rear light voltage problem

Had a flickering rear light that then failed altogether. Bulb checks out OK, so checked voltages on the terminals in the bulb holder and here's the thing. I'm getting 13 volts across one set - presumably the brake light as this works OK, but only 9 volts across the other. Surely I should be getting 12 or 13 across them both.
One other factor - the number plate bulb was not working either - traced that to corrosion in the bulb holder and broken terminal on the tiny bulb. Cleaned the holder up and this is giving 12.5volts but I don't have a replacement bulb yet

Could the number plate bulb and the rear light bulb be wired together in some way so that if one fails the voltage to the other is affected?
Mike

The tail lamp and lighting plate lamp share the same ground. They also share the common hot (yellow wire) from the #10 fuse. If the voltage is different between the two then it's probably the ground from the socket is corroded. Voltage should be the same. THe brake light bulb is dual element, both elements get 12 volts (nominal).

This is a British motorcycle. Bad ground connections are tightly held within its Lucas-derived DNA.

It's congenital.

atomsplitter said:

The tail lamp and lighting plate lamp share the same ground. They also share the common hot (yellow wire) from the #10 fuse. If the voltage is different between the two then it's probably the ground from the socket is corroded. Voltage should be the same. THe brake light bulb is dual element, both elements get 12 volts (nominal).

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Got a new number plate bulb today and as soon as I put that in, the rear light started working again.
Mike

Mike Sands said:

Got a new number plate bulb today and as soon as I put that in, the rear light started working again.
Mike

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I wanted to give feedback on my own rear light problem, just in case someone else has a similar issue.
My indicators started giving intermittent problems - but if I turned her off and back on they started working ok again.
I had put LED headlights in. I searched the forums and asked for advice from members. This is what I found.
1) Looking into this problem I noticed that my rear light and number plate light also didn't work.
- My brake light did operate, however when the indicators stopped working - the brake light and the horn all stopped working too
2) I had taken out the parking bulbs when I put the LED lights in, so I replaced the LED light with the old lights and replaced the parking bulbs
- this didn't solve the problem
3) I looked at the rear cables
- one cable outer had rubbed through but the cable underneath was fine
4) Checked the fuses and both #8 and #10 were blown
- replaced and still not working
5) Pulled out the brake light bulb (twist fitting with tabs clockwise)
- bulb ok
6) Eventually through this site realized how to get access to the number plate bulb. It is a push in fit and needs a slight twist and pull to get the rubber grommet out.
- the bulb was ok BUT the connectors were corroded
- with a small screw driver I scraped the corrosion off so that I could see the copper surface.
7) Replaced the bulb and hey presto good to go

If any one manages to get to this post and this helps solve their problem - I will be a happy man.

Stay safe and on the ride free

Mike Sands said:

Got a new number plate bulb today and as soon as I put that in, the rear light started working again.
Mike

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I have had strange issues that turned out to be caused by the bulb although it looked ok but both elements we coming on at the same time. Any strange issues with lights and I replace the bulb first. It's the cheapest option. (or bad earth)
After that there's a good chance it is the bulb holder or complete light fitting causing it, especially if a printed circuit is involved.

MountainMan said:

This is a British motorcycle. Bad ground connections are tightly held within its Lucas-derived DNA.

It's congenital.

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The old British motorcycles were a great design for electrics. Open ended bullet connections in the open air around the forks and under the fenders.
Unbelievable really as we are not known for good dry weather.
Also don't forget about the compulsory built in oil leaks we had to contend with. Impossible to fix even with new gaskets and sealer.