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Technical how can a coil be tested?

Discussion in 'The Hokey Ass Message Board' started by atch, Jun 19, 2018.

  1. atch
    Joined: Sep 3, 2002
    Posts: 4,373

    atch
    Member

    I'm cleaning in the shop and came across a Delco Remy coil. I don't remember where it came from or what it was off of. It is obviously used. Can I test it? I'd like to know if I should keep it or send it to the metal recycler.
     
  2. henryj1951
    Joined: Sep 23, 2012
    Posts: 2,310

    henryj1951
    Member
    from USA

    Stick yer tongue on it like a 9volt...j/k the experts ell be here any second...lol
     
    HiHelix likes this.
  3. tubman
    Joined: May 16, 2007
    Posts: 4,879

    tubman
    ALLIANCE MEMBER

    If you have a multi-meter, you can do a preliminary test. The primary (between the two smaller poles) should be between 2 to 10 ohms. The secondary (between the tower and ground) should be 6,000-12,000 ohms. Zero ohms on either is an open, and infinite ohms is a short and the coil is no good. Please note these are preliminary tests only. There are other variables, among them heat and vibration. This should be enough to tell you if you should keep it or toss out, though.
     
  4. A coil can pass a bench test and after 5-15 mins run time fail.
    They are tricky rascals
     
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  5. sevenhills1952
    Joined: Mar 14, 2018
    Posts: 907

    sevenhills1952

    You can resistance test it. I just measured one and it was 1.5 ohms from + to --, then 10.68k ohms (10,680 ohms) from either terminal to center tower.
    Also take a plug wire tower to spark plug. Clip lead + to battery, have a clip lead hooked to --. Be sure plug threads touch battery ground, then touch that -- clip lead to battery ground for a second. When you release lead plug should fire. I just tap lead on battery-- about twice a second, plug should fire pop-pop-pop-pop-pop!

    Sent from my SM-S320VL using Tapatalk
     
    Terrible80 likes this.
  6. sevenhills1952
    Joined: Mar 14, 2018
    Posts: 907

    sevenhills1952

    " infinite ohms is a short"
    Well, actually infinite ohms is an open, just like leads in air.
    A short is zero ohms.

    Sent from my SM-S320VL using Tapatalk
     
    Blues4U likes this.
  7. sevenhills1952
    Joined: Mar 14, 2018
    Posts: 907

    sevenhills1952

    Very true.

    Sent from my SM-S320VL using Tapatalk
     
  8. stubbsrodandcustom
    Joined: Dec 28, 2010
    Posts: 1,195

    stubbsrodandcustom
    Member
    from Spring tx

    This is the true "traditional" way, please post a video for us to make sure you're doing it right. hehe
     
  9. atch
    Joined: Sep 3, 2002
    Posts: 4,373

    atch
    Member

    Thanx all. I'll run the multimeter test and let you know the results.
     
  10. Blues4U
    Joined: Oct 1, 2015
    Posts: 4,503

    Blues4U
    Member
    from So Cal

    An open would be infinite ohms, zero ohms implies no resistance at all.
    A short could be between the two coils, which you could check for by checking for continuity between them. Though a short may not present itself at the low voltage of a test meter, yet be present under high voltage of operation.
     
  11. sevenhills1952
    Joined: Mar 14, 2018
    Posts: 907

    sevenhills1952

    You mean like this Model T baby...I just put it on my bench power supply. It was buzzing away...hmmmm...I wonder if it still works, so I quickly touched the high voltage terminal. Thank God I had towels on bench when I dropped it.
    I was feeling like a nap...not now! 20180619_130539.jpeg

    Sent from my SM-S320VL using Tapatalk
     
  12. atch
    Joined: Sep 3, 2002
    Posts: 4,373

    atch
    Member

    Results:

    + to - infinite
    + to tower infinite
    - to tower infinite

    It'll be in my annual load to the scrapper.

    Thanx for the help.
     
    lothiandon1940 likes this.
  13. saltflats
    Joined: Aug 14, 2007
    Posts: 9,916

    saltflats
    Member
    from Missouri

    Check the numbers on it.
    Some restorers are always looking for a certain number (not sure what they are).
     
  14. tubman
    Joined: May 16, 2007
    Posts: 4,879

    tubman
    ALLIANCE MEMBER

    You're right, of course. Sorry for the mix-up. I was posting it and my significant other said I had to leave right then so I didn't get a chance to proof read it. It doesn't make a hell of a lot of difference though; both are bad.
     
  15. Truck64
    Joined: Oct 18, 2015
    Posts: 3,984

    Truck64
    Member
    from Ioway

    My old MoToRs Repair manual talks about a couple things.

    "The coil must be tested at normal operating temperature because defects often fail to show up on a cold test."

    But they aren't talking about a simple ohms test, they go on to talk about "high frequency coil testers". They go on to explain that the secondary windings can have just a few shorted turns. The standard ohms test will pass because it is still in the ballpark range, and, the peak voltage will not markedly change.

    The length of the spark produced will not change either, but, the length of time that the spark lasts will be considerably shortened due to the dampening effect the shorted turns have. So under load, high compression, worn plugs etc, the spark may not be able to provide good ignition. Tough to diagnose, that. Shorted turns will cause the coil to run excessively hot, causing even more secondary turns to short causing eventual failure.
     
  16. Ebbsspeed
    Joined: Nov 11, 2005
    Posts: 4,949

    Ebbsspeed
    ALLIANCE MEMBER

    In pretty much any automotive coil, the primary and secondary are connected to a common terminal, hence there WILL be continuity between the primary and secondary windings. If this were not the case the coil would have to have four terminals on it. If the windings were not connected, magic would be required to produce a spark.
     
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  17. Huh? It’s the magnetic field collapsing over the windings which induces the voltage for spark....no magic and no connection between the two

    Edit. Both are connected through the low side when the points are closed
     
  18. Ebbsspeed
    Joined: Nov 11, 2005
    Posts: 4,949

    Ebbsspeed
    ALLIANCE MEMBER

    The points have nothing to do with it. Both windings are connected INTERNALLY, within the oil-filled (or epoxy filled) coil canister. See diagrams below of two different versions of ignition coils. See the common connection on the "+" terminal?

    Many mistakenly believe that the coil canister needs to be grounded, and also believe that ground is the other "end" of the secondary winding. A coil does not need to be grounded to function. Coils.jpg
     
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  19. Marcosmadness
    Joined: Dec 19, 2010
    Posts: 350

    Marcosmadness
    Member
    from California

    Whenever I hear (or read) about coil problems I remember an "coil industry" article saying that something like 90% of the ignition coils that are replaced had nothing wrong with them... Personally, in my 73 years of working on cars I have had exactly one instance of a coil failing so at least some of them fail. But whenever I suspect a bad coil I think back to that article and say, "the coil is probably good so don't throw it away yet". Usually I find that the problem is in the ballast resistor. The coil of wire in the ballast resistor breaks but still works intermitently.
     
    Atwater Mike and clem like this.
  20. Nope, the canister does not need to be grounded. Ok probably a waste of my time but what the hell! Ever wonder how a coil puts out 30-40,000 volts when we have a six volt or twelve volt system? The primary winding is connected to the positive terminal on the coil which goes to battery voltage, I’m not getting into resistors or resistor wiring, the other end of the primary winding goes to the negative terminal on the coil which is connected to the points. With me so far? Now when amperage is run through a coil of wire a magnetic field builds around that coil of wire, this happens when the points are closed as we have a complete circuit. Store this away..... now on the secondary side there are far more coils of wire than on the primary side, when the points open there is no more current flow in the primary, the magnetic field collapses and the flux lines pass over the secondary windings, stepping the voltage up, which then goes to ground through the secondary side of the ignition.


    Sent from my iPhone using H.A.M.B.
     
  21. Ebbsspeed
    Joined: Nov 11, 2005
    Posts: 4,949

    Ebbsspeed
    ALLIANCE MEMBER

    If this was intended for me then, yes, you are wasting your time. When did the question become "How does a coil work?" I learned all that in the '60's, long before I earned my electrical engineering degree.

    It was you that made two incorrect statements in post #18. Any time I can correct some bad electrical info on this board I will, and that is what I was attempting to do when I quoted your post #18:

    1. "no connection between the two" referring to the primary and secondary windings

    2. "Both are connected through the low side when the points are closed."

    Now I'm going to stop wasting my time.

    With me so far?
    Store that away.
     
    henryj1951 likes this.
  22. Wasn’t intended for you personally, I think there is lots of misconceptions about how some stuff works but as always, someone needs to get their shorts in a knot and take everything as an attack on them, sorry if that’s how you took it, not at all how it was intended to be or I would have said so. The reason I don’t post much here..... now I do have a legitimate question for you, if the primary and secondary windings are connected at the positive terminal, internally of course, what stoops the induced voltage from backfeeding the ignition circuit? Or is it such little amperage we just don’t worry about it? As I said, wasn’t an Attack on you, hard for people to understand how to test stuff if they don’t know how it works.

    And you are correct that my second comment was incorrect, I was trying to type with an eight month old demanding attention! Should have proofread....
    Sent from my iPhone using H.A.M.B.
     
    Last edited: Jun 19, 2018
  23. utahdodge
    Joined: Sep 13, 2008
    Posts: 186

    utahdodge
    Member

    [​IMG][​IMG]

    Can they be (economically) rebuilt? This one is out of my '16 Hudson and although it is running with a modern unit, I would love to have this guy mounted back on the firewall.


    Sent from my iPhone using The H.A.M.B. mobile app
     
    Truck64 likes this.
  24. gatz
    Joined: Jun 2, 2011
    Posts: 1,446

    gatz
    Member

    In recap then... (I need to get this down)

    Primary circuit: 2 to 10 Ω
    Secondary (from tower to case): 6k to 12 Ω
    Short: zero Ω
    Open: infinite Ω

    does voltage affect those values? i.e 6v or 12v

    Didn't the old Sun Engine Analyzers have a coil tester that could detect that?
     
  25. Truck64
    Joined: Oct 18, 2015
    Posts: 3,984

    Truck64
    Member
    from Ioway

    Probably, I dunno. A 'scope would probably sniff out shorted turns in the secondary. All the traces and parade and that stuff. Spark duration.

    I would agree everybody must have kept the coil people happy back in the day replacing perfectly good coils.
     
  26. Truck64
    Joined: Oct 18, 2015
    Posts: 3,984

    Truck64
    Member
    from Ioway

    Maybe talk to some coil (radio) rewinding guys. They had some interesting tools for rewinding oscillator, detector, and antenna coils, and transformers. That's what an ignition coil is basically. Maybe Bubbasignition.com has some ideas. I don't know what your idea is of economical though. Couple hundred bucks for a commercial job. No idea though really.

    Hard to tell from that pic, but it almost looks like you could stuff a modern coil inside it and then make the connections externally. That might be the way to go. Maybe a motorcycle coil or a riding lawnmower, something that will fit and meet the specs.
     
  27. MAD MIKE
    Joined: Aug 1, 2009
    Posts: 463

    MAD MIKE
    Member
    from 94577

    Not resistance values, no.
    Doubling the voltage will double the output voltage, but at the cost of insulation durability. It may work for a bit but it could burn out the coil(internally) if it is used outside of its voltage rating.
     
    sevenhills1952 likes this.
  28. Mr48chev
    Joined: Dec 28, 2007
    Posts: 26,563

    Mr48chev
    ALLIANCE MEMBER

    I have an old coil tester that runs off 12 volts that you connect the coil to and it will throw a spark between a post and a movable wheel setup that lets you see how hot of a spark it puts out. Probably more of a 40's early 50's sales aid but it works pretty well to show you if the coil works or doesn't work. I'l try to remember where I have it stuck away and get a photo or two..
    The info Gatz posted in post 25 looks pretty accurate but every time I tested a coil with a multi meter I had to dig out the book and get the right numbers.
     
  29. Ebbsspeed
    Joined: Nov 11, 2005
    Posts: 4,949

    Ebbsspeed
    ALLIANCE MEMBER

    @57 Fargo I apologize for the slight rant, thought you were throwing shade my way. In any case, here's a bit I copied from an ignition system site which explains towards the end the reason the primary and secondary windings being connected really doesn't impact the electrical system. Also, to satisfy your curiosity as to whether or not there is an internal coil connection between the three terminals take an ohmmeter and put the two leads on any two terminals. Doesn't matter which two, you will see they are all connected. Assuming a good coil, of course.

    From the article:
    The ignition coil is essentially a low voltage to high voltage transformer with about 100 to 1 ratio of windings and voltage. The coil case is not grounded, and both primary and secondary windings inside are "floating" or isolated from the case. The only thing the windings have in common is one end connected to the same primary terminal, and it really doesn't matter much which one. Being a transformer it must have pulsating or alternating current to work. Initial pulsating is done by connecting and disconnecting the primary circuit ground connection. Alternating current then comes into the function in a big way by electrical "ringing" in the condenser at very high frequency. A transformer is not affected by polarity, since it is an alternating current device, so it matters not to the transformer what the input or output polarity may be. Any polarity on the primary side and any polarity on high tension side will produce the same quality of spark.

    Why then do we worry about coil polarity? Because the spark plugs do care which way the electrons are flowing in the high tension circuit. The spark plug has a thermally insulated center electrode (surrounded by ceramic). With engine running the center electrode runs substantially hotter than the exposed end electrode. Design of the ceramic insulator determines how hot the center electrode will run, leading to the designation of hotter or colder spark plugs. As electrons go, they love to jump away from a hot surface and fly toward a colder surface, so it is easier to drive them from hot to cold rather than from cold to hot. End result is a difference of 15 to 30 percent in voltage required to make spark "initially" jump the gap on the plug depending on which way it is going. So the spark plug prefers to see a voltage potential that is negative on the center electrode and positive on the end electrode for the very first hop of the spark. Oddly enough, this has nothing to do with polarity of the vehicle electrical system, but it is influenced by the common connection inside the ignition coil.

    The common knowledge bit about electrons is that they carry a negative charge. For electrical bits (similar to magnetic bits) opposites attract each other and negatives repel. This means the direction of flow of electrons in a car is from the battery negative post through the wiring to the battery positive post (not necessarily intuitive). If you reverse cable connections on the battery the current flows in the opposite direction through the vehicle wiring. For most original functions on the MGA this matters not one whit to anything, as most original equipment in the MGA is not polarity sensitive (except maybe the optional radio). As one end of the primary winding in the ignition coil is connected to one end of the secondary winding, reversing polarity of the coil primary side will reverse the drive direction of the spark current on the output side (even though current in the vehicle low voltage wiring still flows the same way).

    So reversing vehicle electrical system polarity will reverse direction of spark drive. The engine still runs either way, but spark might be more reliable under marginal conditions if you get it right. The simple fix for this is to reverse the two primary wire connections on the ignition coil. Because the output spark is very much higher voltage (20,000v) than the car battery (12v), it doesn't care if the battery polarity is helping or hindering by a meager 12 to 14 volts in battery potential.
     
  30. Truck64
    Joined: Oct 18, 2015
    Posts: 3,984

    Truck64
    Member
    from Ioway

    IMG_0283.GIF

    IMG_0282.GIF
    I remember that page some time ago here:

    http://mgaguru.com/mgtech/ignition/ig104.htm

    A coil hooked up either way will work, but it's kind of a headscratcher if it's an older coil to determine from the markings (+) or (BAT) which is the common connection. If someone converted to negative ground, the coil may have reduced output? I thought coils have a magnetic core, right? I think Bubba mentioned something about this in one of his posts. Even if re-connected right it will show reduced output.

    Analog voltmeter connected as in the pic for testing will allow you to sleep at night, everything is right in the world. 12 volt meter works fine. I didn't do the pencil test, because I got skeered figuring I'd get zapped.
     

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