Diagnostic Methods

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 Digital Multimeter (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 Digital Multimeter (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 Digital Multimeter (DMM), cause significant voltage loss in higher current circuits). Digital Multimeter (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 Digital Multimeter (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 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).