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I'm stuck with a basic electric problem, on my honda cb500. The rear light bulb is a twin-filament for position/brake, 12v 5/21w. It's brand new and tested. The battery has 11.9v.

On the rear light connector, without the light bulb, I get 12v on the brake light wire, but as soon as I put the bulb in, it does not light up, and the voltage drops to 0.00v.

The position light works nice, without any voltage drop.

I don't have any clue on what could be happening, and testing the whole wire harness would take a lot of time, so I'm looking for any suggestion!

Thanks.

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    Check that the fuse is making a good contact and try bypassing the brake switch by touching the 2 connections together, see if the bulb lights then.
    – HandyHowie
    Commented Nov 3, 2015 at 9:43

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I'm assuming that you are testing the brake bulb voltage with the brakes actuated.

There is high resistance somewhere in the circuit causing a voltage drop. For example lets suppose there is 10k ohm of resistance in the brake light switch. When the bulb is out and the switch is activated there is no current flow. A good meter has 20M ohm of resistance and won't load the circuit compared to the 10k. This is how you can see 12v. When you install the light bulb which is around 12 ohms, it loads the circuit. When the circuit is loaded there is current flow and the 10k drops all the voltage. This is why the voltage drops to zero.

To try and find the high resistance, activate the brake switch, install the light bulb and check the voltage in strategic locations starting from the bulb and working your way back to the brake switch and finally the fuse. When you find that the voltage switches from ground to 12v the length between the two measurements is the problem spot. Explore it, looking for broken wires and corrosion in connectors.

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  • Thanks for your help, I did this and found that it was a connector with a metal pin pushed out of its slot. Everything is ok now!
    – TNCreator
    Commented Nov 9, 2015 at 7:34
  • Great information. One of the best trouble shooting suggestions and put me on the problem right away.
    – John
    Commented May 20, 2020 at 4:01

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