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Wire Tapping Rear Brake Wire

Discussion in 'Technical and Troubleshooting Torque' started by Hipster Doofus, Apr 1, 2011.

  1. I've got three accessories I want to tap into the system for power when the key is on.
    An oil temp guage, a volt meter & a gear indicator.

    I know the volt meter reading will change when the rear brake is applied but at least it will show that it is working OK. (think that's OK)

    Will there be any issues tapping the three into the rear brake light wire? I was thinking of joining the three then tapping once into the brake.

    Any suggestions on another wire to tap under the tank? Bike is a FI XJR1300.

    I chose the brake because I'm a noob at this stuff & that one seems the easiest to get to.

    Also would there be any problems with tapping two wires, again joining them & then into an earth? The earth is from a USB charger that is going past where I'll be working.

  2. I'm not sure I want to get into this in any detail, because I'm no electrician...

    I'm not sure you mean the rear brake light wire, or perhaps the tail light wire? The latter would at least supply power as long as the lights were on, which should be all the time the key is on.

    When you talk about a twist and tape join - that should be ok for most things but soldering that joint and insulating would be better. Buying a box of male and female electrical connections of the appropriate size, and a crimping tool ($10 or so) would be an even better and more professional way.

    I'm not sure what you mean by an earth from a USB? Are you talking about something from the power-supply transformer? In any case - it doesn't sound right at all. A bike (or car) electrical system is Direct Current, and one end goes to one side of the battery (generally positive, I think) and the other side of the battery goes to the chassis or frame. That is the 'earth.' To power a device, you take power from a source that's live (12V dc) and it goes to one end of your device, the end with the polarity input which matches the supply side of your wiring loom, and then you connect the other side of your device to the frame, or engine block or something.

    USB connections - on the computer side, are 5V DC, and very low current, and they don't actually have an earth. They are powered, but they're primarily designed as a data interface, not a power source. On the external ones, that come with their own little plug in power supply, the supply side is 240V Alternating current, and you don't open that stuff up and try and use it at all. Mostly, they don't have an earth either.

    It's possible I've misunderstood you, but I'm not getting the sense here that you have a deep electrical background, and I'm thinking I'd get someone along there to have a look, who does know, before you start cutting and connecting and ...
  3. the bike "Earth" is the Negative connection.

    You want to Hook a wire via Solder or a good quality connector, to the Tail light or another "on with key" power line, and run a relay set up. Provide Power Direct from the Positive connection to the battery Terminal to each device through a dedicated relay, not forgetting the appropriate fuse.

    If you are unsure, pay an Automotive Electrician to do, it shouldn't be a big job, and it shouldn't cost to much.
  4. Thanks guys. :)

    I want the rear brake light as when the key is off there is no power, key on the wire has power. If I tap into the tail light it will interfere with the volt meter.

    I want to tap into the wire not tape wires together. After looking around I need what's called a Wire T-tap. No soldering required.

    The USB is a USB charger. Something like a car charger. It has pos & neg (earth) connections that go straight to the battery.

    The only thing of concern is hooking up three things to the one brake wire.
  5. Ok, now I know what you mean about the USB. You had me a bit worried there.

    So protect with a relay. Use your primary circuit as a switching circuit, and feed the real power through the secondary. (And respectfully, with a smile and the best will in the world, if that sentence is gobbledy-gook to you, go find an auto electrician, and show it to him.)

    The right place to hook up the volt meter depends on where and what you want to measure the voltage of. Most just go across the battery, so you can confirm you have 13.8 ~ 14.4 volts while charging, and 10.8 ~ 12.2 when not. A good volt gauge will also show you voltage drop during cranking, but most are not that good. I think what you want here will be a voltage meter that picks up directly from the battery terminals. If you share your power with anything else, that will distort and corrupt your reading.

    Nominally, everything in the electrical loom runs at 12V, but as you've discovered, that's true in only a general sort of way.
  6. The USB charger doesn't need a relay as it has a reset fuse in it. This is it over here.

    The gauges are from Motosens.

    It's no good hooking the volt meter direct to the battery as it will drain it (its illuminated). That's why I want the rear brake wire as it will have power when the key is on although when I brake it will alter the reading but it should give an accurate reading at other times.
  7. You can have a perfectly good and extremely accurate voltage meter that needs no power except the power you're measuring, but those are big and boxy and silly and old fashioned today, and I think you'd struggle to find one anywhere, new. And they wouldn't look right on a motorcycle - at all. Most modern gauges have one pair of inputs for power, and one for sensor. (Or they have their own batteries, like a hand held multi-meter.) When the power is off, there should be no drain through the sensor inputs. So you supply power to your gauges by direct wire from the battery, via a relay. Then you wire up a supply from a circuit that's live when the key is turned - perhaps the tail light. You use that as your switching current, to pull the relay in, and then you have power to your gauges. Total current or drain on the switching circuit is negligible, so no additional steps need to be taken to beef up that circuit, like heavier cabling or heavy duty switches. And your gauges will always have a good clean power supply from the battery. That only leaves you to select where you are going to connect your sensor wires to. They don't have to be straight to the terminal clamps, but if you put them in some places in the electrical system, you will get a reading which is correct for the potential difference between the two points, but not indicative of the voltage the battery is recieving, and that's what you generally want in an automotive voltage meter.
  8. Have you bought your gauges yet? Did they come with instructions? Are the instructions clear? ...

    It sounds logical to me that the people who made the gauges would have thought about this, and their instructions would be the first thing I'd look at. There are probably fifty ways to design a volt gauge, each requiring a slightly different method of wiring. But if the instructions say "... jam this wire into the clamp on the battery and then pinch it up again, and solder this one to the frame." - then that's what I'd be doing.
  9. The only problem with running it off the taillight is that the circuit might not be able to supply enough power to run what you need.

    Much better to use a relay that is activated when the key is turned on, you could use the taillight power to activate the relay, the relay won't draw much current.

  10. Bit the bullet & went straight to the brake line which is LED if that means anything.

    Everything is working OK. Just hope my crimping & tapping holds up.

    Thanks all. :)
  11. did you use relays, or not?
  12. No relays. Instructions that came with the gauges just mention a fuse so that's what I went with.