How do I do a voltage drop test?
Short answer: With the circuit operating, put the meter leads at both ends of one side of the circuit, power side or ground side; the meter reads the voltage lost in that wire, connection or switch.
Voltage drop testing finds resistance that continuity checks miss. An ohmmeter test with the circuit off uses a tiny current, so a wire with only a few strands left can read fine. Under real load, that same wire drops volts and the component starves.
To test the ground side, turn the circuit on, put the black lead on the battery negative post and the red lead on the component's ground terminal or case. The meter shows how much voltage is lost getting back to the battery. For the power side, put the red lead on the battery positive post and the black lead on the component's power terminal.
Typical shop guidelines are around 0.1 volt per connection, a few tenths for a whole low-current circuit, and about 0.5 volt for a starter circuit while cranking, but check service information for specific limits. If the drop is high, move one lead toward the other along the circuit until the drop disappears; the last section you skipped holds the problem. Keep hands and leads clear of belts and fans while the engine runs.
Tools used for this job
General guidance only. Follow the vehicle and tool manufacturer's procedures, and confirm vehicle coverage on the tool maker's list before buying.