Tail light speedo tack light fuse blows

[Tumminelli]

Hi newbie here. Just bought a 1980 CB750 custom. Decent bike but having issue with the lower right fuse blowing instantly. I traced the the brown/blue wire that comes from the fuse which turns into a brown/white as far as I can. Short of tearing the whole wire harness apart not sure how to check for a short. Are there any areas critical areas anybody can suggest to check. It controls the tail light and speedo and tack lights. I tested the light running by running a battery directly and it works.

Any help would be appreciate. I'm not the greatest with electronics.
Thanks Paul

 

[justafarmer]

Check the tail light bulb. It should be a dual filament. PO could have stuck a single in there. The correct bulb should have two contacts on bottom

 

[amc49]

You pretty much wreck a socket by putting the wrong base bulb in it, I'd think somebody's brain would kick in there first when the bulb doesn't fit.

Unplug the harness to back bulb and speedo and tach one at a time and see what makes the short quit. Instrument bulbs can do it pretty easy as they are often crammed in small space to mess up. Easier to simply pop harness loose then when you find the error you can go deeper into that one harness.

 

[amc49]

My bad. Fuse #4 is taillight only, #3 is brake light. The #4 last fuse controls the front side marker lights (NOT front turns) too and not mentioned so they may not be working, I'd look at that for sure.

 

[Tumminelli]

Brake light works fine. Bulbs are the correct ones (2 filaments). Ill try the unplugging think in the morning. I know it is not the key switch I pulled that and it tests ok. Thanks

 

[justafarmer]

You pretty much wreck a socket by putting the wrong base bulb in it, I'd think somebody's brain would kick in there first when the bulb doesn't fit.

Unplug the harness to back bulb and speedo and tach one at a time and see what makes the short quit. Instrument bulbs can do it pretty easy as they are often crammed in small space to mess up. Easier to simply pop harness loose then when you find the error you can go deeper into that one harness.

I know but i have seen it done before.

 

[amc49]

The bulb will not even lock in place, the pins on the ends are timed wrong, one bulb type being dead even and the other staggered. The socket would have to be wildly worn or Chinese to not be nearly to spec ID. I've seen it done too, by a world class idiot. The socket should be wrecked by the time he is done.