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Thread: Compression Test & the Starter Motor

  1. #31
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    glen's Avatar
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    Re: Compression Test & the Starter Motor

    Hey folks, sorry for the delay. Life attacked and I had to put this one on the backburner. I see y'all have been busy here in my absence! And I appreciate all the info. Truth is, there's nothing wrong with my bike that I know of (except for that random clicking noise). I'm just working through the manual in order to learn as much as I can about the machine. Also, when I finally got time to try it again, the parts store no longer had a compression gauge for rent. So, I'll keep you posted after I find another one.

  2. #32
    Site Supporter Jim Van's Avatar
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    Re: Compression Test & the Starter Motor

    Quote Originally Posted by Slydynbye View Post
    OK found the connector, it's tucked up behind the left pocket. You can see it by looking in and up past the valve cover looking forward from the rear after removing the maintenance cover. I think that is probably the best and safest way to disable the ignition.
    Nice to know !! Now I can add cleaning those connections to my yearly list of PM chores to accomplish !!
    IBA #35372
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  3. #33
    Site Supporter Jim Van's Avatar
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    Re: Compression Test & the Starter Motor

    Quote Originally Posted by dwalby View Post
    OK, I'll preface this by saying I know nothing about coils other than the basic electrical properties involved, but I'd like to understand them better.

    The secondary voltage is basically proportional to the primary voltage times the turns ratio, right?

    When the primary voltage is switched off (by the ignition module?) the secondary potential is ready to discharge, but if you don't discharge it what happens? Instead of the voltage dropping as the charge is transferred from the coil to the plug, the voltage just stays constant (approximately??) like a battery with no load. This process repeats itself continually as the ignition module turns the primary voltage switch on and off.

    So how would the ignition module get damaged? (not a contradiction of your claim, its a question in an attempt to learn)

    Sorry, I'm a little late chiming in here ......

    To really understand what's happening, you have to consider the inductance of the coil primary and the action of a capacitor inside the Ignition Module. The I.M. charges the coil primary ( inductor). Energy is stored in the coil primary because of the current ( E= .5 * L * I^2 ) . Then the current source is turned off, but with the help of the capacitor, current continues to flow ( now through the coil primary & capacitor). The current decreases while charging the capacitor. A change of current induces a voltage into the coil secondary. The primary tries to oscillate for a while ( because of the inductor & cap interacting ) and would if there was no load on the secondary and the primary voltage would get a heck of a lot higher if the spark plug was not connected. But with the spark plug connected, Like Bush said, the secondary voltage is limited and the energy is transferred from the primary to secondary and then spark plug.

    By knowing the inductance of the primary & secondary, turns ratio, and some other parameters, a mathematical model using a Laplace Transform can show what happens over time with the spark plug disconnected.

    But Ya, disconnecting the coils from the I.M. by unplugging that connector is the quick & easy way to protect the I.M. when the plugs are removed when doing a compression check.

    With the CV carbs on the ST1100, it seems to me the slides also need to be lifted ( some way ) when doing a compression check. I did a compression check on a Moto Guzzi without holding the throttle open, and I got some really low readings until I realized I needed to open the throttle to get air into the cylinders.
    Last edited by Jim Van; 11-11-2017 at 09:50 PM.

  4. #34
    Bush's Avatar
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    Re: Compression Test & the Starter Motor

    Quote Originally Posted by Jim Van View Post
    ( E= L * I^2 )
    You just made that up, didn't ya?

  5. #35
    Site Supporter Jim Van's Avatar
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    Re: Compression Test & the Starter Motor

    Not really ..... but I forgot to multiply by .5 , so it should have been : E= .5 * L * I^2 For a capacitor the energy stored = .5 * C * V^2

    For mechanical Energy : E = .5 * M * V^2 See the similarity between the three equations ? For an Inductor ( L ) the energy stored is a function of the current squared. For a Capacitor ( C ) the energy stored is a function of the square of the voltage stored in the cap. And for a Mass ( M ) , the mechanical energy stored is a function of the square of the velocity.

  6. #36
    Site Supporter Jim Van's Avatar
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    Re: Compression Test & the Starter Motor

    BTW, a Capacitive Discharge Ignition system works much differently than the above explanation.

    With a CDI system, the ECM charges a fairly high value capacitor, then it is discharged into the coil primary. This cause the primary current to increase , instead of decreasing, but the change in primary current still induces a voltage into the secondary, and a spark is produced at the spark plug.

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