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Conundrum about meaning of "Dwell" and "Dwell Meter"

  1. Apr 14, 2017 #1

    jim hardy

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    In the days of points everybody used a Dwell Meter to check their points gap setting.

    Even this old PF thread is vague as to meaning of "dwell" as regards coil charge time.

    The original Kettering patent used term "dwell" in another sense, namely points closed when engine is stopped which would run down the battery.
    He included a temperature sensitive iron wire to reduce coil current when points stay closed too long, sort of a 1912 PTC thermistor. It doubtless evolved to the ballast resistor we all knew and loved in the 1960's.

    Nowadays "Dwell" is defined as fraction of the time points stay closed, ie coil charging time expressed as degrees of camshaft rotation.

    A logical enough unit, but take a look at what a "Dwell Meter" reports !


    It's just a voltmeter connected across the points EDIT Later i found this was a bad assumption on my part , read on..


    points closed = no voltage
    points open = battery voltage (12 volts)

    so, more time the points are open = more voltage = increasing meter indication
    0 volts = points always closed = coil energized 100% of the time
    12 volts = points always open = coil energized 0% of the time
    If full scale indication = battery voltage 12 volts = points always open, coil energized 0 % of the time
    AND that agrees with the right hand end of the scale - a six lobe cam would be 60 degrees between lobes , meaning 60 degrees indicated dwell says points are open for the entire 60 degrees between cylinders and the engine ought not run because there's no time left to charge the coil between cylinders - the points never close.

    so i think there's widespread confusion afoot.
    I think a dwell meter reports not the number of camshaft degrees points are closed charging the coil,
    instead it reports the number of camshsft degrees the points are open between charging intervals.
    Despite what's written all over the internet.

    EDIT Atleast some dwell meters report correctly, as i found in post further down. Next line is wrong,too .
    Setting points for more dwell indication as indicated by a dwell meter will hurt high speed performance because it shortens coil charge time.

    Next time you have your dwell meter out, put a 9 volt battery across it and see what it reads ?
    I think it will read about ¾ scale, EDIT retracted, see further down
    which is 45 degrees on 6 cylinder scale or 33.75 degrees on 4 oops make that 8 cylinder scale.
    Then check carefully - does widening the point gap make your dwell meter read more dwell angle , or less ?

    I think the scale on dwell angle meters is backward. I'd love to know where i went wrong.
    In one case i found a dwell meter with scale printed in reverse which corrects the error, a few posts down
    I suspect most but not all read correctly. Also found two with active internal electronics to make them report correctly.

    old jim
    Last edited: Apr 15, 2017
  2. jcsd
  3. Apr 14, 2017 #2


    Staff: Mentor

    I don't know the answer @jim hardy, so I guess it is the "curmudgeon's conundrum." :wink:
  4. Apr 14, 2017 #3

    jim hardy

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    Hmmm here's instructions for an ancient marine engine
    Note they measure voltage across the coil not across the points.
    That would give the correct indication .

    I'll check a dwell meter's instruction manual if i can find one online.

    One needs to understand his test equipment in order to not get fooled by it.

    old jim
    Last edited: Apr 14, 2017
  5. Apr 14, 2017 #4

    jim hardy

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    says measure voltage across points EDIT but i was unable to tell from his manual whether it reads dwell or dwell's complement..

    from https://www.scribd.com/document/35604330/TACH-DWELL-Tester-Instructions
    they got it wrong too.
    EDIT I couldnt tell from this manual if it's right or wrong

    This one gets it backward too
    EDIT This one has the reverse printed scale so reads correctly

    So far the Atomic 4 Marine engine guy looks like the clear winner

    It's just something to be aware of.. If the proper setting for your car is midscale it won't matter which way you measure
    but there's a LOT at least some wrong directions in circulation out there.

    If you understand the measurement you can do it with just a voltmeter.

    Doggone right i'm a curmudgeon. :wink: My hero is Uncle Hub of 'Secondhand Lions' .

    old jim
    Last edited: Apr 14, 2017
  6. Apr 14, 2017 #5


    Staff: Mentor

    This image is broken.
    This one is broken, too.
    This might be an oversimplification, because the points don't just go from closed to open and stay there. I'm not an expert in the internal workings of a dwell meter, but I have used them to set points in multiple cars.
    I'm going to guess that the "dwell" that is being measured is the fraction of the maximum possible voltage that is attained before the points close again.
    Again, it's been years since I used a dwell meter (it was on a Chevy V8). I don't remember the exact figure, but I think the dwell angle was about 30 degrees. The points cam would have had 8 lobes, at 45 degrees apart.
    Here's a picture for a V6 engine.
    It seems to come down to how you define "dwell," either as the time the points are open or that they are closed. If you know one, you can calculate the other easily. In the image above, if you set the dwell high (i.e., points closed longer), the time available to charge the coil is less.
    BTW, I have two vehicles that came with points, both are motorcycles. One is a '46 HD WL (45 cu in motor, flathead), and the other is a '48 HD FL (74 cu in motor, OHV). The '48 is now running an electronic ignition, so no points, but the older one is still running points. I don't use a dwell meter on it - I set the timing statically, although I could set the timing on both using a strobe timing light.
    Last edited by a moderator: Apr 15, 2017
  7. Apr 14, 2017 #6

    jim hardy

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    I dont know what that means. I snipped and annotated it with Paint, then uploaded.

    A dwell meter EDIT make that voltmeter connected across the points will report 28 degrees
    A dwell voltmeter connected across the coil will report 32 degrees
    and the engine will run pretty well throughout its normal speed range with either setting .

    Agreed 2000 per cent.
    From https://www.collinsdictionary.com/us/dictionary/english/dwell-angle
    That is the definition i find in every article, fraction of a firing cycle that points stay closed which is the time available for current to build up a strong magnetic field in the coil. They call that "Charging the coil" which to me sounds intuitive. You're building up energy in the coil that you'll discharge into the sparkplug when points open.
    Hmm that comes down to how you define "charge the coil"
    I call "charging the coil" that interval of time when points are closed allowing current to rise building up an intense magnetic field, the 32 degree "Dwell" in your image. When the points open that field collapses forcing spark current into the plug.
    So for me, more dwell gives more charging time. But the instructions for the two dwell meters i found and linked tell one to connect his meter so it reads backward, reporting for your image ( 60 minus Dwell) degrees. Might call it "complement of dwell" or "Dwell's Complement" ..

    Now i suppose it's possible there is collusion between manufacturers of vehicles and manufacturers of dwell angle meters , and the number we're told to adjust for is really "Dwell's Complement" .

    Is there a GM design guy in the house ?

    old jim
    Last edited: Apr 15, 2017
  8. Apr 14, 2017 #7

    jim hardy

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    Here's a home-made dwell meter that properly reports closed time
    note it requires a power source meaning either a three lead hookup or an internal battery.


    The left hand 2N2222 'complements' the points open-closed status and reports it to the meter . So this one displays fraction of time points are closed not open.


    here's an instruction manual for an old dwell meter that had an internal battery
    Sun would do it right.......

    might be i need to revisit those other instruction books ? Will do.
  9. Apr 14, 2017 #8


    Staff: Mentor

    It means that, for me at least, the image is not displaying. I see a small icon of a computer screen with an X across the screen.
  10. Apr 14, 2017 #9


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    Seeing all of the above, I'm glad I stuck to using the auto manufacturer's recommended point gap measurement and let the distributor cam profile determine the number of degrees of "dwell".
  11. Apr 14, 2017 #10

    jim hardy

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    Aha !!! MY MISTAKE !! Look at the scale on this meter -

    zero volts is 100% dwell(45 or 60 degrees) points never open and full voltage is 0% dwell points never close !
    I must apologize for maligning that manufacturer - hereby tendered to Actron https://www.manualslib.com/manual/449419/Actron-612.html?page=4#manual

    This manual that i also cited doesn't give enough detail to know how it works
    so i owe that manufacturer an apology, and a chide for inadequate instruction book. Hereby tendered .

    Likewise this manual doesn't give enough detail to know how it works.
    So i owe him the same apology and chide. Hereby tendered , Actron again, http://dalhems.com/f/d/ebc1a67544af4773b36ba484007bdcd2/cp7605.pdf

    Once again i am hoisted on my own petard .
    Reminding me of my favorite saying, "When all else fails, read the instruction manual . "

    a BIG serving for me -


    Sigh ... this is how a fellow learns ! from his mistakes.... my concept of "Dwell" is a lot clearer now.

    will go back and fix earlier posts now

    old jim
    Last edited: Apr 14, 2017
  12. Apr 14, 2017 #11

    jim hardy

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    ? I dont know what to say , they show for me both in my post and in your quote of it.
    but i've seen that sometimes , for me it's just one of those vagaries of the e-wind .

    Here's links for the two broken images


    Last edited: Apr 14, 2017
  13. Apr 14, 2017 #12
    This is far less reliable when you consider how the components can wear.
  14. Apr 15, 2017 #13


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    Lol... just kidding.[COLOR=#black]..[/COLOR] :olduhh:

    Here are some links...

    Dwell angle...

    Dwell and Timing Explained Even More...

    And, krab had it right...
    Look at all the different definitions the word "dwell" can have, but it usually means... pause, delay, linger, reside, or stay.

    Lol... get the point ?[COLOR=#black].[/COLOR] :oldbiggrin:
    Last edited: Apr 15, 2017
  15. Apr 15, 2017 #14

    jim hardy

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    Thanks Guys for your good natured indulgence. And the great images.

    I'll still take apart my dwell meter (when i find it) to see how it handles " Dwell's Complement", that inverse relation between average volts across points and dwell .
    So far i've seen that reversed scale on the Actron 612 and two with internal electronic inverting circuits.

    OCR's links are spot on

    as are Mark's comments.
    I have a '79 Sportster with points and a kick-starter . Set it static too. With only two cylinders there's plenty of time available to charge the coil.
    Your two antiques sound exquisite.!

    old jim
    Last edited: Apr 15, 2017
  16. Apr 15, 2017 #15
    I've only ever used a dwell meter that was a function on a digital multimeter. The number of cylinders was a selectable setting, like resistance range might be.

    I always imagined it would have a PLC in it that was measuring duty cycle and running a simple internal calculation: duty*360/#cylinders.

    Never thought about how it would work on an analog meter.
    Interesting thread. Thanks Jim.
  17. Apr 15, 2017 #16
    Hi Jim, please re-upload those images. Something went wonky with the original upload. Thanks!
  18. Apr 15, 2017 #17


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    Warning, the majority of this post could definitely be considered "off subject" but just thinking about the issue revived many memories about dealing with the earlier ignition systems in competition applications.

    The primary wearing component on the old standard ignition points was the composite material blade that ran against the distributor cam because wearing of this component affected both dwell and ignition timing. That is the reason that the checking and adjusting of the point gap was a part of the standard periodic maintenance program (as was checking and adjusting the valve stem and rocker arm gaps); in competition, verifying both of these and engine timing was included in every pre-race engine setup. As long as correct lubrication of the distributor cam was maintained wearing of the cam lobes that was the other possible source of a variance in dwell was of little concern.
    In the early '60's when I was most involved in competition tuning, concern about point arc pitting and poor point contact were other primary concerns and resulted in both short term abrasive point cleaning and point set replacements. One of the frequently discussed items on higher rpm engines of the time were the issues of point inertia and "point bounce" vs point spring loading; but, point arc pitting pretty much trumped all other issues when it came to the life of a point set particularly when it came to the higher voltage "performance" coils that were available.
    At the same time, all of my work was on 4 cylinder/4 cycle engines and that probably minimized the need for concerns about the precision of dwell setting.
  19. Apr 15, 2017 #18

    jim hardy

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    Reloaded both as jpg's from my local PF folder

    if they're still wonky please advise and i'll re-"Paint" them

    SORRY for the false direction i started early in the thread

    it stemmed from a long time misconception i've held that "Dwell" is the amount of time points dwell open not closed .

    Glad to know "where i went wrong" all those years ago. Maybe it'll help others understand their dwell meter.
    "It's not what we don't know that hurts us, it's what we know that ain't so."

    old jim
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