Voltage Drop and High Resistance
Diagnosing High Resistance
High resistance in a circuit kills components and causes intermittent failures that are nearly impossible to find without voltage drop testing. A circuit can look completely intact — no breaks, no corrosion visible externally — and still have a connection that is failing internally under load.
When to suspect high resistance
A component that works sometimes but not always under load. A motor that runs slowly or weakly. A starter that cranks slowly on a battery that tests good. Voltage drop test the circuit under load. Work section by section. The section with the highest drop contains the fault.
Where to look first
Battery terminals — corrosion under what looks like a clean post. Ground connections at the engine block, chassis, and body — especially on vehicles that have had paint work. Fusible links and main fuses that have been heat-cycled for years. Any connection that has been disconnected and reconnected multiple times without being cleaned. Aftermarket wiring splices. These are the high-probability locations before you start probing everywhere.