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Testing a Coil?

Discussion in 'The Hokey Ass Message Board' started by bradley01, Oct 13, 2008.

  1. bradley01
    Joined: Jul 13, 2008
    Posts: 31

    bradley01
    Member

    Hey guys. Is there any way to test a coil? I got my '39 all buttoned up, new rear-end gear put in it, and I was ready to fire her up. It started and ran GREAT before I tore the rear-end apart...but now it won't fire. It cranks fine, but I get no fire at the plugs. I'm thinking it is a bad coil, but i'm not sure. Is there way to test it on-car? I am running a 12v conversion, so would it be a 12v coil? Please forgive my ignorance...but I inherited this car and don't know much of the history about it. I do know that it has a positive ground setup. Any type of help or pointers would be greatly appreciated!
     
  2. Shifty Shifterton
    Joined: Oct 1, 2006
    Posts: 4,964

    Shifty Shifterton
    Member

    Coil wire in one hand, other hand on the fender. By the 4th revolution you'll know if that coil works. :)
     
  3. bradley01
    Joined: Jul 13, 2008
    Posts: 31

    bradley01
    Member

    That sounds like a BAD idea! I was hoping for a GOOD idea!
     
  4. bradley01
    Joined: Jul 13, 2008
    Posts: 31

    bradley01
    Member


  5. Saxon
    Joined: Aug 9, 2008
    Posts: 2,157

    Saxon
    Member
    from MN

    Actually It won't hurt, it's a jolt, but it won't hurt. I don't know how many times I've been zapped by a plug adjusting the carb. Man up :]
     
  6. Usually all you have to do is ground the coil wire to the block somewhere, have someone crank it over and watch for a spark, which should be strong and blue. Or swap another coil in quick. I always keep a "known" coil around for such purposes.

    Bob
     
  7. Saxon
    Joined: Aug 9, 2008
    Posts: 2,157

    Saxon
    Member
    from MN

    Could be the condenser as well. Try a different coil.
     
  8. bradley01
    Joined: Jul 13, 2008
    Posts: 31

    bradley01
    Member

    Yea...I don't have another coil handy. I'll go get one and then try that. I'll try grounding the output wire to the block and see what happens first. I'm worried that the points could be bad too. I'm not sure though.
     
  9. 39cent
    Joined: Apr 4, 2006
    Posts: 1,569

    39cent
    Member
    from socal

    oldtimer showed me how he checked a coil. Pulled dist cap off and put a half dollar in the middle and hav someone turn over the engine. ggggg it worked
     
  10. You DON'T ground the coil wire that's pulled from the center of the dizzy cap. You hold it about 1/4" away from ground, and have someone crank it over and look for a healthy spark. There won't be a spark if it's grounded
     
  11. Deuce Daddy Don
    Joined: Apr 27, 2008
    Posts: 5,224

    Deuce Daddy Don
    ALLIANCE MEMBER

    Coils RARELY go bad, also did you know that there is NO DIFFERENCE between a 6 volt coil & a 12 volt coil?????????---------Check condensor & points too!!--------Don
     
  12. jdj9410
    Joined: Sep 4, 2007
    Posts: 324

    jdj9410
    Member
    from Paris TX

    You can isolate it to determine if it is points or coil also. Crank check it like bobss396 said. If there is no spark that tells you it is either coil or points system. To isolate the coil and check points and condenser. Have the coil wire close to a ground like when you crank checked it for spark to jump a gap. Take a jumper wire and clip on negative side of coil. With ignition on to power coil, momentarilly ground other end of jumper wire. If coil is good it will have a blue spark jump gap. It is the points. If no spark it is coil. Your just bypassing the ground function of the points.
     
  13. That's true in theory, the coil doesn't know 6v from 12v or any other volt for that matter. But in practice, back in the dark ages of 6v systems and early 12v there was a difference in construction. Most 6v coils have an internal resistance of 1.5 ohm or less, 12v generally had 3 ohms or so.
    You can use a 6v coil on a 12v conversion but you would need to run a ballast resistor with it.
     
  14. 32fordpickup
    Joined: Apr 20, 2008
    Posts: 30

    32fordpickup
    Member
    from ohio

    Take a test light ground the test light check the positive side of coil should be constant power there.crank the engine while holding the test light on the negative terminal of the coil it should pulse on and off telling you your point are giving a signal to your coil .If it doesn't pulse check your points to make sure they are opening and closing hope this helps.
     
  15. JAWS
    Joined: Jul 22, 2005
    Posts: 1,846

    JAWS
    Member


    You asked how to test a coil. Grab your DVOM and lets get to it.

    1. remove the wiring and keep it in order.
    2. put the meter in the ohms scale.
    3. test across the primary circuit.
    a.That is the positive and negative stud connected to the primary winding in the coil that wraps around the secondary winding. It makes a field around the secondary winding and it collapses when the current is broken. Amplifying the pulse because it is absorbed by the secondary winding, which is much larger, and it is expelled through the coil wire to a ground source, ie distributor cap terminal to rotor to plug wire terminal to wire to plug to block. Whew! OR coil wire to fender or bolt that you have the wire end just held away from...:D

    You should have about .5 to 1.5 ohms resistance on the primary circuit. If you do, then that circuit is good.

    4. Set the DVOM if it does not auto range itself to the 10-20,000 ohm scale
    5. Test the secondary winding in a similar manner. You can use either the negative terminal or the positive terminal, direction of current does not matter. Test across one of them to the terminal for the coil wire. You should see around 9,000 ohms or so give or take a few thousand.

    What you are looking for here is a major inconsistancy, like a shorted winding that would show you basically no resistance or an open winding which will show you OPEN or OL.

    This is only for testing your coil.

    As was suggested earlier, use your test light or meter to find if the points are breaking current as they should or open or closed constantly.

    The point gap should be around 19 thousanths .019"

    Hope that helps..:D
     
  16. str8axleford
    Joined: Oct 14, 2007
    Posts: 92

    str8axleford
    Member

    If you are running points, you should run a ballast resistor. putting 12 volts to the points will burn them. I learned this the hard way on my 69 nova I inherited from my brother when I pulled the engine he blew and put in another. I put on his heads because they were worked, and it burned through the points in a short time. It was one of them instances where my dad just let me do it and laughed when it happened. You know how it is when your 18... your stoked about getting it going... all that hard work. I asked my dad what happened, and he said go look at the chevelle's setup... I looked and saw the resistor, and went and got one. they are (were) less than 10 bucks at shucks. So I got the new points and resistor, wired it up set my dwell, and away I went smokin tires. It had a stock 283 with 194 fuelie heads and it would smoke tires all day... It was a slouch but it was cool. My dad wasnt laughing when I bent his axles in his chevelle though.. It was a 68 ss 427 with a hell of an ultradyne cam through a rock crusher, into 456 gears. Man that thing tore up asphalt and frame rails... Axles too... lol
     
  17. dante81_98
    Joined: Sep 26, 2005
    Posts: 504

    dante81_98
    Member
    1. A-D Truckers

    Ok, so I did this test today and ended up with power to both posts on the coil when the key was on. Neither post would pulse when I cranked it over. Does this mean the points are toast? It is absolutely possible as I haven't done anything with then yet.

    Thanks,
    Chad
     
  18. dante81_98
    Joined: Sep 26, 2005
    Posts: 504

    dante81_98
    Member
    1. A-D Truckers

    anybody? have any advice for a guy just stepping out of the late model world into some older stuff?
     

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