Voltage drop testing

If a circuit is under load and the wiring is healthy, the wiring should not “use up” much voltage. Voltage drop testing is the fastest way to find corroded cables, weak grounds and loose connections.

The core idea (in one sentence)

Measure across the suspect cable/connection while the circuit is working. A high reading means that piece is resisting current and stealing voltage from the component.

Do

Test under real load (cranking, blower on, heated screen on)
Put probes at each end of the cable/connection you are judging
Move step-by-step until the drop “jumps” at the bad point

Don’t

Judge a cable with the engine off and no load
Assume a good alternator/starter is bad before checking drop
Forget that a bad ground can mimic every sensor problem

Quick workflow (starter / no-crank / slow crank)

Battery positive to starter B+ (probe on battery +, probe on starter feed). Crank. High drop = bad positive cable/connection.
Starter case to battery negative (probe on starter housing, probe on battery -). Crank. High drop = bad engine/chassis ground path.
If one side is high, split it further: battery post → clamp, clamp → cable, cable → junction, etc.

Charging system: why alternators get blamed unfairly

An alternator can produce healthy voltage at its terminal but the battery sees less due to drop. Test these under load (lights, blower, heated screen):

Alternator output stud to battery positive
Alternator case to battery negative

Sensor faults & “ghost” codes

Many plausibility and communication faults come from shared grounds. If multiple unrelated sensors look odd, check:

Battery negative to engine block under load
Battery negative to body ground (lights/heater load)
ECU ground pins (only if you know the pinout and can back-probe safely)

Rule of thumb: The exact “acceptable” drop varies by circuit and manufacturer. The useful diagnostic move is comparative: the worst drop is where you focus. If your drop is noticeably higher than similar vehicles/circuits, you found your suspect.