coil voltage testing

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  • Dr Dave
    Member
    • Nov 2010
    • 51

    coil voltage testing

    At a class I attended we saw that one tech was labscoping the B+ side of a coil or coil circuit if had multible coils .And said if you saw mode that a 2 volt spike from the base battery voltage that is a bad coil. We use this a lot if we suspect a bad coil and it seems to work .But I have 2 questions 1) will a bad spark plug or wire cause a spike as well or can we say this is a solid coil test ? 2) what would be the set up procedure to set a trigger to determine which coil it is on a C O P system?
  • badcoupe
    Senior Member
    • Jan 2011
    • 218

    #2
    you need a low amp clamp to measure current draw on the coils by clamping on a wire that feeds all the coils. Then you can backprobe the number coil control side for a trigger to see which coil it is in the firing order

    Comment

    • Joe Rappa
      Snap-on DSD
      • Aug 2007
      • 2050

      #3
      I doubt you could determine a coil is bad only by watching to see of a positive spike appears on the B+ wire. The only thing you MIGHT be able to determine with that test is if the secondary is leaking into the primary. I can't see it being reliable though. Maybe there is something I am missing though...which certainly wouldn't be the first time!
      There are already several good ways to check for bad coils. I'd stick with those.

      Joe
      "You don't build a reputation on what you're going to do"
      Henry Ford

      Comment

      • Dr Dave
        Member
        • Nov 2010
        • 51

        #4
        I,d like to explore this more

        I have done this a lot since I heared about this in a class. It was presented to us as a method used by a tech in a case study.If you monitor the b+ with a labscope any spikes over 2 volts is a bad coil. I did a Tarus to day with arching wires and I saw no spike . I had a mis fire at highway speeds on a 2005 5.4 expedition it had no codes . I hooked into the b+ for the coils and at Idle with no misfires felt saw 1 spike over 2 volts and 3 smaller spikes . with no other diag I replaced all the coils and problem solved and a nice steady battery voltage after the repair. Seems like this method can work .I also did this with a dodge coil pack wish I did it before I changed the plugs . What need to know is how to quickly determine which cylinder it is on a COP system for those customers that can't afford all coils

        Comment

        • crackerclicker
          Senior Member
          • May 2008
          • 400

          #5
          If you don't know how to setup for cylinder ID, then you might want to reconsider using a technique this open for interpretation. Using noise for diagnostics is a slippery slope, so make very sure you know your equipment. The last part of badcoupe's reply is what you're looking for.

          Comment

          • Dr Dave
            Member
            • Nov 2010
            • 51

            #6
            we use this set up alot

            We have had our labscope for about 4 years but have been using it and experimenting as much as we can I have been a tech for about 30 years but never had a scope except the sun smart sciope . I personally have verified a misfire after we did a tune up on a dodge 3.3 2004. We replaced the plugs and wires and it came back the next morning with a notacble mifire I blamed the champion plugs or a bad beldon wire.But before I did anything I scoped the B+ of the coil sure enough there was a 2-3 volt spike. I put a coil on it .the mis was gone and the spike was gone. I have scoped good coils as well to compare.I did this on a 5.4 that was not missing at the time but would mis some times on the highway on light accel. saw multible spikes decided it hould have all coils if possible . the spikes and mis went away now I was wondering if you had some creative ideals to simlply and quickly set a trigger to know which one is mising . But I can and have and are able to set it up to see,I was just looking for different ideals .But if you are not using this method at all. I am one step ahead of you. But you are correct it saying be very carful of noise because I have been fooled before on injector amp ramp alway use battery ground!

            Comment

            • komobu
              Member
              • Feb 2009
              • 46

              #7
              Dave,

              You really have me kind of confused with your 2 volt test. Hook your channel 1 test lead to coil positive, and your neg test leat to battery negative. Set your scope to a 100ms time sweep and your voltage setting on ch 1 to 100volts or use an attenuator. Watch what happens while idling. You will see voltage spikes to about 70 volts on each firing cycle. That is totally normal behavior as you have a primary voltage spike that takes place as the magnetic field collapses.

              As for figuring out which coil is firing, place your amp clamp around the battery positive wire that feeds all coils, or pull the fuse and put in a fused jumper wire and place your low amp probe around it.

              Now place your channel 2 test lead on the negative side of any coil. It doesn't matter which coil it is, as long as you know what cylinder it is. Monitor both channels on your scope, channel 1 being amps and channel 2 being volts. Idle the vehicle and watch what happens. Channel 2 will show a spike every time that coil fires. If you are monitoring a 4 cyl engine for example, you will see for coil ramps for each spike on channel 2. You simply use your firing order to identify which ramp goes to which cylinder. So if you are hooked up to cyl2 and the firing order is 1,3,4,2 your first ramp would be cyl 2since that is what your hooked up to. Your next ramp would be next in the firing order, in this case 1, followed by 3 and then 4. Then you will see another spike on channel2 as cyl2 fires again.

              Hope this is clear enough for you. There are many videos on this on YouTube. Check out scanner dinner on you tube for some pretty decent videos

              Comment

              • fueltrim01
                Member
                • Apr 2008
                • 60

                #8
                Scanner Danner, on U Tube.

                Comment

                • komobu
                  Member
                  • Feb 2009
                  • 46

                  #9
                  Thanks Fuel Trim. That error was caused by auto correct on my iPad!

                  Comment

                  • Dr Dave
                    Member
                    • Nov 2010
                    • 51

                    #10
                    thanks

                    I'll try that to see if I can tell which coil. As far as the 2 v spike theory .I put the lead on the B+ side of the coil and - on the battery like you said. put the scale on 20volts I'm not sure of the sweep. but you should see a steady battery volts if you see spikes more than 2 volts you may have a bad coil. Your looking at it too sensitive. It could even be done with a graphin meter I think , just look into it I sear I have verified a bad coil before and after the repair and never felt the misfire when testing and still saw a spike and no spike with a new coil.

                    Comment

                    • Crusty
                      Senior Member
                      • Dec 2007
                      • 2450

                      #11
                      I think this measurement is referred to as an EMF (backfeed)
                      Under normal circumstances any EMF backfeed should be under control but when current carrying coils have the amperage change voltages can be intoduced (as they should into the secondary windings) in undesirable ways if there are problems within the coils. Clamping diodes are one method of controlling voltages the same way capacitors can control voltages (as in points & condenses) and it's up to the design engineers to build ignition coils with their desirable characteristics.
                      If a vehicle has six coils and ONE shows an EMF I would be suspicious. If all six coils display that same characteristic I might not be so fast to condem all six coils.

                      Here are some expressions to research if one is interested
                      BACK EMF, FLY BACK EMF, COUNTER EMF.
                      Last edited by Crusty; 02-11-2014, 11:14 AM.

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