Measuring Automotive Circuits

Wiring Pin (Terminal) Fit And The Use Of Rotunda Flex Probes 

• To avoid wiring pin (terminal) damage, Rotunda Flex Probes NUD105-R025D or Terminal Probe Kit 29-011A must be used to connect test equipment or jumper wires to pins (terminals).
• Male to female pin (terminal) fit is critical for correct connection and durability.
  • Pin (terminal) fit may be checked by using the mating pin (terminal) to test for normal separation force (a damaged pin or terminal will have very low separation force from the mating pin or terminal)
  • Correctly checking the separation force of small pins (terminals) may require removal of the connector terminal guide/retainer if it adds drag to the pin (terminal) insertion or removal
• Replace damaged connectors or pins (terminals).

Checking Power-Providing Circuits 

• Measuring a power wire with the intended load disconnected using a DMM will only find open circuits (open fuse or wire).
• Recommended practice:  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 and illuminated. A reduction in the voltage present during test-light-loading indicates excessive circuit resistance.
• Recommended practice:  Circuits carrying more than one ampere* should be loaded with a device requiring similar current (e.g., a headlamp bulb may be effective). A reduction in the voltage present during loading indicates excessive resistance.
• *Circuit current is matched to wire gauge/size; Examples:
  • Conductor sizes of 24 gauge (.5 mm) or smaller are generally used to carry approximately 1 ampere (1000 mA) or less. Use of the test light to load these circuits is appropriate.
  • Conductor sizes of 20 gauge (.8 mm) or larger are generally used to carry approximately 5 amperes (5000 mA) or more. Match the substitute load (measure substitute load current first as necessary) to this current level.

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.
• Recommended practice:  Expect less than 2 ohms for most small diameter (18 gauge and smaller) wires.
• Ohmmeter accuracy is limited to circuits carrying less than approximately 5 amperes (this is due to the fact that very small resistances, undetectable by a DMM, cause significant voltage loss in higher current circuits).
• DMM ohmmeter readings are easily corrupted by the normal voltage present (battery connected) in many ground circuits.
  • Recommended practice:  Reverse the leads and check for changes in the measurement. Reversing the DMM lead connections should never change the resistance measurement (unless the circuit contains a semi-conductor). Measurement (non-semi-conductor) differences when leads are interchanged at the test points indicate invalid test results. The presence of voltage corrupts the reading, and causes the meter reading to change when the leads are reversed.

Checking Circuit Continuity 

• Recommended practice:  Expect less than 2 ohms of resistance for most wires.
• Ohmmeter low-resistance resolution (approximately 0.1 ohm) limits its use to circuits carrying less than approximately 5 amperes. This is due to the fact that very small resistances, below the resolution of a DMM, cause significant voltage loss in higher current circuits.
• The DMM applies a small amount of voltage to the circuit or component to calculate resistance. As a result, DMM ohmmeters are very sensitive to any level of voltage present. Voltage present in the circuit will corrupt the DMM reading.

Checking For Unintended Continuity (Shorts) To Other Circuits 

• A DMM ohmmeter may be used to detect undesired circuit connections to:
  • Ground
  • Other unpowered circuits
• Recommended practice:  Expect greater than 10, 000 ohms of resistance between two separate circuits; the best result is an open circuit DMM ohmmeter indication (no detected resistance).
• Shorts to voltage are checked with a DMM voltmeter
• Recommended practice:  Turn ignition on (with battery connected) and measure the circuit for any voltage present (none should be present)

Checking Circuits By Back-Probing A Connector 

• Back-probing should be a testing method of last resort. It 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.
• Do not force test leads or other probes into connectors. Adequate care must be exercised to avoid connector terminal damage while ensuring that 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 Rotunda Back-Probe Pins POMA6411 to assist in making a good test connection and to prevent connector or terminal damage during back-probing.
• Do not test for the presence of voltage at a single point where zero volts is a possible result (you cannot tell the difference between a bad probe contact and a zero volt result).
• Do not test for continuity/opens (using a DMM ohmmeter) between two points (you cannot tell the difference between bad probe contacts and an open circuit).
• Back-probing may be used where the circuit must be analyzed with the voltage-drop method (if the circuit carries greater than 5 amperes and no other means of testing will definitively eliminate circuit resistance as a possible fault). 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 is an acceptable remedy for these testing dilemmas.

Circuit Analysis Using Jumper Wires (Creating Substitute Circuits) 

• Jumper wires may be employed for circuit analysis.
• Cautions: 
  • Always use fused jumper wires - the recommended universal-testing jumper wire fuse is 2-5 amperes; larger fuse ratings should be used only in special circumstances.
  • Use flex probes or equivalent to prevent connector terminal damage (flex probes are not intended to carry higher currents necessary to operate motors such as a cooling fan or blower motor).
  • Follow Service Information testing directions when using jumper wires to avoid component or harness damage due to 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.

Checking Modules 

• Generally, module failure rate is very low and therefore replacement modules usually do not resolve the root cause. Incorrect replacement of a module is often the result of inadequate testing.
  • Understand the correct module function.
  • Make sure programmable parameters are set correctly for the function in question (Refer to Module Configuration for more information).
  • Resolve DTCs first - as directed by Diagnostic Routines.
  • Test all inputs, both hard-wired and networked.
  • Test outputs (see "Checking module switching circuits" below).
  • Check applicable TSBs for module software changes (flash programming).
• Checking module switching circuits.
  • Using the scan tool module-output command function (for example, IDS Output State Control) to activate components is a fast way to confirm an output is capable of being switched on by the module. Testing that reveals normal module-output function confirms the need to analyze the module inputs.
  • Don't apply ground or power directly to module-switched components with jumper wires (unless directed by an article procedure), as the component can be damaged by a direct connection to ground/power.