. A 1928 Model A part on a GS has got to be cool (I think)! Post a photo of that tail light so we all can see!

[salty_monk]

Fuse glowing red and melting is not from a short circuit to ground. If it was a power wire shorted to ground the fuse would blow in a flash as the current would be lotsa lotsa amps like a hundred times the fuse rating. THe fuse glowing and melting is more of an overload, more like only twice the fuse rating.

I could clearly see the fuse go red & melt on the ignition circuit when I had a trapped killswitch wire in the switch casing at the handlebar... That was a short to ground.... maybe not a great ground location but good enough.

Dan

[750bobber]

thanks so much for the responses... so, the socket and bulb are both 12v. through a trial and error method, i definitely found the right connections and everything was working fine... "was" being the operative word.

[750bobber]

Dude, 6 volt, 12 volt, blah blah blah, who cares. A 1928 Model A part on a GS has got to be cool (I think)! Post a photo of that tail light so we all can see!

oh it looks awesome... not to be effed with...

[750bobber]

Do you know for sure what the connections are supposed to be on the "new" tail light?

Is it really supposed to be one wire for tail, one wire for brake and ground through the case?

Are you sure you have the correct wires connected from the bike?

Expiring minds want to know.

.

1st question... no, but a series of trial and error helped me figure out the correct config.

2nd question... that's how my stock taillight was set up. i was following suzuki's lead on that

3rd question... yes, pretty sure

[Steve]

The problem is the 28 vehicle is meant for a 6 volt system. At 12 volts, you are drawing twice as much current through the taillight.

Actually, at double the voltage, it will only draw HALF the current.

.

[750bobber]

so this is what she looks like. check out the action at the axle... pretty tough

[spchips]

Actually, at double the voltage, it will only draw HALF the current.

.

Nope, I am certain it will draw double the current. Say for now that the resistance of the bulb is 2 Ohms. The resistance is fixed and does not change. Originally, the system was 6 volts. To determine the original current:

V/R = Current = 3 Amps

Now, we double the voltage to 12 volts (V=12)

V/R = Current = 6 Amps

[almarconi]

Nope, I am certain it will draw double the current. Say for now that the resistance of the bulb is 2 Ohms. The resistance is fixed and does not change. Originally, the system was 6 volts. To determine the original current:

V/R = Current = 3 Amps

Now, we double the voltage to 12 volts (V=12)

V/R = Current = 6 Amps

Steve must have had a senior moment