Fluctuating neg. to ground on ignition coil

Small Block Mopar Engine

  1. Mattax

    Mattax Just the facts, ma'am FABO Gold Member

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    STM almost certainly = Standard Motor and part number is RU12. Same as on the truck now.
    More Information for STANDARD MOTOR PRODUCTS RU12
     
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    • Stephan D

      Stephan D Well-Known Member

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      Finally got time to do some measurements:

      The ignition coil is getting between 7 and 7.7 volts at + when running at idle, it's marked with 12 Volt, I have yet to remove the ignition coil to see more text on it.

      Resistance between + and - in the ignition coil is 1,5 Ohms.

      My readings at the ballast resistor:
      Lil Red Ballast Resistor readings.jpg

      Battery is getting 14 volt at idle.

      ECU is branded "Standard":
      IMG_6461.JPG
       
    • Mattax

      Mattax Just the facts, ma'am FABO Gold Member

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      All of this stuff is called '12 Volts'. Its '12 V' system as opposed to a '6 Volt' system. But it actually runs at something above that.

      The voltage the coil is getting while running is going to fluctuate with current through the resistor.

      That's ballpark correct. What's the book say it should be?

      I assume the resistance measurements were done with the key off.
      The voltage drop measured indicates current was flowing through the 1.3 Ohm resistor.
      upload_2020-5-22_8-45-16.png

      Probably good.

      So its been replaced.
      Most replacement ECUs do not need the current limited by a resistor. They get power from the run wire and no longer use the 5th terminal in the connector.
       
    • Mike69cuda

      Mike69cuda 64 is the new 17 FABO Gold Member

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      You can run a stock coil for awhile without the ballast resistor and it will work, if you want to do that to troubleshoot. Just jumper it.

      The previous owner of my car ran an unintended full voltage coil life experiment. He the ballast wired incorrectly, so it got full voltage when it was running, and had the ballast in the circuit when it started. He drove it that way for about 4 or 5 years. He said he had to replace the coil about every six months. From this data it appears that it will take awhile (maybe tens or hundreds of hours) for a coil to fail at full voltage.

      When I got the car I figured it out after he told me about the routine coil failures.

      I am not saying you couldn’t possibly do so damage to the coil doing this, but that it is probably unlikely for the time it takes to drive it around to see if that fixes your problem.
       
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      • Mattax

        Mattax Just the facts, ma'am FABO Gold Member

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        For rough comparison two coils were used in '73 Dodge cars.
        upload_2020-5-22_8-57-21.png

        Look up how a points distributor and coil work. Then this will make more sense. All the ECU is doing is converting the signal to electronicly do the same as points.
         
      • nm9stheham

        nm9stheham Well-Known Member

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        To be clear, most newer ECU's no longer need the power through the 5 ohm half of the dual resistor. Look at the connector on the ECU; if there are only 4 pins, then you should not need the 5 ohm half of the dual resistor.

        The 1.3 ohm value of the low resistance part of the ballast is double what it should be. It is supposedly to be 0.5-0.6 ohms cold. This is a typical problem with aftermarket ballasts. When hot, that 1.3 ohms will rise to probably 5 ohms or more (based on ballast measurements I have done in the past.) That is far more than the 2-2.2 ohms of the original ballast when hot.

        If the ECU has only 4 pins, then I would:
        • Disconnect the 5 ohm side of the resistor and see if the system runs
        • If so, then use just a low resistance single ballast to coil + of the correct value. I like to buy OEM ballasts on eBay or the MSD 0.8 ohm cold resistor. OEM Part number 2095501 is one old PN.
        This may not solve every issue for the OP, but it needs to be corrected.

        The coil resistance is good but is not 100% proof that the coil is all good. I have had one that measured 1.5 ohm but was producing weak spark. I replaced it with a Pertronix 40011.

        And just for info: All the above info is from when I solved the exact same issue (rough idle) in the '62 Dart in my Avatar. Wrong ballast resistor and weak coil.
         
      • Mattax

        Mattax Just the facts, ma'am FABO Gold Member

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        Based on the voltage reading, no current was going through the 5 Ohm resistor, so almost certainly its not being used during running.
         
      • 67Dart273

        67Dart273 FABO Gold Member FABO Gold Member

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        This might not even be a problem. Did you try increasing RPM up to "high cruise" maybe blipping it very high and see what is and at about what RPM you get maximum voltage, and what that voltage is?

        The thing is, CURRENT affects this, as well as the alternator (regulator) affecting charging voltage. Current, more of, coil heating, ballast heating, and charging system voltage variation causes more or less voltage drop through the system and through the resistor.

        At quite low idle, the system charging voltage may NOT run at 14. At higher RPM and due to the typical voltage drop in harness, the battery may see HIGHER than 14.

        The coils VARY and as old as these girls are may have CHANGED value as well as having been replaced with a somewhat different coil

        YOU CAN NOT TRUST low value resistance measurements

        YOU CAN NOT figure current based on COLD resistance measurements, because BOTH the ballast AND the coils CHANGE RESISTANCE when heated

        (You cannot even do that with lamps because the cold resistance of a filament is much lower cold than when operating and heated)

        It might be the coil is a little "hotter" than original, that is, more current
        It might be the ballast has a little different curve than the original AND BE SMALLER RESISTANCE WIRE

        If you are starting to get the idea that there are "all sorts" over possible variables......you are right!!!!

        If you are insistant, put the thing on a scope and see what the coil reserve voltage is. Measure from ECU case (and regulator case) to battery NEG and make sure there is no voltage difference at ground.
         
      • nm9stheham

        nm9stheham Well-Known Member

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        Hey, Del,

        You're 100% the last guy I wanna argue with; you do too much good work. I have done the cold and hot resistance checks on the OEM ballast versus a few different SMP RU-series ballasts. I've found that the higher cold resistance ballasts change their resistance by as large or larger ratio than the OEM one. I'd expect that 1.3 ohm cold ballast to be around 5 ohms when hot... vs the 2-2.2 ohms hot for the OEM. That is a real coil current drop.

        And the OP is subtracting his lead resistance from the readings to get more accuracy.

        I cannot see any reason to not put the system into the right configuration as far as ballast resistance. I am willing to bet a lot of OEM systems have been blamed for being bad when it was just something this simple....
         
      • Stephan D

        Stephan D Well-Known Member

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        I finally got time and courage to bypass the 1,3 Ohm side of the ballast resistor with a wire, giving the ignition coil 12 volts temporarily, but there was no change in the jerky idle, so this was not the problem...

        This is how it looks and feels:
         
      • 67Dart273

        67Dart273 FABO Gold Member FABO Gold Member

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        Hard to say from this video. A few random comments

        Might not even be electrical. Might be carburetion problem

        Might be time to get on a scope. If you cannot conjure up someone with a real by Gawd ignition scope, try to find an electronics hobbyist with an electronics test bench scope. Examine the dist. neg (primary waveform) for irregularities, wrap a few turns around the coil wire to get the "parade." You'll have to sync using a pickup off no1 to figure out which cylinder(s) you are looking at.

        If possible, make a "rig" so you can monitor the distributor pickup output and look for irregularities there
         
      • 512Stroker

        512Stroker We are all here because we are not all there.

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        Distributor shaft bushings are wore out.
         
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        • nm9stheham

          nm9stheham Well-Known Member

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          Next suggestoins:
          1. Connect a vacuum gauge to the manifold vacuum and see what it reads
          2. Look down into the carb throats when idling and see if there are any visible fuel droplets in the carb. (There should be none.)
           
        • 512Stroker

          512Stroker We are all here because we are not all there.

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          I used to see this a lot years back with a dwell meter.
          It was a sure sign that the bushings needed to be replaced.
          Caused a radical idle every time.
           
        • Stephan D

          Stephan D Well-Known Member

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          1. Yes, have done this a lot in order to adjust idle mixture for max vacuum, here is a reading, never mind the ugly sound, it's from the air injector tube on the exhaust manifold being exposed with the air cleaner taken off.

          First part is the vacuum in neutral, efter 15 seconds comes the reading in gear:


          I wonder if the jumping vacuum is the cause or a result of another problem?

          I have tried spraying brake cleaner on every possible leak-surface with NO change in idle speed.

          2. I Have looked down the carb at idle and it looks dry from above.
           
          Last edited: Jun 14, 2020
        • nm9stheham

          nm9stheham Well-Known Member

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          I'd suspect that the vacuum reading changes are from the engine's erratic firing. The actual readings look pretty good. OK on the no visible fuel drops in the carb.

          What is your idle ignition timing? Do you have a timing gun? I am not a big fan of maximizing the idle speed with ignition timing... you can end up with far too much idle timing and not know it.

          I like the suggestion on the worn distributor bushings.
           
        • Stephan D

          Stephan D Well-Known Member

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          Yes I have a timing gun, initial timing is now 13 degrees, no knock or pings audible when driving.

          The timing looks stable at idle when using the gun.
           
        • Stephan D

          Stephan D Well-Known Member

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          Is there a simple way to find out if the distributor bushings are worn?

          The engine(and truck) has only 40.000 original miles, seems a little early to wear out?
           
        • RustyRatRod

          RustyRatRod Lemmie see your b00bs. FABO Gold Member

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          Put a dial indicator on it.
           
        • 512Stroker

          512Stroker We are all here because we are not all there.

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          If you adjust the idle mixture screws for max manifold vacuum - it will be lean.
          Try adding 1/4 turn out on both screws, if this helps the idle quality add another 1/4 turn.
           
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          • 512Stroker

            512Stroker We are all here because we are not all there.

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            Pull the distributor cap off and try to move the rotor side to side.
            Should be very very little to no movement.
            Dial indicator would be best as suggested Rusty.
            I dont know off hand what the spec is maybe someone can chime in with a number.
             
          • Stephan D

            Stephan D Well-Known Member

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            Yes, I have tried different methods to adjust idle mixture screws, by now I'm pretty good att finding the sweet spot between lean and rich, but nothing takes the slight misfires away.
             
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            • Stephan D

              Stephan D Well-Known Member

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              OK - I will have a look at axle play at the rotor tomorrow.
               
            • 67Dart273

              67Dart273 FABO Gold Member FABO Gold Member

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              The service manuals used to detail distributor service. Wander over to MyMopar. Even if you don't get the exact year, it will be close to same
               
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              • 512Stroker

                512Stroker We are all here because we are not all there.

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                Great time to do a good inspection of the cap and rotor
                Post more pic's if possible.
                 
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