Welcome to Netrider ... Connecting Riders!

Interested in talking motorbikes with a terrific community of riders?
Signup (it's quick and free) to join the discussions and access the full suite of tools and information that Netrider has to offer.

fluctuating voltage

Discussion in 'Bling and Appearance' started by thecptn, Apr 4, 2008.

  1. Hi all.

    I recently picked up a Yamaha sr185 off ebay, all for large sum of $83, I gave it a look over today, and i found a very unusual problem, the voltage fluctuating very rapidly, from 5 to 19volts, and the voltage seems to do same thing when I have it turned off, but it only goes up to 12 volts, another bizzare thing happens when I place my multimeter over the cdi box, it will do the same thing, even if the other lead is not connected! now the bike is dirt cheap so im not worried, just curious.

  2. More info needed, I think.

    Where are you measuring the voltage? At the battery terminals or somewhere else?

    What sort of frequency does the fluctuation have? Does it alter in magnitude or frequency with engine revs?

    Does the fluctuation coincide with the sound of a relay clicking anywhere on the bike? My first thought is that the indicators are on (or the bike "thinks" they're on) and that the electrical system or battery are sufficiently unhealthy that every time the flasher relay closes the battery voltage drops.

    Like you, I am intrigued.

    You might want to test something else with your multimeter, just to eliminate the possibility of an instrument fault.
  3. Digital multimeters will take all kinds of spurious readings if the leads are poor quality and/or not connecting on one side to a proper ground or "earth return".

    Since they are a digital device they can have voltages induced into them which will produce random figures on the display. Better quality/higher price ones should be more stable.

    You can only measure voltages when both leads are connected, one to the positive source and one to the negative or ground.

    Not all motorcycles use the frame to ground the negative battery terminal. You would need to take a resistance reading between the battery (-) terminal and the frame (find a non-painted section) and should see 0 ohms if the connection is, indeed, made.

    For safety sake (safety meaning reliable readings) I always meter to the battery (-) terminal first, making sure that voltages are available wherever, then try using a frame point for earthing of the multimeter negative lead if the battery is connected to ground via the frame.

    All the best

    Trevor G
  4. Well I tested them on the terminals and frame, it happens when it idles, the most bizzare thing though, it will do it while the multimeter unit is hovering over the cdi unit box without any leads touching the bike! it has had its lights and indicator wiring removed, could it be that? some thing...is very odd, I did use a replacement regulator unit which didnt rectify the situation, over all, I think the entire electrical...no, scratch that, the entire bike needs an overhaul...lol.
  5. I think I'll secong Trevor's comment that you meter might be picking up a false signal from somewhere.

    Might be worth extending the leads and seeing what happens when it's well away from the bike.

    If you're getting a reading withouth both multimeter leads connected, it's more likely the meter than the bike.
  6. That's all we needed to know - it took me a couple of hours to actually post my response because work was busy. The ink from your original post was still fresh when I started writing...

    Oh, the problem is with the multimeter.


    Trevor G

    PS See if it still produces a reading while "hovering" with one lead at least connected to the battery negative terminal.

    See what happens with both leads connected to battery negative.

    You should have no reading on the meter, well, not above zero!

    Then change to Ohms and short the two test leads together - you should have a reading of less than 1 ohm. If higher (let me know what the reading is) you could have several problems...

    Do the same test with the meter on volts, ac or dc, doesn't matter. With the two leads shorted together you should have a zero reading.

    I just threw out 2 digital multimeters the other week because they were not reliable. In many cases a cheap analogue meter with its relatively low input impedance (20,000 ohms per volt) is safer, especially when dealing with earthing issues, than the high impedance, 10 megohm input, digital multis.

    Then again, the simple 12v test light is often possibly even better!