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Discussion Starter #1 (Edited by Moderator)
I just picked up a 1986 Nighthawk 450, and the most pressing issue I need to get resolved is the brake light as it appears to be stuck on. I pulled the cover and only the left bulb is lit, and both filaments in it appear to be lit any time the key is on regardless of brake position. The right-hand bulb is not lighting at all. I tried swapping bulbs and everything remains the same - both filaments are lit in the left bulb and nothing is lit on the right.

Any pointers on where I should start are greatly appreciated.
 

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Lets start with getting the FSM, factory service manual. PM sending in a few
The brake light is actuated by the brake switch on the master cylinder or the rear foot pedal.
The rear brake switch is on the right side of the frame forward and lower than the battery, activated by a spring connected to the pedal.
The front switch is under the master.
 

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Discussion Starter #3
Okay, I'm grabbing those. Do you know, offhand, if the switch is open or closed when the light is on? Just trying to get an idea of what I'll be looking for once I get the wiring diagrams.
 

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Discussion Starter #4
Okay, I grabbed the manuals and got some answers - I think. It looks like the switches have continuity when the brakes are applied, so I don't think it's going to be an electrical problem (other than one socket not working, but they appear to share power and ground wires, so that's probably going to be at the socket). I'll probably get chastised for not mentioning it, but the front brake is dragging. The lever returns, but the pads are still grabbing the caliper a bit. I guess I just assumed the front switch was based on the lever position, but, looking at how the switch is wired into the master cylinder, I guess it could be based on the piston position?

If it's something like that, I guess my brake light is working perfectly fine and is properly detecting that the brake is applied. If that's the case, I'll feel a bit dumb, but I'll have learned something :)
 

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Try cleaning the fluid return (small pinhole at bottom of reservoir to the left of the larger hole)

Front brake switch works on pressure.....
 

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Discussion Starter #6
Pressure makes a lot more sense. I popped the top on the reservoir and the fluid is fairly low and nasty looking, so it needs some attention - probably going to end up ordering a rebuild kit just to be thorough as I assume those cheap master cylinders I see on Ebay are just that - cheap and not something to trust my life with.

If nothing else, it occurs to me that I can disconnect the switch to see if the brake light goes out to kind of narrow things down. I really appreciate the help.
 

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On these models the front brake switch is mechanical, not pressure. It's held by 2 screws to the bottom of the master.
These are some decent masters https://www.ebay.com/itm/Honda-Nighthawk-V4-V65-VT-VF-500-650-700-750-800-Brake-Clutch-Master-Cylinder/301891053689?hash=item464a1be879:g:L2AAAOSwAuNW3nIx:sc:USPSPriorityMailPaddedFlatRateEnvelope!98662!US!-1
You want to look at yours carefully on the main body and see if it says 5/8" 9/16" 15mm, 16mm or 14mm. This link is the 16mm but he has other sizes. I've used one
 

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Fixed the title so to eliminate the confusion of the DOHC CB450 and the SOHC CB450SC.
The pre-1978 models did use pressure switch for the brake light on the few disc brake models. Later years went to the mechanical for the most part
 

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Discussion Starter #10
Thanks for the title fix, longdistancerider. I feel like I'm starting to chase my tail on this one. I verified that the switches seem to be working in the weirdest way possible - when the key is turned to the park position, the tail light is lit up like normal (both filaments on the left bulb only) along with the rear turn lights on solid, but nothing up front. However, pulling the front brake lever or pressing the rear brake pedal when in the park position will turn the front lights on while the brake is held along with the gauge lights. Turn signals work correctly with the key turned to on. Basically, pressing the brakes causes P to act like On. Weird.

I'm going to look at it again tomorrow, but I see that the wiring for the brake switches goes through the turn signal relay (unless I'm reading the diagram wrong). I'm taking a wild guess right now that maybe the "stop, horn, 15a winker relay" fuse. The horn is non-functional, so... maybe?
 

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Under the seat you'll find a wiring harness, follow that to the taillight area where there are bullet connectors used. Look at the wires and be sure that the color codes on both sides of each connector are the same. IE; Green to Green, Brown to Brown.
BTW, Green is ground on these.
Key on you should have power to the Brown, nothing else.
Brake applied you should have power to the Green w/Yellow
Right turn signal on pulse power on the Lt. Blue
Left turn signal on pulse power on the Orange.
 

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Discussion Starter #12
Got a chance to look at it a bit better today, and found the problem. It appears a rodent built a nest in the empty space in the tail behind the taillight and spent some time chewing on the wires. I pulled the tail light cover and the reflective piece in there, and, when I pulled the sockets out, I saw stuff packed behind the socket and the wires on the right-hand socket were chewed up.

At this point, it looks like I have to pull some more stuff off to get access in there, but at least I know what the problem is, or, more specifically, where it is. I'm sure there's just stuff touching in there that shouldn't be.

Thanks, again, for the help. I was scratching my head as electrical stuff is not one of my strengths and I appreciate the advice I got.
 

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Discussion Starter #13
Okay, this will probably be my last update on this thread, but it's definitely the mouse. I pulled the little compartment that's under the seat out, popped the tail light housing off, and found where several wires were touching. I spread them apart, turned the key on, and verified that the left socket is working correctly now. However, the wiring is chewed right at the other socket, so I'm not sure if I'm going to be able to fix it. Does anyone know the part number on the bulb sockets or where I can get similar ones?
 

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