The testing of ground and low reference circuits can be performed in multiple ways using multiple tools. Ground circuits may be tested in the following ways:

• Measuring resistance with a DMM
• Testing continuity with a test lamp
• Measuring voltage drop with a DMM

Each method of testing a ground or low reference circuit requires attention and understanding of the tools and results to ensure misdiagnosis does not occur based on the results:

• A DMM requires a more specific set of conditions on the vehicle when testing, but will accurately measure resistance. A DMM may not identify a ground or low reference circuit that maintains continuity, but is damaged and has compromised load carrying capability.
• Testing a ground or low reference circuit with a test lamp is a good way to quickly determine continuity, but may not identify all instances of excessive resistance.
• Measuring voltage drop with a DMM is a very accurate way to test a ground or low reference circuit, but requires the most time and effort to access components and ground locations.

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.

When using a test lamp, it is important to understand the impedance of the test lamp. A test lamp with high impedance, such as an LED, will illuminate with very little current flow. A test lamp with low impedance, such as an filament bulb, will require more current flow to illuminate.

When testing any ground or low reference circuit, you must understand the purpose and current carrying capacity of the ground or low reference circuit being tested when choosing an appropriate test lamp. If the ground or low reference circuit is for a high current device, such as a blower motor, using a high impedance test lamp may indicate a good ground or low reference circuit because the current required to illuminate a high impedance test lamp is much less than the current required to operate the component. You can determine the current carrying capacity of a ground or low reference circuit by referencing the circuit gauge in the schematic or connector end view. Larger gauge wires have a high current carrying capacity.

The recommended test lamp and the test lamp referenced in all diagnostic procedures is EL-35616-200 . This is considered a high impedance test lamp that utilizes a filament bulb, but does provide more load than an LED.

Test lamps do have limitations when measuring a high resistance condition:

• High impedance test lamps may illuminate with up to 200 Ω present on the ground or low reference circuit
• Low impedance test lamps may illuminate with 20–30 Ω present on the ground or low reference circuit

It is important to remember that if a test lamp illuminates, there still may be a high resistance condition present. This is especially important when using a high impedance test lamp to test any ground or low reference circuit or when using any test lamp to test a ground circuit with a high current carrying capability.

Voltage drop is measuring the difference in potential (voltage) at two points. Using a DMM to measure voltage drop on a ground circuit will identify both continuity and current carrying capability. Because voltage drop is measured using the actual component and electrical loads seen on an intact circuit, it is very accurate in determining and identifying any faults on the ground or low reference circuit.

Measuring voltage drop with a DMM on a ground or low reference circuit can be performed in the following ways:

• Low Reference Circuit – Back-probing the low reference circuit terminal at the component connector and back-probing the low reference circuit terminal at the control module connector
• Ground Circuit – Back-probing the ground circuit terminal at the component connector and probing the actual ground location

With the DMM properly connected to the circuit being tested, the component can be activated and the voltage drop observed. Typical voltage drop should be less than 1 V, but may be higher in some instances. A large voltage drop would be an indication of a damaged ground or low reference circuit.

Additionally, ground circuits may be tested by measuring voltage drop between the component and negative battery terminal. If there is acceptable voltage drop when measured between the component and ground location, but excessive voltage drop is present when measured between the component and negative battery terminal, this may indicate a poor ground connection at the ground circuit. This may be caused by:

• Loose ground nut or bolt
• Damaged or corroded ground ring terminal
• Damaged or corroded ground stud or bolt
• Painted ground stud or bolt location