electrical problem on 73 Interstate

Greetings,
On my Interstate I can move the key to the first position and have a red Ignition lamp in the headlight, blinkers, front and rear brake light. When I try to operate the horn the fuse blows. Before it blew their was a rather anemic buzz from the horn. I have disconnected it for the time being.
When I move the key to the second position, as soon as I turn on the headlight, it lights briefly, gauge light too, then blows. BTW the paper inside the fuse says 35 but if memory serves this does not mean to use a 35 amp fuse. Where to begin? Many thanks.
Glenn in NY

Updating this post a bit. With the headlight two way switch vertical and the key in the second position...... the headlight, high, low,tail light, blinkers and brake lights all work fine. When I flick the headlight switch to the left, the fuse blows. Do you think the problem may be with the way the headlight switch is wired? Perhaps wired backwards? Isn't the headlight 'On" position to the left? Thank you.

For the horn, I would look for a direct short from the horn push to the horn. Abraded insulation, bullet connector not fully seated, etc. It sounds as if a bare section of wire is touching metal.

For the headlight issue, I would start at the headlight and work back to the toggle switch and then to the handlebar switch. It sounds like there is a short there also.

The original fuse is a "slo-blo" fuse. I have a hard time finding these and generally use a standard 20 amp fuse.

Sport75073 said:

BTW the paper inside the fuse says 35 but if memory serves this does not mean to use a 35 amp fuse.

Click to expand...

35A is the old Lucas (blow) rating.


Auto fuses are generally continuous rated so if not replacing with the Lucas blow rated type then use either a 15A or 20A continuous fuse.

Thank you for the tips. Glad I didn't put in a 35a continuous. Dont want to let the smoke out!

Take a look at your rear brake light switch wiring. I have had one short out against the exhaust pipe.
John in Texas

I would open the headlight shell and let the headlight and guts hang out. Inspect all the connections inside the shell or switch connections. Make sure no wires are bare or in contact with the shell. Try the key and switch again with a new fuse in the holder, while observing the wiring in the shell. If there's no visible or audible arcing to ground and the fuse blows again, I would move to look at the switch blocks, and particularly the horn switch...

Clean up all the ragged connections. I really like to use heat shrink coverings on connections and dielectric grease. It gives that extra level of corrosion and shorting protection. Make sure the headlight isn't pressing any wires up against the 3 position switch when it's reassembled back to the shell.

Undo the switch blocks and let them hang so you can observe their guts. Turn the key on again. Observe the switch block wires for some sort of arcing, then check to see if the new fuse blew again during that test. If it didn't blow, jiggle the switch block and see if you can get it to blow the fuse. Clean the connections, make sure there are no ragged wires and reassemble. Be aware that bolting the switch block back up can pinch wires.

Personally, I don't like those barrel fuse connectors. I think they corrode and cause intermittent electrical flow. That sort of flow acts like a spot welder and can burn up fuses just because their connection is poor. I spliced a blade style fuse in it's place and run a 20 amp blade style fuse in it and have no issues with that set up

Beyond those suggestions, electricity is logical. Try looking at the schematic, and disconnect a circuit at a time to see if you can isolate the problem... If your horn is acting weird, I'd disconnect that first... but NOT at the horn itself because that still leaves the switch with power in the circuitry...

Also, the crapiest thing on a norton is the chromed plastic directionals. Their grounds are screwed into the plastic/chrome base of the fixture. I always run a ground wire back through the stalk of the directional to the frame to give them a better connection. This way they either work or they blow the fuse. The poor grounding can cause intermittent problems which (as you are finding out) are the hardest to track down. Corroded bulbs in their sockets can do weird things too, check them all, clean them up, dielectric grease them, and reinstall.