Testing for Voltage Drop

Resistance testing (Ohms) on an automotive circuit is practically useless for finding high-resistance faults. A circuit with a single strand of copper intact will read 0 Ohms, but will immediately fail to run a 15A fuel pump. You must test the circuit while it is working.

The Concept

Voltage drop measures the amount of voltage "used up" by resistance in a wire or connection. In a perfect world, a wire uses 0 Volts, leaving all 12V for the component (like a lightbulb). If a corroded connector has high resistance, it acts like a resistor and "steals" voltage. The bulb gets dim.

Expected Voltage Drop Calculator

Calculate the natural theoretical voltage drop of a copper wire over distance.

(Calculator assumes the ground return path is the same length)

How to Perform the Test

  1. The circuit must be powered and under load. The component (fan, pump, light) must be turned on. If the circuit is open (off), there is no current flow, and therefore zero voltage drop, regardless of corrosion.
  2. Set your Multimeter to DC Volts.
  3. Place the Red lead on the Battery Positive post.
  4. Place the Black lead on the positive input terminal of the component (e.g., the power wire at the fuel pump).
  5. The meter will read the exact amount of voltage lost between the battery and the pump. A reading of 0.2V is excellent. A reading of 2.5V indicates a severe problem (corroded fuse block, bad relay contacts, or damaged wire).

Repeat the process on the ground side. Put one lead on the Battery Negative post, and the other on the ground wire at the component. Bad grounds are the cause of 80% of automotive electrical issues.