Relative compression testing

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  • Trindaddy
    Senior Member
    • Jan 2010
    • 339

    Relative compression testing

    Hello everybody. Can someone please take the time to outline what exactly the set-up is to test for cranking amperage as in a relative compression test. I went nuts trying to do this tonight using the Modis Ultra and was very frustrating. I would like to see starter draw and a trigger to id the cylinder at the same time. I am using the Blue-point EETA501B high amp probe around the starter wire and channel 2 (on 2 channel scope) on the primary wire for coil #1 on this 06' F150 5.4 liter. First, I can't move the waves into the center of the screen, it won't allow me to move it low enough to see the tops of the amp waveform! Second, I can't seem to get the trigger for the coil to show! All I get is a constant cranking voltage of about 10v. Anyone ever do this with ANY Modis? Please advise.

    Neil
  • greasybob
    Senior Member
    • May 2008
    • 1590

    #2
    How fast of a sweep rate are you using ? try about 2 seconds Do you have your trigger set to channel 2 and in range ? Maybe the video I did will help http://www.youtube.com/watch?v=zKvxGLtsDJY

    Comment

    • Trindaddy
      Senior Member
      • Jan 2010
      • 339

      #3
      Thanks Greasy. I found some good stuff on the link but none of the fellas on the net use another channel to find a trigger cyl and follow the firing order! Why bother doing the test if you don't know which cylinder is suspect? I was using 10 secs at first, then..I forget. I tried a few. Channel 2 trig set at 20v. All i could get was constant cranking volts and no trigger. A couple of these guys are using a low amp probe and "ac coupling". Now I'm really confused.

      Comment

      • greasybob
        Senior Member
        • May 2008
        • 1590

        #4
        I would set the trigger on a down slope at about 5 volts. You are on the control side of coil right ?

        Comment

        • greasybob
          Senior Member
          • May 2008
          • 1590

          #5
          Try setting the amp probe to the 40 amp setting and set the modis scale to 40 amps with it ac coupled and adjust it from there to get the amplitude you want on the screen. I looked up a picture of the amp probe you're using, do you have both the amp probe and the test lead grounded at the modis ? I'm saying this because the plug from that amp probe is one piece and doesn't allow the grounds to be stacked the way the newer amp probes do. I had this on my first amp probe and had to cut apart the leads and solder in a separate ground (banana style) jack.

          Comment

          • Trindaddy
            Senior Member
            • Jan 2010
            • 339

            #6
            I tried both sides. looks the same, about 10.5 (cranking) volts. What does the angle of the trigger icon do? I thought it triggered the recording when scope sees the move of the channel's signal in that direction, right? Why just 5 volts on the coil driver? Its pulling 12v down.

            Comment

            • Trindaddy
              Senior Member
              • Jan 2010
              • 339

              #7
              Originally posted by greasybob
              Try setting the amp probe to the 40 amp setting and set the modis scale to 40 amps with it ac coupled and adjust it from there to get the amplitude you want on the screen. I looked up a picture of the amp probe you're using, do you have both the amp probe and the test lead grounded at the modis ? I'm saying this because the plug from that amp probe is one piece and doesn't allow the grounds to be stacked the way the newer amp probes do. I had this on my first amp probe and had to cut apart the leads and solder in a separate ground (banana style) jack.
              I think your onto something here. Yes, its screwy. I had to jump the leads of the amp clamp to a set of channel one leads, then, to get the voltage of the coil primary on ch2, I had to piggy another lead to batt ground from the Modis. Doesnt seem right to me. Never "mixed" grounds before.

              Comment

              • greasybob
                Senior Member
                • May 2008
                • 1590

                #8
                I'll get jumped on for this but the trigger just keeps the pattern your interested in seeing in the same spot on the screen while the entire event is still being recorded, Makes things more viewable. Since the 12 volt supply is getting pulled down to zero on the control side setting the trigger at 5 volts on a down slope should establish a good starting point for the pattern you want to view unless of course the coil is not being fired. I'm a lot better working the scope than I am at describing it.

                Comment

                • sandt38
                  Senior Member
                  • May 2012
                  • 187

                  #9
                  Use your amp clamp on channel one and the trigger on channel two. Be sure when you set your trigger up that you select channel two as the trigger, and yes you want to use a falling edge trigger. I like to use injectors as my trigger.

                  Comment

                  • greasybob
                    Senior Member
                    • May 2008
                    • 1590

                    #10
                    I was messing around with some settings on my scope with my different amp probes. Try setting your scope to 500mV with it ac coupled. That seemed to work best with my amp probe on the 40 amp setting which I believe yours should have. As far as what to use for a trigger , with Fords having your gas pedal at wide open throttle should be clear flood mode and the injectors won't fire but your coils will. Trigger off of number cylinder coil and just follow the firing order to the matching cylinder. Might take some practice to figure out the correlation. A one second sweep also worked well.

                    Comment

                    • Trindaddy
                      Senior Member
                      • Jan 2010
                      • 339

                      #11
                      Thanks boys! Give me a few days to try this again. I have come to realize that the worst time to try something new is when you are under the gun for results. I need to try this when my head is clear on some down time. I'll be back.

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