Using an Insulation Multimeter

Similar to typical 12V vehicle systems, loss of isolation on the high voltage system of the Volt, ELR and Spark EV can be as simple as a direct conductor-to-chassis short. However, unlike 12V systems, the potential within high voltage systems means that insulation breakdown is also a cause for loss of isolation. Insulation breakdown typically occurs only when high voltages and/or current is present.


An Insulation Multimeter (EL-50772) (Fig. 12) must be used to test the isolation capability of high voltage components and circuits. An insulation breakdown cannot be diagnosed with a typical digital multimeter (DMM) because high voltage is not used by the DMM when measuring resistance. An Insulation Multimeter uses its own built-in high voltage.


F12 meter 1

Fig. 12


TIP: Always perform the high voltage disable procedure as published in the appropriate GM Service Information before servicing high voltage components and wiring and always wear the proper personal protective equipment.


Diagnosis with an Insulation Multimeter


The EL-50772 Insulation Multimeter tests the isolation capability of high voltage components and circuits. For correct test results, the functions of the Insulation Multimeter must be used properly.


Plug into the correct ports – It is critical that the correct ports (1, 2) are used for insulation testing. This is easy to get wrong, especially if the EL-50772 is used for standard DMM testing too. Do not use ports 3 and 4 for insulation testing. (Fig. 13) Also, if port 1 and port 4 are mistakenly used, continuity testing will still function, but the insulation test will always appear to pass.


F13 meter 2

Fig. 13


Always test at the 500V range – With the test leads not connected to anything, press and hold the insulation test button to know how your meter displays an infinite measurement. The EL-50772 will display 550M Ω when measuring an open circuit while set to the 500V range


See how your meter displays continuity – With the test leads connected together, press and hold the insulation test button. The EL-50772 will display 0.0 Ω.


A good ground reference is needed – When possible, use an alligator clamp to attach the ground lead. Typically, the reference point will be to the vehicle chassis, except cable testing, which requires connection to the cable shield termination at the connector and not to the chassis.


Verify the ground connection – Prior to every test measurement, verify the ground connection at a suitable ground location near the test point. The EL-50772 should display 0.0 Ω, indicating continuity.


Large capacitance or inductance testing – When testing components with large capacitance or inductance, such as certain modules and larger motor stators, it may be necessary to hold the insulation test button for a few seconds until the maximum resistance value is displayed.


More Information


For more information about loss of isolation testing, U.S. dealerships can check out the March 2015 edition of the Emerging Issues seminar (10215.03D). It includes a video demonstration using an Insulation Multimeter.


Also refer to the recent TechLink article Loss of Isolation Testing on High Voltage Systems


The EL-50772 Insulation Multimeter (Fluke 1587) is available through GM Dealer Equipment. Equivalent tools also are available from other sources.


– Thanks to Keith Newbury and Paul Radzwilowicz


Updated March 10, 2017

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  1. Thanks. The model number has been updated.

  2. Kevin Flaharty says:

    EL-50772 Insulation Multimeter is a Fluke 1587 not a 1857 in case someone would like to go to the Fluke site and check specifications.