I really struggle with electrics !!!
My brake light comes on with the ignition and stays on all the time until the ignition is switched off.
The RED warning light on the cluster operates as it should ie. OFF once each of the brake switches are operated
With ignition ON I have power to both wires at the rear brake switch all the time so that appears to be the problem.
What are the steps to try to track down the "short" ??
I have checked the earth wires under the tank and as they are bundled together and all other functions on the bike are normal I don't see how it would be there.
From here do I go to the BMU and if so what do I check and how

The first thing to do—although now it's the second or third— is go to the brake pedal and view how it activates the switch there. It's a plunger switch and it might be stuck inward and in need of some cleaning and convincing to release outward, or it might need replacing altogether. Attached is an example from Euro Motoelectric. Also, inspect and clean the switch wire's harness connector, too.

Yes, the BMU connector is a good place to troubleshoot. (But a PITA to get to.)

Take the BMU out and then test the following at the connector:

The Gray/Red wire should only have 12V when the front brake switch is activated.

The Gray/Green wire should only have 12V when the rear brake switch is activated.

However, what I would do first (because it's easiest) is disconnect the rear brake switch connector under the right battery cover (gray/green and green/black wires) to see if the brake light goes out when you disconnect that. If that makes the brake light go out then you have an issue with the rear brake switch.


With the BMU disconnected, shorting from the gray/red wire (front brake switch) to the gray/yellow wire (brake light) should light up the brake light. Shorting from the gray/green (rear brake switch) to the gray/yellow wire should light up the brake light.  If both of those work with the BMU disconnected then your BMU is bad.

Same problem on my RS...but heres how I solved it.

I got this 83 RS about a year ago after it sat a long time.

Working through it realised it needed lots, one of the things being brakes overhaul. All done, but bulb warning light on, replaced switches and solved.....but brake light on all time.

Narrowed it down to rear by turning on ignition and noting the warning triangle would go out as normal by using front brake only > problem is somewhere in rear brake. 

Further investigation revealed the pedal was not making contact with the brake switch pin. There is an adjuster on the pedal just below the brake switch.....but also my spring for the brake pedal went walk about. As I know from a trip to France with 88 some years back, no spring the pedal sags and brake light on all the time.

A no cost fix. Just lift the pedal up while the ignition is on and see if the brake light goes out......

You also need the correct tension spring. mine was too light and every time I hit a decent bump the brake light would flash on.
Regards Martin.

Hows it going Packo? Have you disconnected the brake light switches yet, the back brake switch connects under the battery cover and is a sight easier to get to than the front that requires the tank be shifted. If you disc them and it's still on then the BMU needs checking as DUCK said. With the ignition on and the park light on it will have power, it's green black and the stop light wire is green yellow and only has power when a brake switch is activated via us using the brake lever or dirt's stopping the switch pin from moving out free and stopping power.
Grey Green rear brake switch to bmu and grey red front brake switch to bmu.

Hope it's solved. It's the 83bm you're talking about right?

[quote="duck"] Yes, the BMU connector is a good place to troubleshoot. (But a PITA to get to.)

Take the BMU out and then test the following at the connector:

The Gray/Red wire should only have 12V when the front brake switch is activated.

The Gray/Green wire should only have 12V when the rear brake switch is activated.

However, what I would do first (because it's easiest) is disconnect the rear brake switch connector under the right battery cover (gray/green and green/black wires) to see if the brake light goes out when you disconnect that. If that makes the brake light go out then you have an issue with the rear brake switch.[quote="duck"]

I did the above checks as suggested : Grey/Red wire had 12V all the time
                                                     Grey/Green only 12V when rear brake switch activated
With these results I then disconnected the front brake switch connector under the tank & the brake light went out
Confirmed that it was definitely a faulty front brake switch and now have a new one on order

Thanks to all who replied for your help

In the meantime you might want to soak the front brake switch with penetrating oil.  I had a problem with my "choke" switch, and that got it loosened up again.