Testing Ground and Low Reference Circuits
Because a test lamp can light with up to 200 Ω in a ground or low reference circuit they are not recommended to be used for this type of test. A low impedance test lamp can light and indicate the circuit is good when there is as much as 20-30 Ω present, while a high impedance test lamp can light with up to 200 Ω present in the circuit. This is why the Digital Multi-Meter (DMM) is used when validating a ground or low reference circuit continuity.
When using the 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. This may cause 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 vehicles 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 would be a control module housing (if the control modules housing is metal and grounded), door jamb striker (if attached to metal), under dash metal frame work, 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 Ω with the ignition ON, and drop to 15-25 Ω after the ignition is turned OFF. The reading will drop below 10 Ω after 30-40 seconds, then below 5 Ω after 60 seconds. Once the vehicle completely goes to sleep (generally 3-10 minutes) the reading will drop below 0.3 Ω.
The following list of conditions may need to be met to ensure a valid continuity reading on a ground or low reference circuit:
| • | Key out of the ignition switch (when not equipped with keyless entry and remote start) |
| • | Retained Accessory Power (RAP) OFF (open and close the driver door after ignition OFF) |
| • | Battery charger set at a 2 A or less charge rate |
| • | Scan tool not communicating with any vehicle control module (in some cases it may need to be disconnected from the DLC) |
| • | Headlamps OFF (auto headlamps disabled) |
| • | Any accessory that can work when the ignition is OFF |
| • | Wait up to 60 seconds (after all other listed conditions are met) |
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.
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