Dynamic Circuit Resistance Testing

Making Voltage-drop Measurements 

A voltage-drop test measures the loss of power or voltage in a circuit. Losses can be measured on the ground or power (negative or positive) circuits of any device.

• Measuring voltage-drop requires
  • A voltmeter connected at the beginning and end of the suspect circuit.
  • An operating, or attempting to operate, circuit. Power must be on and available to flow.

• The polarity of the voltmeter lead connections should follow conventional current flow.
• A zero-volt reading indicates bad voltmeter connections or the component has not been turned on.
• A small amount of voltage indicates normal circuit loss. In 12-volt circuits, this is usually less than 0.5 volts (expect less than 5 percent of circuit operating voltage).
  • Voltage indications greater than 0.5 volts indicate abnormal voltage loss. Abnormally high voltage drop indicates bad wires or connectors are causing high circuit resistance.

Voltage In Voltage Out 

The purpose of a circuit is to supply source voltage and a good ground to a load so that the load can function. A Voltage In Voltage Out test is another way to measure voltage drop across a load. This test measures the amount of power or voltage in the circuit going into and out of a load to help determine which side of the circuit as well as the location of the circuit fault.

• Measuring Voltage In Voltage Out requires
  • The negative lead of the voltmeter connected to ground or battery negative terminal.
  • An operating, or attempting to operate, circuit. Power must be on and available to flow.

• Using the positive terminal of the voltmeter, measure the power or voltage on the power side of the load and then followed by the negative side of the load.
  • Power side should be within 0.5 Volts of battery voltage.
  • Ground side should be greater than 0 volts but less than 0.5 volts.
  • 0 volts or source voltage indicate an open circuit.