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Discussion Starter · #1 ·
Okay, so I thought I should start a new thread to focus on a specific question about my Sprint Sport repairs.

The brake light went out. I found that one of the bulbs was burnt out and that the wiring at the front master cylinder was pulled out so I fixed those issues and now the front brake will activate the light but the rear brake will not.

Now here's the crazy thing. I started doing some testing. There are 3 connections going to the rear light. 1 is ground, 1 is always hot, and 1 is triggered off of the brake switches. When I hook a meter up to the one that is triggered off of the brake switches and I hit the front brake, it reads 12v. When I hit the rear brake it reads 12v. But when I hook the connections up to the lights, the front brake triggers the light but the rear brake does not...

I thought if they were both sending 12v across the wire then the lights should light. I tested resistance and the resitance across the wire from the rear brake is greater (100ohms) than the resistance from the front brake (3 ohms) but if the voltage is the same, shouldn't the light light up the same? I don't get it.

Any advice from those who know electricals would be much appreciated.
 
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(Skip to the bottom for a quick, blather-free, synopsis.)

I saw similar results with voltage testing when my rear brake switch wouldn't trigger the light. I haven't got a whole lot of electrical knowledge, so I just said eff it & bled the rear brake. Problem solved, for a while, mostly, sort of.

My rear brake was also half seized & generally in poor shape. I failed my first inspection this year due to bad rear pads, so I finally got around to putting on the "new" rear brake I got off eBay & cleaned up. The difference in feel is amazing: it's got some feel to it now! I came from a drum rear bike to this one, so I had no idea what a rear caliper was supposed to feel like. One side effect of the replacement caliper is that the rear brake light goes on every time I hit the pedal, no question.

I also have here a K&L replacement brake light switch that I got on eBay for under $20 shipped. I never put it on, since the switch on there started working about the time I got it.

I suspect that thoroughly cleaning the original caliper would've given similar results. (Maybe not, as my caliper may be slightly bent, but that's another story.)

Long story short, try bleeding the brakes. If that doesn't work, pull the caliper & clean the pistons. Probably wouldn't hurt to replace the seals while you're in there, but I didn't need to.

HTH,
-Kit
 

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It would appear that you have a bad connection in the wiring of the rear brake light switch, hence the 100R resistance.

When you measure the voltage with your multimeter, the multimeter does not draw any current through the wire (or only a millionth of an amp) hence there is no voltage drop along the cable. When you connect it to the bulb, the bulb will try to draw nigh on 2A (assuming its a 21W bulb) and the 100R resistance you have measured will drop the 12v to very little. Basically check the wiring. Disconnect the brake light switch and measure the resistance across its terminals, should be zero ohms.

There is also an in-line fuse in the brake light wiring, it is in a fuseholder that is integral in the wiring bundle, beneath the seat - have a look at that.
 

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Discussion Starter · #4 ·
I hooked up the meter directly to the pressure switch and pushed down on the pedal. Meter ready about 30 ohms. But the harder I pushed, the lower the resistance went. If I stood on the pedal and jumped up and down I could drive the meter to below 2 ohms.

So I hooked the wiring back up and replicated the experiment. If I pushed down on the pedal really hard, the light would actually light up about 60% of the time.

Next I'm going to try to jumper from the switch directly tho the light and see what happens just to see what happens. I suspect the results will be the same and the problem is at the switch.

Kit - I may try a bleed but the pedal feel right now is really good (much better than the front actually). I guess it would be cheap insurance.
 

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Discussion Starter · #5 ·
Jumpering produced the same results, must be the switch - 28 bucks at bikebandit. Ah well. Not a hard fix.
 
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