Ground testing using a scope

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  • jpgarrido
    Junior Member
    • Dec 2006
    • 10

    Ground testing using a scope

    Ground testing using a labscope.

    I often get the question of how much of a voltage drop is acceptable for a given circuit. Years ago I learned a spec of 30-50mV per amp of current flowing in the circuit. After testing lots of grounds of the years I found this spec to be very good.

    I also use a scope to test grounds while doing a ?Wiggle? test of harness and connections. The MODIS/Vantage Pro has a nice long record depth to make this much easier.

    Now when testing sensor reference grounds I do not use the Voltage drop method of checking them. They are extremely low current circuits (so they don?t create an accurately measurable voltage drop) so an ohmmeter is preferable for checking ground circuit integrity.

    And while we are talking about sensor grounds many sensor grounds today are not at battery negative potential (Key off or KOEO) or engine block ground KOER. Many sensor grounds, sometimes referred to as sensor low or reference low, are ?floated? above engine ground so don?t be surprised to see some voltage potential offset in those circuits.

    Below is a capture of a bad alternator ground circuit found during a wiggle test. Channel 3 shows the noise created by the alternator ground being loose. Look at the cursor readings for channel 3.

    Happy hunting
    Attached Files
    Jim
  • kmcneil
    Junior Member
    • Dec 2007
    • 1

    #2
    floating grounds

    I'd like to here more about floated grounds i've heard of it but don't realy understand it

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    • KM@PROAUTOTECH
      Senior Member
      • Oct 2007
      • 358

      #3
      Grounds

      I would be weary of ground tests being done with resistance. There is no load on the ckt when you check resistance. Computer related grounds should not exceed .10 volt, be it KOEO or KOER. You may have some Chrysler products that reach .20 only because everything that is shared by the ATM ckt, and possibly operating at the same time. The most basic example of resistance being a problem for ckt. integrity can be demonstrated with a no start that appears to be a battery or starter, in which both are replaced and the battery cable was tested for resistance, which passed, then the starter and battery replacement was justified possibly with the logic "one took at the other". Problem still present, only to find that with the insulation from the cable removed only one stran was connected, hence why the cable past the resistance test, but won't handle the proper amount of VLT. or amps for that matter. Some will argue with computer related grounds or sensor grounds to do your volt drop between the sensor and the closest ground, not the battery, what if that is the problem, that has become your base line and you won't even find the ground problem.

      Comment

      • Crusty
        Senior Member
        • Dec 2007
        • 2450

        #4
        Grounds

        KM is absolutely right about resistance tests. Most Asian testing relys on checking for continuity. This is just a quick go/no-go. Voltage drop testing is much more accurate and reliable ESPECIALLY on an active load circuit.
        Are the "floated" grounds intentionally given a higher bottom voltage in an attempt to overcome the problem of oxidized ground connections and only used on PCM/BCM or other module sensors only?

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