'81 CB750K Running brake light wiring issue

[JMC]

Hello Fellow Riders,

I have researched all over the forums and still cannot find a solution that works on my end.

My running brake light does not work in position 1. I'll try to make it simple:

Running headlight - works
Instrument panel lights - work
Brake light in position 1 with brake depressed - works
Running brake light in position 1 - does NOT WORK
Brake light in position 2 - works

I have checked the main fuses, cleaned all connections with contact cleaner and light brass brush to remove gunk/corrosion.
I have put new bulbs in the brake light, just in case that was the issue.

Of note, I do not currently have a headlight assembly, just the bulb. The wiring harness was new from vintagecb750 (the PO's harness had been altered just about everywhere).

See picture for my wiring configuration under the seat. I have "deleted" the turn signals.

Hopefully, this is a simple fix and I'm just oblivious.

 

[JMC]

Here's a better picture.
Ignore the color of wire in the middle. I created/soldered those using wires I had on hand.
The main wiring harness connections are connected to corresponding wires on the tail light harness.

 

[brettp]

Is it a stock rear light?

 

[JMC]

Is it a stock rear light?

Yes, it came off of an '81 CB750 Custom donor bike that I have.

 

[brettp]

Then it sounds like your running light filament might be burned out.

 

[JMC]

Then it sounds like your running light filament might be burned out.

They're brand new bulbs (as in yesterday) from O'Reilly.

 

[brettp]

If I remember right the brake and running lights share the same ground. I would be looking for continuity along the wires to that running light.

 

[mrtwowheel]

There's a group of connectors stashed under the tail/brake light too, been the area of my bad connections a few times.

 

[Loose Chain]

Also don't assume just because something is new that it is good. New doesn't mean it works. Test the new bulbs.

 

[JMC]

There's a group of connectors stashed under the tail/brake light too, been the area of my bad connections a few times.

I was in there last night. Clearly 40 years of dirt, but I disconnected, cleaned, and reconnected everything.
Maybe it's a break somewhere along the brake line harness to the main harness.

 

[brettp]

Wires 40 years old turn brittle and break and ponch points. Not uncommon at all. That's why I suggested tracing continuity. I'm sure you'll find the culprit.

 

[Bo82CB]

Wires 40 years old turn brittle and break and ponch points. Not uncommon at all. That's why I suggested tracing continuity. I'm sure you'll find the culprit.

I agree but as I stated before, if ONE wire strand is good and all other broken you will still get GOOD continuity! True continuity test is voltage drop.
Just a thought, can you run new test wire if it is straight wire and not spliced one!?

 

[JMC]

I agree but as I stated before, if ONE wire strand is good and all other broken you will still get GOOD continuity! True continuity test is voltage drop.
Just a thought, can you run new test wire if it is straight wire and not spliced one!?

That's a good idea, especially since I've deleted the turn signals anyway. I will try that.

 

[JMC]

I finally resolved this last week (and am kicking myself for it).

After all the connections I cleaned and locations I checked, it was the fuse box. I HAD checked the fuses, but the "brackets" that hold the fuses are a bit worn.
There's a sweet spot and the position of the fuse just needed to be adjusted up slightly. I couldn't believe it.

On the bright side, I learned a whole lot about wiring!

 

[mrtwowheel]

The best thing you could learn about wiring................Buy a good Ohm meter, read the little booklet, learn how to test and follow the voltage. Should make more sense to you now after this experience.