How is a voltage drop test done with a multimeter?
Set the multimeter to the DC voltage range and touch the two ends of the connection to be tested while the circuit is UNDER LOAD. If the reading is below 0.2 V the connection is good, between 0.2-0.5 V it must be cleaned and retightened, above 0.5 V it is a fault. On the chassis path the target is in the order of 0.05 V; a resistance measurement is no substitute for this test because it hides a bad connection.
The voltage drop test is the most reliable method of hunting a bad connection. Its logic is simple: almost no voltage drops across a good conductor even when current flows through it, whereas a resistive point generates a voltage across itself when current passes.
Steps:
- Set the multimeter to the DC volt range and select a low range.
- Run the circuit and make it draw the real load. A value measured with no load is meaningless.
- Put the probes on the two ends of the section you want to examine: fuse input-output, terminal input-output, battery negative-device negative and so on.
- Interpret the reading against the thresholds: below 0.2 V is good, 0.2-0.5 V is borderline, above 0.5 V is a fault.
- Narrow down the section where you found a large drop by splitting it in two; continue in whichever half has the drop.
Why we do not measure resistance: a corroded contact looks conductive at the microamperes an ohmmeter drives but collapses at 20 amperes. Likewise an ohmmeter cannot be used while the circuit is energised. As a reference, on the chassis cable running from the battery negative to the engine block the target is below 0.2 V, and on a good chassis path it is 0.05 V.
Full coverage of the topic: DC motor and driver fault diagnosis matrix