Testing Tail Light Problem

[Red Fthr]

So I've been using the following tail light on my Honda CB 500 F k0 :
http://www.dimecitycycles.com/vintage-cafe-racer-caferacer-bobber-brat-chopper-custom-motorcycle-lighting-parts-black-cats-eye-taillight-60-0694.html

Everything worked fine when I hooked the light and then the running light started working intermittently, and then not at all. I changed bulbs thinking it was the filament; bulb is fine. I then tested my stock light and received the same results. My brake lights work just fine and the parking light works fine as well, my running light refuses to come on though. I then broke out my Multimeter to test wires and got the following results:

With headlight on, I performed test where the ignition switch connects to the harness and where the brake lights (green/yellow), running light (brownish redish), and ground connect to the fender. Both test sections received similar results.

Ignition switch connection results:

DCV (20)

Running Light: 0.05
Instruments: 0.05
12v (blk): 11.21
Red wire:11.21
------------------------------
Parking Light (with switch turned to parking) : 12.08
------------------------------

Resistance (Ignition switch turned to running position)

Running Light: 1
Instruments: 1
Parking: 1
12v (blk): -1
Red wire:-1

Resistance (Ignition switch turned to parking position)

Running Light: 1
Instruments: 1
Parking: -1
12v (blk): 002
Red wire:-1

I popped open my ignition switch and noticed some of the connections have been worn. I figured this was my problem, so I bought a new switch; got the same results with new switch. What on earth can be going wrong? What else should I be testing and how?

Below is a diagram of my simplified harness:

 

[Sonreir]

For the resistance measurements, what points were you measuring between?

 

[Red Fthr]

Between a ground and mentioned wires

 

[Sonreir]

OK... that probably won't tell you too much.

I'd focus your efforts on the tail light and the power going to the tail light.

If you do a resistance test between the tail light and the tail light ground, and again between the brake light and the brake light ground (often the same ground as the tail light ground), this would be a good place to start.

 

[Red Fthr]

Wires at taillight results

Resistance:

Running = 1 (no beep)
Brake = 1 (beep when engaged)
Parking = 1 (beep)

20 DCV:

Running: 0.18
Brake: 11.40
Parking: 12.55

 

[Sonreir]

Running light and parking light are the same, so maybe I'm misunderstanding the issue?

 

[Red Fthr]

Exactly. The only thing that makes a difference is the parking light works when the ignition switch is turned to the parking setting, but when the ignition switch is on the run setting I don't have a running light. This made me think it was a worn out switch so I bought another, but I still recieceved the same results as with the old switch.

 

[Sonreir]

Trace the brown wires to your headlight. Likely something is just unplugged.

 

[Red Fthr]

Ok, yes, there is a brown wire that splits off from the ignition switch and goes to the tail light and the headlight. However, there's nothing in my headlamp for the brown wire to connect to (unless that's what that extra spot on the relay you wired up for me is for?) and nothing in the diagram shows the brown wire connecting to anything other than the taillight and ignition switch. I'm sure I'm missing something here.

 

[Sonreir]

It's been a while, but I seem to recall there being a double-female bullet connector on one end of either the speedo or tach backlight? I think that's where it joins in.

 

[Red Fthr]

There's a ground and an instrument (brown/white wire) male wire that connect to the speedo and tach. The brown wire on my harness, going into my headlight bucket is female. It has to go to something haha. I can't find it on any diagrams though.

 

[Red Fthr]

Unless, somehow the brown/ white wire (instruments) acts as the running light switch via the headlight high/low beam control. Which in my mind makes sense being that your instrument lights and taillight only comes on when you turn on your headlamp. Does that make sense?

 

[Sonreir]

Yup. So on the ignition switch, when you're in the 'park' position, the brown and the red wires are joined, ensuring you have power to the tail light. When in the 'on' position, the brown and the brown/white wires are joined. This means the tail light only comes on when the headlight is turned on. The headlight switch provides power to the brown/white wires which feed the backlights for the gauges and runs back through the ignition switch, to the brown wire as well.

 

[Red Fthr]

Got it all sorted out. Thanks again for all your help.