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My tail light fuse blows as soon as I turn on the ignition...

vasi0011

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I'm tracking down an electrical problem in my 1978 Honda CB750a Hondamatic. The tail light fuse (7A) blows immediately when the ignition switch is turned on. I've determined that there is no short in the tail light, and it's working perfectly if it is connected to a 12V source. The tail light circuit from the tail light fuse grounds out when I turn on the ignition.

The wire going from the 7A fuse is a solid brown wire. I'm also having a problem with the fuel gauge illuminator light not lighting, and it is wired with a brown with a white stripe wire. I think these two problems are related. The solid brown wire grounds out when the ignition switch is turned "On".

I'm wondering if anyone knows how to test the ignition switch. I took it out to check it, but not sure how to check its functions. Does anyone know what the "P" position on the ignition switch is supposed to do?


Can anyone help? Thanks.
 
Do you have a wiring diagram? For just the ignition switch, even if no diagram, it's just continuity check with a meter, one end to the juice side, check each wire at different click.
 
Had the same problem with my 1978 Hondamatic 750a, and found that the "tail light" fuse also runs the lights inside the speedo. Look for shorts or unconnected wires inside the speedo housing that run the dash lights.
 
I'm tracking down an electrical problem in my 1978 Honda CB750a Hondamatic. The tail light fuse (7A) blows immediately when the ignition switch is turned on. I've determined that there is no short in the tail light, and it's working perfectly if it is connected to a 12V source. The tail light circuit from the tail light fuse grounds out when I turn on the ignition.

The wire going from the 7A fuse is a solid brown wire. I'm also having a problem with the fuel gauge illuminator light not lighting, and it is wired with a brown with a white stripe wire. I think these two problems are related. The solid brown wire grounds out when the ignition switch is turned "On".

I'm wondering if anyone knows how to test the ignition switch. I took it out to check it, but not sure how to check its functions. Does anyone know what the "P" position on the ignition switch is supposed to do?


Can anyone help? Thanks.
The ignition switch probably isn't your problem. Probably a short in your speedo dash lights. "P" position allows you to take the key out of the ignition, and have the tail light still lit. It's if you have to leave your bike by the side of the road at night, and don't have to leave the key in the ignition to warn other drivers that your bike is there. Doesn't ever happen, so no one ever uses it. You can test it by turning your key all the way to the right to "P", and remove your key. The tail light should stay on, but no other electrical functions should be working.
 

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