Measuring Automotive Circuits

Checking Power-Providing Circuits 

Measuring a power providing circuit for voltage with the intended load disconnected will only identify open circuits (for example, an open fuse or circuit). This measurement practice will not identify excessive circuit resistance. 

• Circuits carrying approximately 200-1000 mA may be loaded with a 250-350 mA test light. Measure circuit voltage with a DMM while the test light is connected. A reduction in the voltage present during test-light-loading indicates excessive circuit resistance.
  • Conductor sizes 24 gauge (0.5 mm) or smaller are generally used to carry approximately 1000 mA (1 ampere) or less. Use of a 250-350 mA test light to load these circuits is appropriate.

• Circuits carrying more than one ampere should be loaded with a device requiring similar current (for example, a brake light bulb). A reduction in the voltage present during loading indicates excessive resistance.
  • Conductor sizes 20 gauge (0.8 mm) or larger are generally used to carry 1 ampere (1000 mA) or more.
  • Using a voltage-drop measurement is best practice and more accurate for higher current circuits.

Checking Ground-Providing Circuits 

The best method of checking ground circuits is to measure the circuit voltage drop during component operation (or attempted operation).

• An ohmmeter may be accurately used if the battery has been disconnected. 
  • Expect less than 2 ohms for most small diameter (18 gauge and smaller) wires.
  • Disconnecting the battery is critical because DMM ohmmeter readings are commonly corrupted by the normal voltage present (battery connected) across body and chassis ground circuits.

Checking Circuit Resistance or Continuity 

• Expect less than 2 ohms of resistance for most wiring harness circuits.
• A standard DMM ohmmeter's low-resistance resolution (approximately 0.1 ohm) limits its accurate use to circuits carrying less than approximately 5 amperes. This is because very small resistances, undetectable by a standard DMM, cause significant voltage and power loss in higher current circuits.
  • A voltage drop measurement is required for higher current circuits.

• Standard DMM apply a small amount of voltage to the circuit or component to calculate resistance. As a result, these ohmmeters are very sensitive to any level of voltage present. Voltage present in the circuit will corrupt the DMM reading.
  • Recommended practice: Reverse the leads and check for changes in the resistance measurement. Reversing the DMM lead connections should never change the resistance measurement (unless the circuit contains a semi-conductor).

    Measurement differences when leads are interchanged at the test points indicate invalid test results. 

Checking For Unintended Continuity (Shorts) To Other Circuits 

A DMM may be used to detect undesired circuit connections to:

• Ground
  • Recommended practice: Disconnect both ends of the circuit. Measure the resistance between the suspect circuit and ground. Expect resistance greater than 10, 000 ohms.

• Other unpowered circuits
  • Recommended practice: Disconnect both ends of the two circuits. Using an ohmmeter measure between the two circuits. Expect resistance greater than 10, 000 ohms.

• Powered circuits
  • Recommended practice: Disconnect both ends of the circuit. Turn on ignition/run power (key on). Using a voltmeter, measure the voltage between the suspect circuit and ground. Expect no voltage.

Checking Circuits by Back-Probing a Connector 

Back-probing should only be employed where a diagnostic step requires a circuit to be tested under actual operating conditions. Back-probing is a risky testing method due to the uncertainty of the probe connection and the possibility of damaging terminals

• Back-probing may be used where a circuit must be analyzed with the voltage-drop method. All voltage-drop tests will measure a small amount of voltage (expect less than 5 percent of circuit operating voltage).
  • A zero-volt result indicates incorrect test conditions (no current flow or bad back-probe connections).

• Occasionally, module failure mode behavior will change the operation of a circuit when it is opened for testing. Back-probing allows testing without altering normal module function.

Back-probing may be employed for circuit analysis if the following cautions are carefully observed:

• Do not force test leads or other probes into connectors. Adequate care must be exercised to avoid connector terminal damage while ensuring good electrical contact is made with the circuit or terminal. Failure to follow these instructions may cause damage to wiring, terminals, or connectors, and subsequent electrical faults.
  • Use back probes specifically designed for the purpose to assist in making a good test connection and to prevent connector or terminal damage during back-probing.

• Back-probing is the wrong test for a single point test for presence of voltage. When zero volts is a possible result, you cannot tell the difference between a bad probe contact and a zero-volt result. Disconnect the circuit and test normally.
• Back-probing is the wrong test for circuit continuity or opens (using an ohmmeter) between two points. You cannot tell the difference between bad probe contacts and an open circuit. Disconnect and isolate the circuit, and test normally.

Circuit Analysis Using Jumper Wires (Creating Substitute Circuits) 

Jumper wires may be employed for circuit analysis if the following cautions are carefully observed:

• Always use fused jumper wires -- the recommended universal-testing jumper wire fuse is 5 amperes or less; larger fuse ratings should be used only when the load requires them.
• Use flex probes or equivalent to prevent connector terminal damage.
  • Flex probes are not intended to carry high current (greater than 5 amperes). Do not use them to connect power for cooling fans, blower motors, or other high current devices.

• Follow diagnostic test directions carefully when using jumper wires to avoid component or harness damage caused by incorrect jumper connections.
• Never repair a circuit by adding a new wire in parallel to the old one (overlaying the circuit) without fully understanding what caused the circuit to fail. Always find, examine, and repair the fault to correct the root cause and to repair any adjacent wiring that has been damaged.

Making Voltage-drop Measurements 

A voltage-drop test measures the loss of power or voltage in a circuit. Losses can be measured on the ground or power (negative or positive) circuits of any device.

• Measuring voltage-drop requires
  • A voltmeter connected at the beginning and end of the suspect circuit.
  • An operating, or attempting to operate, circuit. Power must be on and available to flow.

• The polarity of the voltmeter lead connections should follow conventional current flow.
• A zero-volt reading indicates bad voltmeter connections or the component has not been turned on.
• A small amount of voltage indicates normal circuit loss. In 12-volt circuits, this is usually less than 0.5 volts (expect less than 5 percent of circuit operating voltage).
  • Voltage indications greater than 0.5 volts indicate abnormal voltage loss. Abnormally high voltage drop indicates bad wires or connectors are causing high circuit resistance.

Using Module PID 

Electronic modules connected to a network usually offer diagnostic scan tool measuring of internal data or operating values. This data is known as a parameter ID or PID.

• Using a diagnostic scan tool, PID input values, output states, and diagnostic states may be read.
• Monitoring PID information in the datalogger function of the diagnostic scan tool allows detailed, accurate testing without vehicle disassembly and cumbersome equipment.
• Datalogger displays are the "behind the scenes" information for almost every function on a modern vehicle.
• Controlling most module outputs with a PID is possible. For example, an output state command PID for a cooling fan, engine RPM, and many other module outputs are commonly available.