Voltage Drop Testing the bad connection ohms can't see
You measured the wire, it read good resistance, and the circuit still doesn't work. That's because resistance is measured cold, with no current. A connection that's fine at rest can choke under load — and only voltage drop catches it, because it tests the circuit doing its actual job.
Start here
Why voltage drop beats an ohmmeter
Resistance is tested cold and dead. An ohmmeter pushes a tiny current through a de-energized circuit — a corroded joint can still read near zero and then fall apart under real amperage.
Voltage drop is tested hot and loaded. With the circuit working, you measure the voltage lost across a connection. Loss = resistance where it matters.
The meter goes in parallel — one probe each side of the connection you're testing (or across the whole feed/ground). You're reading what the circuit throws away as heat.
Rule #1: if the circuit has to carry current to fail, test it carrying current. Ohms lie; drop tells the truth.
The numbersWhat passes, what fails
Ground side≤ ~0.1 V across a ground path — grounds should be nearly perfect
Feed / positive≤ ~0.2–0.5 V across the whole feed — includes connectors and the fuse
Single connector≈ 0 V (a few mV) — a good clean connection drops almost nothing
Switch / relay contacts≤ ~0.2 V across the closed contact — more means burnt contacts
Whole circuit budgetfeed + ground combined stays low — every 0.1 V lost is load the component never gets
Where to probeThe connections that hide high resistance
Grounds & ground straps
#1 culprit
Body and engine grounds corrode, loosen, and paint-over. A bad ground fakes a hundred different faults.
Probe from the component's ground terminal to the battery negative with the load on. Should be a few mV, not tenths of a volt.
The positive feed
Under load
From battery positive to the component's power terminal, load running. This catches every connector, splice, and the fuse in between.
High drop here starves the component — dim lights, slow motors, a fuel pump that can't build pressure.
Connectors & splices
Divide & conquer
Found drop in a feed? Walk the meter down it — probe across each connector to split the circuit in half until the drop is isolated.
Green crust, spread pins, and heat-discolored plastic are where it lives.
Switches & relays
Burnt contacts
A switch or relay that clicks but drops half a volt across its contacts is toast internally — the contacts are pitted.
Common on headlight switches, blower relays, and starter solenoids that "sometimes" work.
Where techs go wrong
The traps
Testing with no load. No current, no drop — you'll read 0 V across a bad joint and call it good. Turn the circuit ON.
Trusting an ohm reading. "Continuity is fine" means nothing under 20 amps. Resistance and voltage drop are not the same test.
Ignoring the ground. Half the "no power" complaints are actually a bad ground. Test both sides, every time.
Back-probing sloppily. Probe the actual terminals, not paint or a nearby stud, or you'll add your own drop to the reading.
MethodFind the loss in four moves
Load the circuitTurn the component on so real current flows. Voltage drop only exists while the circuit is working.
Test the ground side firstComponent ground to battery negative. If the ground's bad, nothing downstream reads right — fix it before you go further.
Test the feed sideBattery positive to the component's power terminal. Any big number means loss between here and there.
Split the bad side in halfWalk the meter connector-to-connector until the drop lands between two points — that's your corroded joint or burnt contact.
The unfair advantage
Not sure what "too much" is on this circuit?
Specs vary by circuit and current. Tell Kate what you're testing, the amperage it carries, and the drop you measured, and she'll tell you whether it passes, which side to chase, and the next probe point. No more guessing whether 0.3 V is fine or a failure.