Topic: Tail/Brake light electrical question

[BigBoi]

Hey all,

Having a small issue with the tail light on my '72 CB750K. When the lights are off on the bike, the brake lights work perfectly when I apply the brake.

When the light are on, the running light for the tail works, but the brake lights don't light up much at all when I apply the brake. Just barely a difference and certainly not nearly as bright as when the lights are off.

Can someone tell me what voltage I should be expecting at each of the wires when the lights are on? This may give me an indication of what's up.

Thanks!

[HondaMan]

This may just be a poor ground connection. When the shell of the socket gets rusty, the resistance goes up quite a bit, resulting in OK light with low-current running filaments, but less current when loaded with the brighter brake side.

Check the condition of the socket and the GREEN wire and it's connector, normally found up under the seat, near the frame's crossbar. It may be crudded up.

[BigBoi]

This may just be a poor ground connection. When the shell of the socket gets rusty, the resistance goes up quite a bit, resulting in OK light with low-current running filaments, but less current when loaded with the brighter brake side.

Check the condition of the socket and the GREEN wire and it's connector, normally found up under the seat, near the frame's crossbar. It may be crudded up.

All connections are new since my rebuild (like 6 months ago). What bulb am I supposed to have in there? Also, what voltage am I looking to have across both connections? (Green is ground, should be zero?)

So the running light should be how much? 5V or 12V?

[HondaMan]

This may just be a poor ground connection. When the shell of the socket gets rusty, the resistance goes up quite a bit, resulting in OK light with low-current running filaments, but less current when loaded with the brighter brake side.

Check the condition of the socket and the GREEN wire and it's connector, normally found up under the seat, near the frame's crossbar. It may be crudded up.

All connections are new since my rebuild (like 6 months ago). What bulb am I supposed to have in there? Also, what voltage am I looking to have across both connections? (Green is ground, should be zero?)

So the running light should be how much? 5V or 12V?

It should measure as 12 volts (give or take a volt) between the GREEN wire and either of the two others (with the brake on). The bulbs are all 12 volt bulb filaments. The thinner one is the running lamp, and the thicker wire inside is the brake light. (The thicker that nichrome -compound wire is, the lower it's resistance.  )

[BigBoi]

If anyone runs into this issue...Hondaman was right (big surprise), it was a bad ground. It was actually in the tail light assembly, where the ground wire connects to the actual casing.

Thanks for you help!

[HondaMan]

If anyone runs into this issue...Hondaman was right (big surprise), it was a bad ground. It was actually in the tail light assembly, where the ground wire connects to the actual casing.

Thanks for you help!

Rusty?

[BigBoi]

Yup!