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Author Topic: no lights and no ignition on cb400f  (Read 951 times)

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Offline 62edward

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no lights and no ignition on cb400f
« on: September 11, 2018, 05:57:34 AM »
I am working on a friends 1975 CB400F. I had it running fine and everything worked. we decided to install new coils and wires since the old ones were taped together. After I installed the new parts I tied to start the bike and it blew the 15 amp fuse I replaced that fuse and checked the others. The 7 amp fuses appear to be fine , checked with a ohm meter. There is no power to the 7 amp fuses but there is to the 15 amp. There is no lights and no ignition. Is there a breaker or diode before the 7 amp fuses? There is power at the solenoid. Any help would be appreciated.
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Offline calj737

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Re: no lights and no ignition on cb400f
« Reply #1 on: September 11, 2018, 07:17:38 AM »
Did you ascertain the cause of blowing the 15 amp fuse? Simply replacing it doesn't fix the problem.

As a polite reminder, on these Hondas BLACK is a switched power circuit, NEVER a ground. Solid GREEN is a ground.

In the headlight bucket, BROWN and/or BRW/WHT should have power with the key switch moved 1 position from OFF. Also BLACK.
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Offline Bodi

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Re: no lights and no ignition on cb400f
« Reply #2 on: September 11, 2018, 12:39:49 PM »
The 7A fuses power through the 15A fuse so no surprise if they are dead if the 15 is blown.
Undo the power wires to the coils (or switch kill to off) and try with a new fuse. No problem = trouble with the coils I think.
Where did you get the coils? Many bikes have coils that look the same but have very low resistance for factory electronic ignition systems... those would draw enough power to blow the 15A fuse.
15A fuse directly powers the coils (through kill switch), brake light, signals, horn, a few other things when key is ON, and powers just the tail light (and front parking light if you have one) with key in "P".
7A "HEAD" fuse powers the headlight but comes off the 15A fuse and keyswitch.
7A "TAIL" fuse powers the tail light (with key ON, not in "P"), also comes off the 15A fuse and keyswitch.
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Offline 62edward

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Re: no lights and no ignition on cb400f
« Reply #3 on: September 13, 2018, 04:35:13 AM »
Thanks for the help. I wasn't aware of how the fuses worked. I unhooked the coils and hooked them back up checked some other connections and now it works. Not sure why it blew the 15 amp in the first place, maybe to do with the new coils. Thanks again.
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