Testing brake light switch

cbertozz

cbertozz

Hero member
Location
Australia
Getting close to a road test!

Is there an easy way to test the brake light switch? Neither front nor back seem to be working.

Thanks in advance!
 
So used multimeter to see what the difference between the front lever pressed and not pressed.

With no pressure on the front brake lever I get 13v on one prong and 12v on the other. When I press the brake lever the 12v side goes up to 13v.

Does this mean the switch might be stuck on?
 
cbertozz said:
I get 13v on one prong and 12v on the other.
Click to expand...

Which "prongs" are you talking about? I assume you have the globe out and are putting your meter probes into the globe socket. One prong is for the brake light and the other is the tail light. The earth connection for both is via the outer part of the globe holder. You need to work out which prong does what job.

Brake light prong should read 0V when the ignition is on and the brakes not activated, and go to 12V (or thereabouts) when you activate front or rear brake. Tail light prong should also read 12V if the ignition switch and light switch are on, or if the key is in the "park" position, otherwise 0V.

If you have permanent 12V at the brake light prong, there's a problem with your switch circuit. Probably a stuck switch but not necessarily. Could be a crossover somewhere between brake and tail light wiring.  If your brake light isn't working despite a stuck switch, the globe is likely fucked. Easy enough to check both globe filaments with a battery and a piece of wire.

To check either brake light switch, un-plug the wires from it and use the lowest resistance scale on the meter to test the switch continuity. There should be no continuity with brakes not activated and a connection when they are. I like meters with a beeper on the low resistance scale for this sort of testing. You don't have to look at the meter, just listen for the beep when there's a connection.

 
I noticed that when I press the brake levers the main filament in the light seems to get slightly brighter, rather than the other stop light.
 
The outer ring is the brake light, and it goes to 13v when either brake lever is pressed. The glove appears to be in tact and I've tested it with two tail light units and on neither does the brake light illuminate. So seems like current is getting through.
 
Have also tried closing the loop on the rear brake with a paperclip and the brake light doesn't illuminate, which suggests it's not the switch. Maybe an earth.
 
cbertozz said:
The outer ring is the brake light, and it goes to 13v when either brake lever is pressed. The glove appears to be in tact and I've tested it with two tail light units and on neither does the brake light illuminate. So seems like current is getting through.
Click to expand...

So re-reading your notes Cam, Something is wrong as the outer is carrying current from brake switch. Pretty sure I tried switching but will do again.
 
Thanks for listening while I figured it out with the info Cam gave me. Turns out you should never trust the colouring of a cheap tail light unit!!! All good now, thanks again.

I love that I've been able to learn something useful again!
 
Good news. for the inexperienced in auto electricery tis always better to use a test light rather than a multimeter as a meter whilst will give you volts the circuit may not be carrying enough current through it to light the bulb.
There has been a few threads in the past about stop light switches and the uninitiated falling foul with a multimeter... you should be checking for voltage drop if you must use a meter not just voltage at a particular point. Then again what would I know.........
 
Brett said:
Good news. for the inexperienced in auto electricery tis always better to use a test light rather than a multimeter as a meter whilst will give you volts the circuit may not be carrying enough current through it to light the bulb.
There has been a few threads in the past about stop light switches and the uninitiated falling foul with a multimeter... you should be checking for voltage drop if you must use a meter not just voltage at a particular point. Then again what would I know.........
Click to expand...

As it happens I bought a test light, so much easier. When you don't know how the things are built its doubly hard. Cam's clarification of the way the tail light is configured helped me to work it out. It's hard when the black wire isn't the earth/ground wire!!!!
 
Sorry, didn't get back to this thread until now.

If I'd seen this comment yesterday: "when I press the brake levers the main filament in the light seems to get slightly brighter" I'd have told you to it was a bad earth. Might have saved you a bit of faffing about.

Not to worry, you got there in the end. All part of the learning experience.  :D

Bit of a coincidence. I was fiddling about with my Guzzi this morning and noticed that the brake light is on all the time ...

Cheers,
Cam
 
Dellortoman said:
Sorry, didn't get back to this thread until now.

If I'd seen this comment yesterday: "when I press the brake levers the main filament in the light seems to get slightly brighter" I'd have told you to it was a bad earth. Might have saved you a bit of faffing about.

Not to worry, you got there in the end. All part of the learning experience.  :D

Bit of a coincidence. I was fiddling about with my Guzzi this morning and noticed that the brake light is on all the time ...

Cheers,
Cam
Click to expand...

Yes but to a guy who's more used to fiddling with computer code and data than wiring it's very rewarding to able to fix this stuff (with your help).

Sounds like some brake light switch adjustment needed?
 
cbertozz said:
Sounds like some brake light switch adjustment needed?
Click to expand...

Haven't got around to investigating it yet, but I don't reckon switch adjustment is the issue. Especially since they're hydraulic switches that are non-adjustable. If a switch has malfunctioned, it's a chuck out and replace job.

Brake light appears to be powered by the head/tail light circuit. So a more likely cause is owner stupidity. I may have plugged the wiring together incorrectly last time I had it apart. I'll have to shuffle some bikes around in the shed to get a closer look. It's bound to be something simple.
 
Dellortoman said:
Haven't got around to investigating it yet, but I don't reckon switch adjustment is the issue. Especially since they're hydraulic switches that are non-adjustable. If a switch has malfunctioned, it's a chuck out and replace job.

Brake light appears to be powered by the head/tail light circuit. So a more likely cause is owner stupidity. I may have plugged the wiring together incorrectly last time I had it apart. I'll have to shuffle some bikes around in the shed to get a closer look. It's bound to be something simple.
Click to expand...

I missed off the smiley face, I wouldn't presume to give you advice on that sort of stuff  :LOL:
 
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