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I have a 1972 Honda CB350F and I got the new tail light from DimeCity. I think it was called the Buffalo style Medium tail light. The tail light turns on and works fine when I switch on the headlight, but whenever I activate the brakes (brake light), the fuse gets fried within a second.

I hooked it up the exact same way as my stock tail light (which works perfectly fine with no trouble). Here are some pictures of the connections with labels on the wires. (Sorry for the jumbled mess)

Stock Tail light connection:

-G = Ground (Solid Green Wire) 
-P = Power (Solid Brown Wire) 
-L = Switch (Green Wire with Yellow Stripe) 

Old brake light wiring

New Tail light connection:

-Black wire -> Ground 
-Red wire -> Power 
-Yellow wire -> Switch 

New brake light wiring

  • Welcome to the site. Could you compare the resistance of the old brake light versus the new one. I suspect that the new brake light current is too high. Also, what is the fuse rating? Thanks for a great question – Zaid Feb 4 '16 at 6:25
  • The resistance are as follows --Stock light: (-Power to GRND = 2.7ohms) (-Switch to GRND = .7ohms) --New light: -(Power to GRND = 2.6 ohms) (-Switch to GRND = 2.3ohms ) – PitchMod Feb 4 '16 at 7:12
  • And the fuse is 15A – PitchMod Feb 4 '16 at 7:18
  • It looks like the old switch is ground-side while the new switch is power-side. I suspect this would explain it but am not sure how you can fix it. Hopefully someone else can chime in. – Zaid Feb 4 '16 at 7:56
  • One last thing, what are the resistances between Switch and Power wires? – Zaid Feb 4 '16 at 8:02
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Remove the tail light bulb then hit the brake and see if it still blows the fuse.

If the brake light works, then the combined bulbs is too much for the fuse. Check the wattages of the new bulbs.

If it still blows remove both bulbs. If it still blows you could have a short in the housing.

Re-install the old one to make sure you didn't introduce a short in the bikes wiring while working on it.

I find that 0.7 Ohm resistance on your old light odd. That doesn't seem right. You're just checking the resistance of the bulbs and they should be about the same. Was it working ok? Check those resistances again on both units with the bulbs out. All should be infinite (open circuits).

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