1975 MK3 rear brake light

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Hi,

Complete electrical dumbo, so be gentle!!

I have a standard MK3 ignition switch, when I switch the ignition on (no lights) the brake light works fine, front & rear brake switch work as intended.

When I move the ignition switch to lights on (lights to clocks, front & rear light on etc.) the rear brake light does not work? Using the front & rear brake does not activate the brake light.

Everything else seems OK.

So, anything obvious to check before I start stripping the entire wiring loom!!!

Have a good festive brake!!
 
click said:
When I move the ignition switch to lights on (lights to clocks, front & rear light on etc.) the rear brake light does not work? Using the front & rear brake does not activate the brake light.
Click to expand...

Assuming this is a recent problem (is it?) then have you done any work on the bike lately and if so, what?
If not, then possibly a bad tail lamp earth wire connection so I suggest you check the bulb holder 'earth' wire is securely connected to the bulb holder and harness red (at the connection point normally beneath the tail lamp fairing).
 
Hi L.A.B.,

Nothing done recently, it might have been a long standing issue that I'm only noticing now! I rarely ride the bike with the lights on!

I'll check any earths to ensure they are OK. The fact the brake light works fine with the ignition on makes me believe the earths are fine, but I'll check just to be sure.
 
click said:
I'll check any earths to ensure they are OK. The fact the brake light works fine with the ignition on makes me believe the earths are fine, but I'll check just to be sure.
Click to expand...

Check all three tail lamp wires (brown, brown/green and red) are connected to the harness correctly and that there isn't a feed and earth crossed over?

Edit: Have you checked the bulb and holder?
 
Last edited:
As always, fantastic help L.A.B..

I'll check everything a little later today & give some feedback
 
Test the continuity at the ignition switch pins, the lights should be connected to pin 3 with continuity between it and pin 1 with the switch in the on/no lights position, if you lose continuity when switching the key to the lights on position then problem is in the switch itself

https://www.oldbritts.com/51_121015.html
 
acadian said:
Test the continuity at the ignition switch pins, the lights should be connected to pin 3 with continuity between it and pin 1 with the switch in the on/no lights position, if you lose continuity when switching the key to the lights on position then problem is in the switch itself

https://www.oldbritts.com/51_121015.html
Click to expand...

Thanks for the reply.

The strange thing is, I have full functioning lights but the brake light is not working but if I just turn the ignition to on (with no lights) the brake light works!!

I need to go out to a cold garage to check, which is the reason for my delay in checking things out!!! Once I get a few things done I'll head out:rolleyes:
 
acadian said:
Test the continuity at the ignition switch pins, the lights should be connected to pin 3 with continuity between it and pin 1 with the switch in the on/no lights position, if you lose continuity when switching the key to the lights on position then problem is in the switch itself
Click to expand...

The tail lamp feed (brown/green, terminal 3) from the description appears to be working normally.

It's the brake light that doesn't work when the ignition switch is turned to the 'Ignition and Lights' position.
The brake light (switches) feed is (white) from terminal 2 at both Ign. switch positions. If there was a problem with the T2 switch feed at the 'Ignition and Lights' position then other electrical items also wouldn't work.
 
Last edited:
I'm guessing your tailight leads are switched.
The running light should be the dimmer of the two, the brake light the brighter.
If switched, the running is the brighter and drowns the brake.
Should be reversed.
Easy enough to check. Pull the lense and observe the filaments lighting.
If they both operate, switch the tailight leads.
 
L.A.B. said:
The tail lamp feed (brown/green, terminal 3) from the description appears to be working normally.

It's the brake light that doesn't work when the ignition switch is turned to the 'Ignition and Lights' position.
The brake light (switches) feed is (white) from terminal 2 at both Ign. switch positions. If there was a problem with the T2 switch feed at the 'Ignition and Lights' position then other electrical items also wouldn't work.
Click to expand...

Right.

Then the OP should probably test the brake circuit bullet at the rear fender under both conditions, though I'm not sure how this could be an issue with the light itself, the brake light is only one circuit, it either has power or it doesn't... maybe, as suggested, he does have the running/brake leads mixed up
 
MichaelB said:
I'm guessing your tailight leads are switched.
The running light should be the dimmer of the two, the brake light the brighter.
If switched, the running is the brighter and drowns the brake.
Should be reversed.
Easy enough to check. Pull the lense and observe the filaments lighting.
If they both operate, switch the tailight leads.
Click to expand...


Bingo!!!

I just observed the filaments with the red lens off, the dimmer (5W) filament is activated by the brake light switches. When I put the ignition switch to 'ignition & lights' the 21W filament lights up drowning out the 5W brake light filament, MichaelB, very well diagnosed over the internet!!!

I'll switch the bullets & this should solve the problem.

I only have the Norton around 10 years and I'm finding this issue now!!!!

Appreciate all the help, simple fix in the end :D
 
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