Tail light shorting with stop lamp

[Graydonfisher]

I own a 1979 XS650 Roadster, originally from Switzerland, with the stop lamp warning light above the neutral light. It's pre electronic ignition with a single fuse.
Last night I was riding home with lights on, all working fine. I stopped for a while, then started up again to continue my journey. When putting lights on, the warning lamp illuminated, immediate thoughts was bulb had blown, however, I didn't have brake light either. While ridding occasionally the tail light would illuminate and the warning lamp would extinguish, however, if I applied the brakes, both would go out again.
Sorry to write this essay, but thought it might help.
It would appear, that when the lights are turned off, when activating either brake, the stop lamp illuminates, but dim. I think it may be the tail light filament? The brake lamp warning light illuminates as it should. If I remove the bulb, it then flashes as it should.
I have checked all the electrics from front to back and cannot find the problem. Electrics isn't my area, so any help is appreciated.
Thanks,
Graydon

 

[fredintoon]

Hi Graydon and welcome,
If the bulb checks out OK when it's in your hand but acts up on the bike, it's the bulb's earth path not working.
If the bulb ain't earthed the tail light filament earths itself through the stop light filament except when you hit the brakes.
Then both filaments see current, there is no earth path and they don't light up.
Use your multi-meter set to Ohms to check for continuity.
One probe on the bulb base. T'other probe to the bulb holder, then to the earth wire and then to the frame.

 

[Graydonfisher]

After many hours of testing the connections in the headlamp, I've now tried what you suggested, and you were right, no continuity at the bulb holder. It was a slightly corroded joint in the earth wire from the lamp to the frame. Works perfectly now, many thanks

 

[GeorgeOC]

Hello gentlemen, thanks for the above posting. I have recently ran into a similar issue. In the above post, Graydon are you referring to the bulb holder for the tail light, or the head lamp?

 

[fredintoon]

Hi George,
Graydon's problem was a faulty ground on the stop/tail light bulb holder.

 

[GeorgeOC]

Fred, thank you. I just checked the connectivity for from the stop/tail bulb holder, and the ground at the frame/battery box. Checked ok.

 

[fredintoon]

Fred, thank you. I just checked the connectivity for from the stop/tail bulb holder, and the ground at the frame/battery box. Checked ok.

Hi George,
suggest you re-check. Meter set to Ohms. With bulb in place in stop/tail light on bike.
One probe on bulb's outer shell, t'other probe on battery negative terminal.
Look for reading of one Ohm or less.

 

[GeorgeOC]

Thanks for those instructions Fred. Just rechecked. I am showing 3.9ohms when connecting those at those two points. Is that within acceptable range??

 

[fredintoon]

Thanks for those instructions Fred. Just rechecked. I am showing 3.9ohms when connecting those at those two points. Is that within acceptable range??

Hi George,
hmmm, eff nigh 4 Ohms seems a touch dodgy to me.
Try this:-
check your spare stop/tail light by twisting it's two power wires together, touching those wires to your bike battery's +tive terminal
and it's ground wire to the battery's -tive terminal. Both filaments should light up bright.
Then
Touch the stop/tail's two power wires to your bike battery's +tive terminal and it's ground wire to the base of the bike's stop/tail bulb.
If the bike's stop/tail ground path is OK the spare stop/tail's filaments should light up bright.

 

[Graydonfisher]

Hello gentlemen, thanks for the above posting. I have recently ran into a similar issue. In the above post, Graydon are you referring to the bulb holder for the tail light, or the head lamp?

It was a dirty connector on the earth wire to the tail light bulb holder