Strange Low Amp Probe Phenomenon

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  • greasybob
    Senior Member
    • May 2008
    • 1590

    Strange Low Amp Probe Phenomenon

    Had a 2010 Cadillac SRX 2.8 with a misfire. After doing several ignition and compression tests I came to the conclusion that it must be an injector issue. This is one of the few GM non direct injection vehicles that does not have a flow test for the injectors in the scanner functional tests so went to a wave form test for the injectors first. There are 2 pink/black wires in the injector harness to supply power to the injectors and one of the injector control wires is also pink/black. I by mistake had my low amp probe around all three injector wire when I went to check injector 3 which is the one with the pink/black control wire. As you can see in the screen shot below the control wire is grounding the injector but the current ramp is missing. I thought i had found my problem. Before I confirm anything though I have to check things a little further as my misfire code was for cylinder 5 not 3. Sure enough I figured out that when you have your low amp probe around both the current supply and the ground wire of the same circuit the current moving in both directions seems the cancel each other out as far as the low amp probes ability to sense that current. Hmmm. As you can see in the next screen shot when I corrected the low amp probes position to just the 2 supply wires and still back probed the ground wire the current ramp is back on the screen.

    So back to the vehicle in question. All injectors looked good in both current and voltage wave forms so I did a manual flow test by ground each injector with a piece of wire and found that the number 5 injector did not flow any fuel when grounded. The resistance on all injectors was also similar. The vehicle had been to a GM dealer ship before it came to me for second opinion. They had done a coil swap and when that did not change the misfire they recommended to the customer that the engine would need repair or replacement !!! Geez guys, put a little effort into your diagnostics.
    Attached Files
  • Witsend
    Banned
    • Nov 2012
    • 2942

    #2
    Good job Bob. Seems like most dealers have their techs on flat rate and just care about total hours booked.
    Seems to me some older GMs offered fuel injector balance test that could be run right off a scan tool key on engine off with a fuel gauge (if the system holds line pressure) The scan tool could activate the chosen cylinder injector for a specific time and you could write down the pressure drop of each injector and compare. Cycle the key off and then back on to restore line pressure after each injector tested.
    Seems, GM took this valuable feature away. I think with more and more cars lacking fuel pressure test ports and just having fuel rail pressure sensors, why isn't this test available on all models of cars? Maybe they're scared of potential liability from a chance hydrolock damage could occur ?

    Comment

    • SnapOnKid
      Senior Member
      • Jan 2011
      • 884

      #3
      Nice work Bob!!! Some dealers are better than others. It's just unfortunate they don't invest more in there techs and their education.

      I know someone that works the dealer technical support hotline. Some of the so called "techs" that work at some of the dealers... The best part is when they want to back charge the manufacture for their own mistakes.0

      Comment

      • BRIAN617
        Senior Member
        • Dec 2016
        • 153

        #4
        Bob, I'm curious in your particular situation I'm gathering that the control and amperage tests shows the coil to be functioning properly, and the problem is mechanical internally. If you really slow that voltage test down and zoom in could you see the absence of the injector pintle bump?


        Comment

        • greasybob
          Senior Member
          • May 2008
          • 1590

          #5
          Are you referring to the "no hump, no dump" (cue witsend humpty dumpty joke here) injector wave form ? You are correct in this instance as you can see in the 2 screen shots below. I've been told not totally trust this analysis alone as it wouldn't show a plugged or damaged injector tip. An injector flow test is still worth using if possible for confirming the problem.
          Attached Files

          Comment

          • Witsend
            Banned
            • Nov 2012
            • 2942

            #6
            Are you referring to the "no hump, no dump" (cue witsend humpty dumpty joke here) injector wave form ? You are correct in this instance as you can see in the 2 screen shots below. I've been told not totally trust this analysis alone as it wouldn't show a plugged or damaged injector tip. An injector flow test is still worth using if possible for confirming the problem.
            Today 09:11 AM
            Thought it was "No Humps , equates to No Baby Bump , which could make one a Chump

            Comment

            • BRIAN617
              Senior Member
              • Dec 2016
              • 153

              #7
              Thanks Bob. I haven't been able to capture that type of injector failure yet. Cool to see you did.

              Comment

              • Crusty
                Senior Member
                • Dec 2007
                • 2450

                #8
                Bob, I believe you're showing the VOLTAGE showing the indication of the pintle CLOSING.
                If you have an AMP clamp on the injector, as it RISES you can see the injector starting to open then the "pintle hump" as the injector actually starts to move. (I have some captures but they're not easy to find in the many captures I have. I'll TRY to find some examples)
                BOTH are good tests, both the VOLTAGE and the AMPERAGE, but both are strictly showing the electrical activity, and indicate the physical pintle movement, but, as Bob says, the actual FLOW thru the injector could be restricted, or not atomising properly.
                The hydraulic pressure test is still needed, and that alone only tells us how much fluid is passing thru but not the quality of the atomization.

                Comment

                • Crusty
                  Senior Member
                  • Dec 2007
                  • 2450

                  #9
                  Here's a couple of screens. The yellow trace is the AMPS. The Green trace is the VOLTAGE.
                  The opening/closing activity I may have mixed up but the screens you've shown look like voltage, rather than an actual amperage measurement.
                  The "pintle hump" can be seen in both.

                  If one were to overlay the two, the AMPS shows the opening, and the voltage pintle hump occurs after the amperage pintle hump.

                  These are some early captures from 2007.
                  Attached Files

                  Comment

                  • Witsend
                    Banned
                    • Nov 2012
                    • 2942

                    #10
                    If the pigtail of #3 fuel injector was accessible under all that fantastic plastic to gain access to separate 2 wires and place an amp probe around one of them . I would think that a mechanics stethoscope would just as quickly tell me that the sound of a pintel moving on a neighboring working injector would make a more distinctly satisfying louder clicking noise than the suspect #3 injector that be more muted , but a noid light be just as bright comparing the circuits of the two injectors. Beats dragging out the scope unless buried under a plenum and all you can access is a general loom or PCM connector.

                    Comment

                    • greasybob
                      Senior Member
                      • May 2008
                      • 1590

                      #11
                      Here is the bad injector current and voltage as fired with my SparkOmatiC home made coil and injector firing machine. I'll compare it to a good injector tomorrow if I can find one. The Caddy is fixed and gone.
                      Attached Files

                      Comment

                      • tech25
                        Senior Member
                        • Oct 2017
                        • 172

                        #12
                        Originally posted by Crusty
                        Here's a couple of screens. The yellow trace is the AMPS. The Green trace is the VOLTAGE.
                        The opening/closing activity I may have mixed up but the screens you've shown look like voltage, rather than an actual amperage measurement.
                        The "pintle hump" can be seen in both.

                        If one were to overlay the two, the AMPS shows the opening, and the voltage pintle hump occurs after the amperage pintle hump.

                        These are some early captures from 2007.
                        you are correct about the injector operation, regarding pintle movement

                        Comment

                        • greasybob
                          Senior Member
                          • May 2008
                          • 1590

                          #13
                          Ok, here you go, a working injector with 2 humps. One in the current ramp and one in the voltage. Compare to the inoperative injector with no humps. This is a 2013 suburban 5.3, so voltage and current draw may be different.
                          Attached Files

                          Comment

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