Measuring Automotive Circuits: Notes

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.