Fluctuating neg. to ground on ignition coil

Small Block Mopar Engine

  1. Stephan D

    Stephan D Well-Known Member

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    While trying some more suggestions for troubleshooting my idle vibrations, I connected a tachometer instrument to neg. terminal on the ingition coil and the other wire to ground as instructions said.

    While reading the tachometer I see very signifcant fluctuations, see video.

    Is this correct or what might be wrong with the ignition?

     
  2. Murray

    Murray FABO Gold Member FABO Gold Member

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    I would check with a better quality tach. Is this points ignition?
     
  3. Stephan D

    Stephan D Well-Known Member

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    No, it´s Mopar original electronic ignition since 1979.

    The attached tach in the video is vintage as well, I have a tach in the car that also show these fluctuations, but much less significant.
     
  4. Mattax

    Mattax Just the facts, ma'am FABO Gold Member

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    It depends, but if I recall the previous posts, this is a stock 1978 360.
    We're seeing idle fluctuating between 1250 and 1500 rpm. The movement of 250 rpm is a little more than I'd like to see but not surprising in conjunction with your other observations.
    That isn't the correct slow idle speed though. It might be correct for an engine warming up after kicking it down to the 1st step.
    Has the AIR injection tubing been fixed or plugged?
     
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    • halifaxhops

      halifaxhops It's going to get stupid around here! FABO Gold Member

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      Check if the reluctor gap is the same on all "lobes" Might be a bent shaft.
       
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      • Stephan D

        Stephan D Well-Known Member

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        The the tachometer is for 4 cylinder engines and the reading should be multiplied by 0,5 to get 8 cylinder figures, which would be 650-750 that it is, so the movement is approx 125 rpm measured, it does not change that much in reality, just the tachometer which made me curious why it does.

        The AIR injection has been taped closed temporarily.

        I have looked down the carb with a flashlight at idle, and it looks dry from above, no fuel dripping visible.
         
      • Mattax

        Mattax Just the facts, ma'am FABO Gold Member

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        Righto. I see. I stand corrected.
         
      • Stephan D

        Stephan D Well-Known Member

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        I very much appreciate your input on this idle issue!

        I measured the dual ballast resistor today, Primary side is slight out of bounds, measured 1.9 Ohm (should be 1.12 to 1.38 ohm) and the auxiliary side measured 5.2 Ohm, which should be OK.

        Not sure how the primary side 1.9 Ohm would show up when running at idle.
         
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        • Mattax

          Mattax Just the facts, ma'am FABO Gold Member

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          In theory, yes. That's because the less current through the coil means less energy available to the spark plug.
          In reality I don't know. If there is enough voltage to initiate and enough current to build the flame kernal enough to get a a good burn, then more spark energy won't help. But if its having trouble developing a good burn, then having more energy should help.
          This could be seen on an oscilliscope, but I understand you don't have one. Most people don't.
          While you have the ohmmeter out, check the coil primary resistance.

          The other thing to experiment with is whether changing the fueling smooths out the rpm differences.
          We don't know the 'best' for that engine, rpm and timing, but we know its close. Close enough to make small changes and observe the effect.
          If slightly richer than current setting results in slightly smoother engine, then check it in gear for rpm and vacuum drop. I'd like to see less than 75 rpm drop going into gear.
          If slightly richer causes reduction in rpm, then its definately too rich.
          If slightly leaner causes an in increase in rpm, then try shifting into gear (same comments as above).
          If slightly leaner causes more variation in rpm, and over all drop in rpm, then its a little too lean.

          Depending on the results, I might be very tempted after doing the above to set timing at 10 or 11*, same rpm, and repeat these tests.

          I know its a lot of fuel to use testing. But it you want to fine tune a not quite stock engine setup, its what has to be done. Next time you'll have more experience and knowledge, but first time we have to hear, see and smell it for ourselves.

          This is following up on this post by @nm9stheham
          Compression test on a 360, result OK or not?
           
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          • nm9stheham

            nm9stheham Well-Known Member

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            When you measure the ballast resistances, first connect your ohmmeter leads directly together several times and take readings of just the ohmmeter leads themselves. Average those readings and then subtract that from the readings of the ballast resistance.

            Ballast resistances are to be measured when cold. 5 ohms for the high side is OK; 1.9 ohms for the low side in not OK. It should be around 0.6 ohms cold. It will increase to 1.5-2 ohms when fully hot, which takes about 1 minute of running.

            A high value on the low resistance side of the ballast will reduce spark energy as Mattax says, and that could cause irregular operation. And if the spark plug wires get high resistance or even if the spark plugs get a high resistance inside, that will reduce spark energy.

            An erratic ground on the ignition module body can cause irregular spark, and the reluctor gap like mentioned by halifaxhops has to be steady. It could be a bent shaft or a worn distributor shaft bushing.

            Finally, if the leads from the reluctor to the ignition module are reversed, then the timing can be jumping around, and you might have misfires.
             
          • Stephan D

            Stephan D Well-Known Member

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            I actually bought a new ohmmeter as the old one showed 2 ohms when shorted out, the new one showed 0,5 ohms, which I deducted from the readings.

            The ballast resistor was hot when I measured it, will do another measue with a cold resistor.

            Will measure ignition cables and take pictures of all the spark plugs within the next days.
             
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            • Stephan D

              Stephan D Well-Known Member

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              Did another measure of the primary side, now cold, measured 1.3 ohms.

              But I don't know if its a thermal ballast resistor or not, think I have to remove it to be sure.

              Should it be thermal or not?
              (Another big question)

              Very often the car idles very good after being shut down for 10-15 minutes, fooling me to think problem is solved, could that be the ballast resistor cooling down and lowering resistance?

              Ballast Resistor.jpg
               
              Last edited: May 19, 2020
            • nm9stheham

              nm9stheham Well-Known Member

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              Yes, it is a thermal ballast resistor. The internal heating when running is what causes it to increase cold resistance from cold to hot. Cold means around 25 or 30C.

              One test you could run is to use a wire jumper while it is running and hot and the irregular idle is occurring. Just use the jumper to short the low resistance terminals of the ballast and see if that improves the idle quality. While you are in the garage, measure the primary resistance of the coil + to coil - as suggested, with all wires disconnected. It ought to be 1.5 ohms + or -.

              BTW, this looks like a replacement ballast resistor. Some of them are not true OEM replacements.
               
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              • Stephan D

                Stephan D Well-Known Member

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                Great suggestion!

                I will try to temporarily bypass the low resistance terminals when hot in a couple of days and see what happens with the idle.

                No risk of frying anything vital??
                 
              • nm9stheham

                nm9stheham Well-Known Member

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                There will still be the resistance in the coil primary to limit current. When cold, the total resistance is around 1.5+0.6=2.1 ohms, and the coil by itself with the ballast bypassed for a short time should be around 1.5 ohms.

                This test may tell you if the problem is something limiting the coil primary current.

                And, BTW, coils can be tested but that is not a guarantee. They can be erratically breaking down and partially shorting inside. The only real way to determine if they are 100% OK is by substitution. The same applies for the ignition module; none of us had a good test for those, so we have to try a substitute.

                The reluctor coil resistance can be measured when disconnected. IIRC, it should be in the range of 300 to 500 ohms. But I am not 100% sure of that number; maybe someone else can recall for sure.

                And you ought to put your voltmeter on the + feed to the ballast from the ignition switch when running cold and hot, and see what voltage you have. The ignition switches can get faulty as well as the connections.

                Let us know what voltage is across the battery terminals when idling at 1500 RPM or more. Measure that both hot and cold.
                 
              • 67Dart273

                67Dart273 FABO Gold Member FABO Gold Member

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                Please don't get carried away on ballast resistance. Much better to measure coil + and it varies all over the place. But at a steady RPM the voltage should be steady. It varies from somewhere around 8 with engine stopped and key in run, to as much as 12 or a bit more when the charging system is at 14V

                Ballasts are cheap. Frankly if you really are convinced you have an itermittent somewhere, and YOU ARE CERTAIN it is not the wiring path---which is far more likely, then the only real solution is to pick the cheapest parts and replace them

                And this comes from a guy who does not like throwing parts at a problem

                What to do in the meantime?

                With engine stopped, key in run, monitor the voltage to the "key" side of the ballast. It should be "same as battery" but often is not, due to wiring problems/ voltage drop in the harness

                Wiggle test the harness, especially the ballast and bulkhead connector

                Remove the ECU scrape around the bolt holes and remount, using star lock washers. Same for voltage regulator. Both MUST be grounded to same potential (voltage) as the battery. MUST

                Check all connectors in the ignition system. Work the ECU connector in'/ out several times to scrub the terminals and feel for "tightness and inspect with a light for corrosion, same for ballast terminals and same for dist. pickup connector

                If you can beg borrow or otherwise get an ignition or any other kind of oscilloscope, taking a look at coil neg may give some clues.
                 
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                • Mattax

                  Mattax Just the facts, ma'am FABO Gold Member

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                  Being in Sweden, I don't know how quick and easy it will be to locate a dual ballast, which is needed since its the original ECU.

                  The resistor for the coil can be bypassed for a short period of time. Normally the biggest risk is overheating the coil.
                  But with the dual ballast and original ECU it wouldn't hurt to look at how its wired. Running should not be an issue but starting might be.
                  I'm not really sure how those 5 pin ECUs get power during start and if the lack of resistance could hurt it.

                  I think safer is to get a normal 0.5 Ballast resistor if you can't find a dual with the proper resistances.
                  Then put jumpers to the 0.5 ohm resistor and the 5.0 Ohm resistor.

                  There are ECU testers, but they are rare to here, Iprobably more so in Sweden. Some are just for Chrysler, but others have adapters for all major ECUs from the 70s and 80s. Anyway. One thing at a time. The resistor is probably out of spec, investigate that first. If you got a '79 Dodge truck shop manual, or even a generic shop manual for the truck, the coil, ballast and other specs should be in there. Sorry I have nothing that new for mopar.
                   
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                  • Stephan D

                    Stephan D Well-Known Member

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                    I actually did a minor part parts of your suggested measurements earlier today, it´s Mopar electronic, not breakers;

                    Volts at ignition coil + at idle:
                    Ignition Coil  Volts Engine ON.jpg

                    More measurements to come.

                    View attachment 1715531503
                     
                  • nm9stheham

                    nm9stheham Well-Known Member

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                    What is the 2.5 ohms? Coil + to coil -? With a 0.5 ohm lead resistance, then would that be 2.0 ohms in the coil?
                     
                  • Stephan D

                    Stephan D Well-Known Member

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                    I actually live near one of the most well assorted US cars parts stores in Sweden, so I can pick one up easy, but they sadly have no specifications on the resistance, so I might end up with another one, with the same specs as the one I have, well well...

                    From their cataloge:
                    Ballast Resistor Hansen.jpg
                     
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                    • Stephan D

                      Stephan D Well-Known Member

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                      Sorry, I messed up the pictures I took today, so I removed that one, not sure of what it shows.
                       
                    • Mattax

                      Mattax Just the facts, ma'am FABO Gold Member

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                      Well @nm9stheham thought the one on the truck looked non-orignal.

                      Do they have any single resistors? If you get one that is 0,8 Ohms and put in parallel with the 1.3 Ohms then together that would be equivalent to 0,5 Ohms. Sort of a pain but maybe better than hunting or waiting. ??
                       
                    • nm9stheham

                      nm9stheham Well-Known Member

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                      OK, the voltage at coil - looks reasonable. So no clear problems there.

                      And MSD makes a 0.8 ohm ballast (like mentioned above) that works well. PN 8214.
                       
                    • 67Dart273

                      67Dart273 FABO Gold Member FABO Gold Member

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                      You are on the wrong track. Don't sweat the resistance. For one thing the coil might not be "per original" and your ohmeter might not be that accurate. Low resistance measurements are difficult to deal with, few meters are accurate down low, and there are variances, anyhow, especially in this Chineseoatioized replacement world

                      Where a ballast would show up problems, if it simply was wrong resistance, is heating or failure of the coil or ballast, or missing at high speed if too high resistance.

                      The voltage you posted running.........this is meaningless if you don't know what the batttery was doing. That is, if it's idling slow enough that the charging voltage is sagging, then "that could be normal." If however the battery was right up to 14V when you took that reading, then it's borderliine low. However, it might be a problem, such as harness voltage drop or it might be the coil is a little "hot" and sucking it down. That is, a low resistance high draw coil

                      Also take a voltage reading on the other side of the ballast and compare it to battery. Those two should be withing a couple tenths of a volt and often are not

                      To put this another way. With a good fast idle, the battery warmed and charged to normal, the system (at battery) should be 14V

                      Supply voltage at key side of ballast should then be 14V

                      If your reading was taken under these conditions it is borderline low but not to get excited.
                       
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                      • nm9stheham

                        nm9stheham Well-Known Member

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                        FWIW.. I have found the exact opposite.... crappy low RPM operation, especially at idle and when warming up. IMHO, the OP needs to see if he has the right parts in there.

                        I have a /6 where the wrong ballast was put in there by the P.O., and it had idle problems all the time... its resistance was something like 7 ohms hot! Ran fine when revved... because the alternator voltage came up just enough to charge the coil just enough to fire the plugs adequately. That ties in with your comment on measuring if the system voltage is adequate at idle.
                         
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