Measuring Resistance with a DMM
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 vehicle power mode in Vehicle ON or Vehicle in Service Mode and drop to below 5 Ω as individual control modules go to sleep. Once the vehicle completely goes to sleep, the reading will drop below 0.3 Ω. The time it takes for controls modules to go to sleep will vary based on the control module and the vehicle environment. Some control modules may also periodically awaken in Vehicle OFF, then go back to sleep again.
The recommended DMM and the DMM referenced in all diagnostic procedures is EL-39200-B . This is a highly accurate DMM, able to measure and report very low resistance values. Inexpensive or low quality DMMs may not have the resolution to display low resistance values, which may result in a misunderstanding of the test result and a potential misdiagnosis.
The following list of conditions may need to be met to ensure a valid continuity reading on a ground or low reference circuit:
- Vehicle off
- Transmitter out of range of the vehicle
- Retained Accessory Power off (open and close the driver door after vehicle 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)
- All entry doors closed
- Headlamps off (auto headlamps disabled)
- Any delay lighting off
- HVAC after blow off
- Any accessory that can work when the vehicle power mode is Vehicle OFF
- Wait up to 60 s (after all other listed conditions are met)
Locking the doors with a remote transmitter can greatly reduce the time to achieve a valid ground or low reference continuity test result.
Some low reference circuits that use a driver to isolate the circuit while the ignition is off. This is done in most cases for accidental reverse battery connections or reverse polarity jump starting. When this driver is present, the ignition will need to be on to validate the circuit.