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Voltage drop testing — the test most techs skip
A resistance check pushes about a milliamp through a corroded joint and reports it as fine. Put 20 amps through that same joint and it drops two volts and gets hot. Voltage drop is the only test that finds this, and it is the reason experienced techs stop chasing intermittent electrical faults.
Measure across, under load
A voltage drop test puts the meter in parallel with the thing you suspect — a cable, a connector, a ground strap — while the circuit is running and carrying its normal current.
The reading is the voltage being wasted across that piece. A perfect conductor drops zero. Every tenth of a volt you read is energy turning into heat at a joint instead of doing work at the load.
Why ohms lies here
A corroded connection is not a fixed resistance. It behaves like a tiny film that breaks down under current. At the milliamp your ohmmeter uses, it might read 0.3 Ω and look acceptable.
That same 0.3 Ω passing 20 A drops 6 volts and dissipates over 100 watts. The circuit has to be loaded for the fault to appear, and that is why the meter must be in volts, not ohms.
Halving finds it fast
Do not probe every connection in order. Put one lead at the battery and the other halfway down the circuit. If the drop is there, the fault is in that half. If not, it is in the other.
Split the bad half again. Four or five moves narrows almost any harness to a single connection, no matter how long it is.
How to test it
- Load the circuitTurn the load on and leave it on. An unloaded circuit will read zero drop even when badly corroded.
- Meter across the suspectRed lead on the upstream side, black on the downstream side of the same wire, connection or ground.
- Read while it runsCompare against the limits below. Anything over means that piece is eating voltage the load should be getting.
- Do the ground side tooGround faults are more common than feed faults and produce identical symptoms. Test battery negative to component case the same way.
Numbers to work to
| Per connection | under 0.1 V | a single terminal, splice or plug |
| Per wire or cable | under 0.2 V | end to end, loaded |
| Ground path total | under 0.1 V | component case to battery negative |
| Whole circuit budget | under 0.5 V | battery positive to load, feed side |
| Starter cable, cranking | under 0.5 V | each side, measured while cranking |
Common mistakes
- Testing with the load off and concluding the circuit is good.
- Checking only the positive side. Bad grounds cause dim lights, slow motors and phantom faults just as often.
- Backprobing a connector so gently that you read the drop of your own probe contact instead of the joint.
- Chasing a drop that is really the load itself — measuring across the bulb, not the wire, correctly reads near battery voltage.
Drill this until it's automatic
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