The Voltage Drop Method
The professional technique for locating hidden resistance in cables, connectors and switches.
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What you will learn
- Perform a voltage drop test
- Interpret the result
- Decide the next test
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Voltage drop is the voltage lost across a resistance while current flows. Place the meter across a suspected cable, connector or switch while the circuit works, and read the difference. A healthy connection loses almost nothing; a faulty one steals voltage from the load.
The method is powerful because it tests the circuit as a whole under real conditions. It finds partially broken cables, corroded terminals and pitted switch contacts that a continuity test happily passes.
- Confirm the circuit is operating and drawing current.
- Place the red probe on the supply side of the suspect component and the black probe on the load side.
- Read the voltage; a significant reading identifies the resistance.
- Move along the circuit to narrow down the exact cable, terminal or switch.
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Key points
- The circuit must be operating
- Voltage drop is proportional to resistance
- Compare the drop with the expected value
All values shown are training examples. Real thresholds depend on the manufacturer, engine, temperature and system — always confirm with the vehicle service documentation.
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