Brake light on, until I apply the brakes

ProblemChild · Jun 1, 2015

Title pretty much says it. 1970 Commando with only a rear brake switch. I removed the front switch and taped off the leads as it never worked that well anyway. All other switches and lights appear to be working properly. So, the brake light stays on all the time until I depress the rear brake peddle, at which point the brake light goes out. Checked out some connections, and attempted to use the wiring diagram and some logic, both of which are not strong suits of mine at times. Any thoughts of where to look?

ewgoforth · Jun 1, 2015

I have a 1973 850, but I'm pretty sure this part is the same between bikes. Pull the wires off the switch and jump them together with a short piece of wire. Does the light go on when you make the connection and off when you break it? If so, just replace the switch, there's something flaky about it. If not, there's something really odd about your wiring.

Torontonian · Jun 1, 2015

Re :wink: verse the brake switch connections ?

comnoz · Jun 1, 2015

That generally means that the bulb is not grounded to the chassis. Corrosion around the bulb can do it or the wire that connects to the outside shell of the socket is not making contact with the frame ground of the bike. Jim

DogT · Jun 1, 2015

I'd pull the 2 brown wires off the brake switch and take a piece of bare wire and short them together, that's what's supposed to happen when you depress the brake, the 2 brown wires get shorted together. See what happens. It may be that the switch is defective. The switch is pretty basic and can wear out strangely. Could be a grounding problem like Jim says too, but every time I've had the grounding problem, it just doesn't work. If you've got the early bike, 69-70, the ignition switch has to be on for the rear brake lamp to work.

ProblemChild · Jun 2, 2015

So, I had first attached a ground wire from the Zener ground to the rear fender as it originally had before to supplement the ground to the back. I then disassembled the tail light and inspected—everything appears correct including the braided wire from the housing. Why I chose to do the easiest test last, I don't know. I used a jumper at the switch and sure enough when jumped the light is on and unplugged the brake light goes off. So, time for a new switch. I am just surprised that a switch that goes bad would work opposite. I would think either always on or always off. Thanks everyone for the suggestions. This keeps me from going down the rabbit hole as I am wont to do when I overthink things.

acadian · Jun 2, 2015

I don't see how it's the switch (based on what you've described), the switch only interrupts the circuit, and doesn't care which one. Have you tried to simply switch the bullets around for the brake and running lights in the rubber, lucas junction thingy near the taillight?

ProblemChild · Jun 2, 2015

acadian said:
I don't see how it's the switch (based on what you've described), the switch only interrupts the circuit, and doesn't care which one. Have you tried to simply switch the bullets around for the brake and running lights in the rubber, lucas junction thingy near the taillight?

I could certainly revisit grounding, and don't understand how a bad switch could do that, but the light has been working properly for a little while so something changed. To your point I did check and try reversing wires just to see what would happen. Another switch is on the way to either fix or rule that out.

DogT · Jun 2, 2015

Take the switch apart and it's easy to see how it works. Yes, the switch is open when depressed (the switch, not the brake) when you press the brake the switch shorts (the switch is not depressed). It won't matter which way the wires go on the switch, the switch is a simple non-grounded make/break switch. It has to be that way so the front brake switch can operate independently. The switch mechanism is kind of funky, so strange things can happen in there. Same with the front one. I've always been tempted to replace the front (and the rear one) with a micro-switch and get rid of that rubberery switch there. It doesn't help with the front TLS brake either.

ProblemChild · Jun 2, 2015

DogT said:
Take the switch apart and it's easy to see how it works. Yes, the switch is open when depressed (the switch, not the brake) when you press the brake the switch shorts (the switch is not depressed). It won't matter which way the wires go on the switch, the switch is a simple non-grounded make/break switch. It has to be that way so the front brake switch can operate independently. The switch mechanism is kind of funky, so strange things can happen in there. Same with the front one. I've always been tempted to replace the front (and the rear one) with a micro-switch and get rid of that rubberery switch there. It doesn't help with the front TLS brake either.

I had assumed switching wires didn't matter, and by looking at the wiring diagram it seemed so, but when I start second-guessing myself anything goes. I need to learn more about basic electrics for sure. The reason I haven't a front brake switch on it for my TLS is it never seemed to activate in the useable area anyway. Always required squeezing the lever to the bar to activate. One less thing to worry about and one less thing between my hand and that front brake.

acadian · Jun 2, 2015

ProblemChild said:
DogT said:

Take the switch apart and it's easy to see how it works. Yes, the switch is open when depressed (the switch, not the brake) when you press the brake the switch shorts (the switch is not depressed). It won't matter which way the wires go on the switch, the switch is a simple non-grounded make/break switch. It has to be that way so the front brake switch can operate independently. The switch mechanism is kind of funky, so strange things can happen in there. Same with the front one. I've always been tempted to replace the front (and the rear one) with a micro-switch and get rid of that rubberery switch there. It doesn't help with the front TLS brake either.

Click to expand...

I had assumed switching wires didn't matter, and by looking at the wiring diagram it seemed so, but when I start second-guessing myself anything goes. I need to learn more about basic electrics for sure. The reason I haven't a front brake switch on it for my TLS is it never seemed to activate in the useable area anyway. Always required squeezing the lever to the bar to activate. One less thing to worry about and one less thing between my hand and that front brake.

Look inside the taillight housing, in there you should have one of these:

Brake light on, until I apply the brakes

The brake and running light wires coming from the main harness should match up with those going to the bulb carrier. Just pull out those two bullet connectors and switch them around, see if that solves your problem. The problem is back in this area, not at the switch.

ProblemChild · Jun 2, 2015

acadian said:
ProblemChild said:

DogT said:

Take the switch apart and it's easy to see how it works. Yes, the switch is open when depressed (the switch, not the brake) when you press the brake the switch shorts (the switch is not depressed). It won't matter which way the wires go on the switch, the switch is a simple non-grounded make/break switch. It has to be that way so the front brake switch can operate independently. The switch mechanism is kind of funky, so strange things can happen in there. Same with the front one. I've always been tempted to replace the front (and the rear one) with a micro-switch and get rid of that rubberery switch there. It doesn't help with the front TLS brake either.

Click to expand...

I had assumed switching wires didn't matter, and by looking at the wiring diagram it seemed so, but when I start second-guessing myself anything goes. I need to learn more about basic electrics for sure. The reason I haven't a front brake switch on it for my TLS is it never seemed to activate in the useable area anyway. Always required squeezing the lever to the bar to activate. One less thing to worry about and one less thing between my hand and that front brake.

Click to expand...

Look inside the taillight housing, in there you should have one of these:

The brake and running light wires coming from the main harness should match up with those going to the bulb carrier. Just pull out those two bullet connectors and switch them around, see if that solves your problem. The problem is back in this area, not at the switch.

This may be an early Commando thing (?), but it doesn't have one of those. The wires leave the bulb and housing and actually connect to the main harness along the side of the subframe loop next to the hook for the air pump. These wires are color coded and matched. I switched them, just in case, and no change.

DogT · Jun 2, 2015

Pre-71 Commando's don't have that 4 way connector. There's 2 separate wires (brown and brown/green) and connectors for the tail lamp and the stop lamp. Ground (+) is supplied through the frame and mudguard. I think you've proved it's funky switch operation by using the shorting wire. But I've been wrong before.

BTW, that TLS can be greatly improved by arcing the shoes and centering the drum while tightening the axle. I did a write up on that a few years ago. Removing the switch in the cable will help too. Mine is actually useable now.

ProblemChild · Jun 2, 2015

DogT said:
Pre-71 Commando's don't have that 4 way connector. There's 2 separate wires (brown and brown/green) and connectors for the tail lamp and the stop lamp. Ground (+) is supplied through the frame and mudguard. I think you've proved it's funky switch operation by using the shorting wire. But I've been wrong before.

BTW, that TLS can be greatly improved by arcing the shoes and centering the drum while tightening the axle. I did a write up on that a few years ago. Removing the switch in the cable will help too. Mine is actually useable now.

I read through your write-up quite recently. Haven't gotten to arcing yet but I did know about the centering process while tightening. I did get some shimming material and wrapped/secured around the ends of the shoes where the cams actuate and has made a huge difference already. The bike stops better than it ever has.

DogT · Jun 2, 2015

My shoes were originally making maybe 20% contact at best. When I got done it was over 90% and the diff was huge. Of course it still fades after a high speed stop, but it's better than dragging your feet. I used to plan about 3 miles in advance for a stop. Now I feel like I've got a real brake.

gortnipper · Jun 2, 2015

acadian said:
Look inside the taillight housing, in there you should have one of these:

The brake and running light wires coming from the main harness should match up with those going to the bulb carrier. Just pull out those two bullet connectors and switch them around, see if that solves your problem. The problem is back in this area, not at the switch.

Damn, I wish these things were on the parts list, that makes sense now. My bike came without signals, and I just added them back on recently and it is a bit of a mess back there without that, and ironically I had just ordered one of those to replace the one under the tank that is rotting out.

Sigh.

gortnipper · Jun 2, 2015

ProblemChild said:
BTW, that TLS can be greatly improved by arcing the shoes and centering the drum while tightening the axle. I did a write up on that a few years ago. Removing the switch in the cable will help too. Mine is actually useable now.

I read through your write-up quite recently. Haven't gotten to arcing yet but I did know about the centering process while tightening. I did get some shimming material and wrapped/secured around the ends of the shoes where the cams actuate and has made a huge difference already. The bike stops better than it ever has.[/quote]

DogT said:
My shoes were originally making maybe 20% contact at best. When I got done it was over 90% and the diff was huge. Of course it still fades after a high speed stop, but it's better than dragging your feet. I used to plan about 3 miles in advance for a stop. Now I feel like I've got a real brake.

The best thing I have done was to use the RGM MZ Gold shoes - no need to arc them and the are the best shoes I have ever used on the rear.

http://www.rgmnorton.co.uk/buy/rgm-rear ... d_2471.htm

http://www.rgmnorton.co.uk/buy/rgm-fron ... n_2441.htm

o0norton0o · Jun 2, 2015

those switches suck. I have a box of broken ones because the rubber cracks and breaks releasing the plunger. It's a simple continuity switch, BUT sometimes if the stop screw isn't adjusted perfectly, or if the switch isn't centered perfectly on the bump that depresses it, then it actually makes contact (Lighting the stop lamp) when the brake lever is up and breaks contact when the break lever is down... It's just a cheap fininicky plunger type switch that needs careful alignment.

ashman · Jun 2, 2015

Replace the switch with a early Honda dirt bike brake light switch and you won't have any more problems with the old switch, I had long life out of the original switch till it fell apart and have gone through 2 switchs since, the new ones don't seem to be up to it these days, the frist replacement fell apart within a year on the bike and got just under 2 years on the second one.
Like others have said it could be your earth on your rear light that is your problem.

Ashley

ProblemChild · Jun 2, 2015

ashman said:
Replace the switch with a early Honda ...

Blasphemer!

Actually I have seen folks do that, and those new switches do seem to break down in no time. I've already got a new one on the way to test but sneaking in a better made switch may be in my future. Thanks for the pic and suggestion.

Brake light on, until I apply the brakes

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