Taillight but no Brake light

[slodoug]

1983 CB650SC
Brake light doesn't come on but taillight does. I have tested all fuses and all are ok. I followed test of taillight indicator light and that is fine. Indicator comes on if the bulb is removed or either tail or brake wire are removed. I have tested 3 1157 bulbs (2 used 1 new), all work as taillight and both light up the brake filament if inserted backwards so bulbs are ok, fuses ok, something is not triggering the brake wire. Front brake switch has 12v to ground. If I short that with a paper clip, nothing. Rear brake switch has 12v to ground, nothing. I get like 9v voltage on the brake light when those switches are pulled. I'm thinking it has to be a bad brake light sensor box. What else can I try?

[Volker_P]

Welcome here!

"brake light sensor box" sounds special so this is probably something special of the 1983-on DOHC. So I am sorry I cannot help here directly.
Possibly one can simply unplug and bypass this part to see whether the remaining parts of the brake lights circuit work as they should as if it was a "traditional" circuit. I would suspect that the diode is in this sensor box, too, so you might have to check rear and front brake switch action separately.

Another option is following the cables from the brake light back to the next connector. Then supply 12V directly to the unplugged cable and see if you get a brake light that way. If this works, plug together again and follow the cable to next accessible point and repeat. This way you may detect a weak/broken cable to the rear.

Too low voltage at the bulb could also mean a short from the cable to the frame so the main current does not pass the bulb.

In any case you should have a wiring diagram and some idea what you do to avoid possible damage to other electronic parts.

[slodoug]

brake light fixed, bad fuses. but now my rear blinkers still dont work.

[Volker_P]

Independent of your present problems it might be a good idea to find time now for a comprehensive check of the 40 year old wiring harness. This means to have a close look at all the wires, disconnect and disassemble each connector for inspection and cleaning of the contacts. Often also worth a look at the rear side of the fuse holder.

If you have some idea how to handle plastic clips and very small parts I found that Honda switches of these times typically can be disassembled and cleaned out and lubricated again quite reasonably, too. For me a better solution compared to new but cheap replacements of poor quality.

Sure this is a lot of work but it really helps avoiding annoying electric issues on the road.

[VintageRider]

I took volker’s advise on checking all connections and eve though everything was working there were a couple that looked bad/not safe including main fuse that seems to have been bypassed



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[slodoug]

got the fuse box out, one of the metal tines holding the fuses in broke so i ordered a replacement. Would love it if someone has a step by step DIY for relocating and replacing that archaic box.

[Volker_P]

This is quite exactly what I had in mind with regard to the surprises you do not want to detect during trying fixing electric issues on the road.

I think that this main fuse box looks as it does as a consequence of some other electric issue that was not fixed for a quite long time.
Now I would check really every electric thing at this bike.