voltage at the coil

Discussion in 'Inboards' started by James Francis, Jun 6, 2009.

  1. James Francis
    Joined: Sep 2004
    Posts: 21
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Cape Cod

    James Francis Junior Member

    I replaced the coil and ballast on My chris craft 305K and I was wondering what the voltage should be at the coil with the engine running? I put the leads on the positive and negative side of the coil and I was getting a around 4 volts???


  2. CDK
    Joined: Aug 2007
    Posts: 3,324
    Likes: 148, Points: 63, Legacy Rep: 1819
    Location: Adriatic sea

    CDK retired engineer

    The readout depends on the equipment you use and whether the engine runs or not. With the engine not running and a normal volt meter 4 volts may be correct.
    To see what really happens you need an oscilloscope. You'll see 12 volts just after the contact closes and over 200 when they open.
  3. Rangerspeedboat
    Joined: Apr 2009
    Posts: 120
    Likes: 3, Points: 0, Legacy Rep: 17
    Location: Texas

    Rangerspeedboat Senior Member

    Does the engine run good? If so, dont try to fix what isnt broken.
  4. broke_not
    Joined: Jun 2008
    Posts: 60
    Likes: 3, Points: 0, Legacy Rep: 53
    Location: North Dakota

    broke_not Junior Member

    4 volts sounds too low, but the voltage coming from the ballast resistor depends on the voltage being sent to the resistor. What is the voltage at the battery with the engine running?

    Back when everything used breaker points ignition systems, we typically saw from 7 to 10 volts going to the positive terminal on the coil with the engine running....in other words at least half of what the battery voltage was with the engine running. (Assuming the charging system is working properly of course and is at ~14 to 14.5 volts.) These readings were with the common lead on the meter to ground and the positive lead to the positive coil terminal.

    Having access to an oscilloscope would be nice, but back in the day when breaker points were the norm, many of the Mom and Pop neighborhood garages would have been out of business if a 'scope were required for troubleshooting.

    If your voltage at the battery is normal when running, you may have too much resistor there, or there may already be a resistance wire buried somewhere in the harness.....

  5. James Francis
    Joined: Sep 2004
    Posts: 21
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Cape Cod

    James Francis Junior Member

    Thanks for all the answers. First, the engine fires up and runs perfectly with the original mallory flat cap single point and condenser. I have only tested voltage at the coil ( leads on positive and negative) with the engine running and the ballast resistor in line (mallory # 700) and the voltage reads about 4. I just thought was a little low.

    Thanks again.
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.