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Customs Coil - Electrical Test

Discussion in 'The Hokey Ass Message Board' started by 64impala, Aug 20, 2016.

  1. 64impala
    Joined: Sep 14, 2012
    Posts: 79


    Mallory HEI

    With the ignition set to the off position, what type of reading should I get on either a voltage meter or regular light test?
  2. If the key is in the off position you should have no power at the coil. If it still had power then you couldn't turn the ignition off. Hope that's what your asking.
    loudbang likes this.
  3. What are you looking for? If you want to check the coil, it needs to be removed to accurately check it. As Mallory doesn't publish any specs for their coils, I'd look for something in the .3 to .5 ohm range on the primary side, and between 7K to 9K ohms on the secondary side. It should read 'open' between the primary and secondary.
    loudbang likes this.
  4. do you still have the ignition ballast resister ?

  5. 64impala
    Joined: Sep 14, 2012
    Posts: 79


    yes, I'm running a ballast resister.

    When the ignition in the "off" position, I'm getting a 0.0 voltage reading on both positive & negative side of the coil.
    When I connect a light tester on the coil + & -, the light comes on, is that normal.
    Both the coil and ballast are cool to touch....
  6. If you're using a continuity-type tester (with a battery to power the light), that would be normal...
    loudbang and Mr48chev like this.
  7. Mr48chev
    Joined: Dec 28, 2007
    Posts: 32,352


    You should only have power to the coil when the key is on or when the starter is turning the engine over and the resistor bypass is clicked on.

    I'd agree with Crazy Steve in that your "light tester" is a battery powered continuity tester and yes you should have continuity though the primary winding of the coil.
    To go along with what Steve said on testing the coil with an ohm meter scroll down this page an there is a diagram snowing testing both the primary and secondary windings

    Now for the big question, why are you wanting to test the coil and what is the problem you are trying to figure out?
  8. 64impala
    Joined: Sep 14, 2012
    Posts: 79


    The original problem was that the engine was starting and running fine for years, no issues.
    My original Mallory coil failed as did my optic module and ballast, all 3 were replaced about a year ago.
    Then recently, the engine would start, run for as long as 10-12 or as short as 5 mins (just warming up) and the engine would just die (no sputtering, nothing).
    I replaced a 16 year old distributor cap, rotor and cracked spark plug wires.
    Engine started, but same results, fuel pump tested good and no overheating.
    The problem occurs randomly, and did not occur today.
    Voltage meter check on coil and optic module passed, just looking for more clues.
  9. 64impala
    Joined: Sep 14, 2012
    Posts: 79


    Mallory Parts:
    Coil: 29440
    Distributor: 4748201
    mallory coil.jpeg mallory distributor.jpeg
    Joined: Dec 27, 2009
    Posts: 1,270


    I have had nothing but "issues" with the Unilite Distributor.
    The modules "suck".
    Just my $ .02.

  11. 6-bangertim
    Joined: Oct 3, 2011
    Posts: 404

    from California

    Unlite or HEI?

    On 2 different GM vehicles with HEI, I've seen the module cut out when hot. One would restart immeadly, the other would need to cool down for several minutes, maybe up to an hour to fire off - man, it drove me NUTS!!! Replaced the coil, no change. A new module cured it. :)

    When the car shuts down, connect a test light between the coil (-) and ground. Have a helper turn the key ON - the test light should be BRIGHT. Have the helper crank the engine - the light should cycle between bright and dim, module OK. If the light remains BRIGHT, module is BAD.

    Do you have a spare coil to swap out on a test run? Me thinks it's NOT a coil issue - the module has the same function as a set of points - to interrupt current in the coil primary, induce it into the secondary.
    Good Luck, Tim
  12. Ebbsspeed
    Joined: Nov 11, 2005
    Posts: 5,999


    Most ignition coils have a common internal connection between the primary and secondary windings, so the above statement is not always true.
  13. That's true for single-tower external coils (the typical round coil), but much less so for other types.

    And I'll agree with Tim; it's probably a module....
  14. 64impala
    Joined: Sep 14, 2012
    Posts: 79


    Coil Reading this morning (Engine cold/)
    With the key on, voltage positive reads: 11.74
    With the key off, voltage negative reads: 0.0
    Battery: 12.50
    Mallory Optic Module Test: Passed Voltage reading open & blocked optic.

    Additional Tests:
    - Started engine cold: Started with no issues, ran for 10-12 mins. with no failures.
    - Restarted engine warm: Started with no issues, ran for 5-6 mins. with no issues.

    -Started engine cold: Started with no issues, ran for 5-6 mins, engine cut out.
    -Attempt to restart warm failed.
    -Attached in-line spark tested to random plugs, no spark.

    Is it necessary to test spark from coil wire to distributor cap?
    If so, what tools are required and procedure (spark tool)?

    Will adding thermal grease to internal distributor assist in elimination of possible problems or will it hurt it at this point.

    I do have a Mallory optic module spike protector installed, would removing that help diagnose the problem?

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