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  .

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 Digital Multimeter (DMM), cause significant voltage loss in higher current circuits.
• The Digital Multimeter (DMM) applies a small amount of voltage to the circuit or component to calculate resistance. As a result, Digital Multimeter (DMM) ohmmeters are very sensitive to any level of voltage present. Voltage present in the circuit will corrupt the Digital Multimeter (DMM) reading.

Checking For Unintended Continuity (Shorts) To Other Circuits 

• A Digital Multimeter (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 Digital Multimeter (DMM) indication (no detected resistance).
• Shorts to voltage are checked with a 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 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.

• 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 an 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 a service information procedure), as the component could be damaged by a direct connection to ground/power.