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Thread: 51 M38 - over my head

  1. #51
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    good stuff - thanks!

    I have tried putting my light bulb meter on that center well - and have not gotten a reading - it seems as if when the gap points are open - that should be giving a reading - do you think I should try your other method of checking?


    to me it seems like I have a bad coil - seems unlikey since this a new rebuilt distribtuor - but as I am reading your notes - it may well be bad

    thanks again - I appreciate the notes and encouragement!

  2. #52
    Super Moderator LarrBeard's Avatar
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    "I have tried putting my light bulb meter on that center well"

    Even if the coil and points were working perfectly, you would not see anything with the light. But,if you got a jolt that knocked you on your backside - that would be a good sign.

  3. #53
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    I got a new coil -same results....
    Dumb question first - as I am getting readings with my bulb test across both sides of the coil - that power is gong to a the condenser and then to the rotor - right? - should I get a reading on the end of the rotor? I am not

    I am thinking when the rotor hit the contacts the circuit is complete and shoots to the plug - but it seems the grounded light bulb would do the same

    When turning the motor and watching the gap opening, it doesn’t appear to open when passing the contacts in the cap of the distributor- and I can’t turn the distributor enough to make that work. It seems that gap should be open as the rotor is passing the contacts -

    Could the distributor be set wrong?

    I think the distributor could only go in one way - but perhaps I should put it and try to reset another way?

    I see some stuff about resetting the oil pump but that seems like a big job and I don’t think this one has been altered

    Thanks again!
    Last edited by Will Johnson; 04-05-2020 at 05:17 AM.

  4. #54
    Super Moderator bmorgil's Avatar
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    Will, I don't know if you have seen this but start here. https://willysjeepforum.kaiserwillys...Ambulance-M170 Page 167 begins the Ignition Section for the M38 24 volt ignition. This is the A1 but I think it is the same for all the 24 volt systems.

    The basics are this:

    When the ignition is on 24 volts positive is applied to the positive side of the coil Primary.
    Think of the coil as two parts, one side makes energy the "Primary". One side discharges this energy the "Secondary". There is more to it than this but, think of it this way.
    The points provide the Negative "ground" for the coil Primary. Think of them as a switch to ground.
    The condenser provides a clean voltage break. It does not handle much current. Unless it is shorted, its not the problem.
    When the points close the coil's primary has a complete circuit and the Secondary side "fills with Energy (voltage)".
    The instant the points open the storage of energy in the secondary side of the coil is so large it must exit somewhere.
    It needs a path to ground. It discharges this energy out the coil wire and into the cap.
    The rotor is the director of the energy pulse. The coil wire transfers the energy to the rotor. Wherever it is pointed, the voltage is headed that way.
    The spark plug wire takes it to the plug where the energy is dissipated across the gap of the plug to ground (negative).

    If you look at the circuit diagram, use your meter to be sure you have the correct voltage where it needs to be.
    Last edited by bmorgil; 04-05-2020 at 08:31 AM.

  5. #55
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    Thanks - I have reading and stuff to check and report back

    Thanks for your patience - and sharing of knowledges

    Best

  6. #56
    Super Moderator LarrBeard's Avatar
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    Maybe some help ..

    Quote Originally Posted by Will Johnson View Post
    Thanks - I have reading and stuff to check and report back

    Thanks for your patience - and sharing of knowledges

    Best
    The high voltage spark from the coil (the contacts in the center) get carried from those center contacts to the rotor by a bar in the distributor cap. With the cap off, there is no connection from the high voltage to the rotor.

    As the rotor turns, it does not actually contact the connection to the individual plugs; there is a small gap that the spark jumps across from the tip of the rotor to each plug terminal.

    Your light won't tell you anything on the high voltage (secondary) side.

  7. #57
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    thanks for the info! so the power somehow goes up to the cap - I am betting thru the hole in the middle of the coil - via a spring in the cap - then out to the contacts in the cap - correct?
    not sure I completely understand - but somehow the rotor makes contact with those contacts in the cap and then out to the plugs

    If I am off base - let me know

    It seems looking at when the gap is open and I am getting the power to the contacts, they are past the point where they would be hitting the rotor -
    I can't adjust the distributor to a point where that would

    this why I think the distributor may be set wrong

    check my photos -

    thanks for your knowledge and patience

    stay safe
    Last edited by Will Johnson; 04-08-2020 at 01:33 AM.

  8. #58
    Super Moderator bmorgil's Avatar
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    Will, I think you have it. The rotor "sweeps" past the contacts on the cap. They are seldom lined up perfectly. As Larry pointed out, they do not actually touch. When the spark travels via the "Bar" (or coil wire in a non military), from the coil to the cap (the instant the points open), The energy travels down the cap through the center electrode. From there it travels to the rotor directly through the rotors contact at the middle. It then travels across the rotors electrode and whichever Distributor electrode is closest, it travels to. Then out the cap and so on to the plug the rotor is pointed to. As you adjust timing, you move the position of the rotor in relation to the points opining point, retarding or advancing the timing.

    First set the point gap, then set the timing. Check the firing order. Keep us posted!
    Last edited by bmorgil; 04-08-2020 at 06:19 AM.

  9. #59
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    thanks - I am starting to understand - I assume this concept work on old tractors - I also have a 50's International Harvestor tractor my son is working on - I am hoping I can use what I am learning on that

    back to my issue - I have the gap set, and firing order - but the I can't get the timing right - the distributor won't advance or retard enough either way for rotor to be in the direction of the cap contacts, when they are in the open position.

    that is why I think I have set the distributor incorrectly or perhaps the oil pump is set wrong...

    thanks again for your help!

  10. #60
    Super Moderator LarrBeard's Avatar
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    M38A1 Distributor Cap

    I was finally able to find some good pictures of the M38A1 distributor cap. Yes, the spring fits down into the “hole” terminal of the coil and there is a bar in the cap that brings the spark over to the little button in the center of the cap. That button rides on the tab atop the rotor.
    Attached Images Attached Images

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