Testing Ground and Low Reference Circuits

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A test lamp can light with up to 200 Ohms in a ground or low reference circuit, so it is not recommended to be used for this type of test. Instead, use a Digital Multi-Meter (DMM) when validating ground or low reference circuit continuity. Updated information about testing ground and low reference circuits is now available in the Service Information (SI).


TIP: For 2007 and later vehicles, click the Circuit Testing link in any SI diagnostic procedure, then select Testing Ground and Low Reference Circuits.


When using a DMM, there are many vehicle conditions that can affect the ground and low reference continuity testing. If these conditions are not met, a ground or low reference circuit test can fail on a good circuit, causing longer diagnosis time and incorrect component replacement.


Any current flow through a ground or low reference circuit while being tested will skew the DMM continuity reading, or display a reading higher than when there is no current flowing.


Performing a ground or low reference circuit continuity test in reference to the vehicle's battery negative terminal will have the highest risk of failing a good ground or low reference circuit test than any other ground reference point. The best ground test points are a control module's housing (if the housing is metal and grounded), door jamb striker (if attached to metal), under dash metal framework, engine block, or body ground studs (other than where the negative battery cable is attached).


A typical DMM ground or low reference circuit continuity reading can be as high as 100 Ohms with the ignition On and drop to 15-25 Ohms after the ignition is turned Off. The reading will drop below 10 Ohms after 30-40 seconds, and then below 5 Ohms after 60 seconds. Once the vehicle completely goes to sleep (generally 3-10 minutes) the reading will drop below 0.3 Ohms.


Conditions that may need to be met to ensure a valid continuity reading on a ground or low reference circuit include:

·       Ignition Off

·       Key out of the ignition switch

·       Retained Accessory Power (RAP) Off

·       Battery charger set at a 2 Amp or less charge rate

·       No scan tool communication with a control module

·       All access doors closed

·       Headlamps Off (Auto headlamps disabled)

·       Any delay lighting Off

·       HVAC afterblow Off

·       Any accessories Off that can work when the ignition is Off

·       Wait up to 60 seconds (after all other listed conditions are met)


TIP: Activating the door lock function with a key fob can greatly reduce the time to achieve a valid ground or low reference continuity test result.


- Thanks to Dave Nowak

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This page contains a single entry by Blog Admin published on May 1, 2010 12:00 AM.

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