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2) EVERYTHING HAS RESISTANCE. The wires, connectors, and switch contacts that make up a circuit all have some
resistance and, as its name implies, resistance opposes (resists) current. In an undamaged, properly designed circuit,
normal resistance is small enough that it doesn’t keep the load from working.
UNWANTED RESISTANCE IN THE CIRCUIT REDUCES THE AMOUNT OF ELECTRICAL ENERGY DELIVERED TO THE
LOAD.
Causes of unwanted resistance include: loose connections, corroded connections, broken wire strands, pitted relay
contacts, and other physical damage that resists current.
SOME CIRCUIT PROBLEMS CAN BE SPOTTED QUICKLY WITH THE NAKED EYE. Some won’t be so easy to identify.
Even if you cannot see visible causes for unwanted circuit resistance, the voltage drop test will find them. Symptoms of
unwanted resistance are problems like a light bulb that glows dimly instead of shining brightly, or a motor that turns too
slowly.
CURRENT THROUGH RESISTANCE RELEASES HEAT. (Remember this the next time you toast a slice of bread!) If we want
heat to defrost a piece of glass or warm our leather seats with an electrical heater, resistance is a good thing; a desirable
voltage drop. On the other hand, a hot battery cable and slow-turning motor indicate wasted electrical energy.
VOLTAGE DROP TEST EQUIPMENT
Voltage drops are tested with a voltmeter or scope. (A test light indicates whether voltage is present, but it doesn’t measure
the amount of voltage.)
• Voltmeters measure and display the difference in voltage levels between the voltmeter test leads.
• The black test lead connected to the meter COM port should be considered the “reference.” This is the baseline; it is
the starting point for the measurement. Voltage at the red probe tip connected to the meter VOLT port is compared to this
starting point and displayed on the meter as a number. (Reversing the test leads won’t hurt a digital meter, but we may see
a negative voltage value displayed.)
VOLTAGE DROP TESTS
The voltage drop test has incredible advantages over testing resistance with an ohmmeter. A test with an ohmmeter may
show continuity, but will not indicate if a damaged wire is capable of carrying the needed current. A similar situation
would occur with a wire that is too small to conduct the needed flow of electrons. Such an undersized wire would show
perfect continuity with an ohmmeter, yet become a restriction in a circuit with heavy current. Due to the inconclusive
results and the need to disconnect the components, many consider troubleshooting with an ohmmeter a potentially
misleading waste of time. Ohmmeters do have their place, like testing the resistance of the motor relay pull in windings,
but for most troubleshooting, the voltage drop test is the way to go.
Electrical voltage drop varies according to current flow. Unless you operate the circuit so current flows through it, you can’t
measure voltage drop. Because an ohmmeter’s battery can’t supply the current that normally flows through most circuits,
ohmmeter tests usually can’t detect restrictions as accurately as a voltage drop test.
