Voltage Drop Testing
An electrical diagnostic method that measures the voltage lost across a circuit or component due to resistance while current is flowing.
Voltage drop testing is a highly effective diagnostic technique used to locate hidden resistance in automotive electrical circuits. Unlike resistance testing (using an ohmmeter on an unpowered circuit), a voltage drop test is performed with the circuit fully powered and operating under load. It measures the difference in voltage between two points in a circuit using a digital multimeter (DVM) set to DC Volts.
According to Ohm's Law, voltage will drop across any resistance in a circuit. In a healthy circuit, nearly all the voltage should drop across the load (such as a starter motor or headlight bulb). If a high voltage drop (typically more than 0.2 to 0.5 volts) is measured across a wire, fuse, connector, or ground strap, it indicates excessive resistance caused by corrosion, loose terminals, or frayed wires.
For example, a starter motor that cranks slowly could be caused by a bad battery, or it could be due to a voltage drop in the battery positive cable. By testing between the battery positive post and the starter motor terminal during cranking, a technician can see if voltage is being lost in the cable itself. Resolving voltage drops ensures electrical components receive full operating voltage.