Diagnosing Automotive Faults by Clever Use of Multimeter Voltage Drop
For maintenance technicians, diagnosing faults by measuring voltage with a multimeter is commonplace. However, fault diagnosis based on voltage drop is less common. The so-called voltage drop refers to the voltage reduction generated when current flows through a resistor (wire, connection point or switch), namely the difference between the supply voltage and the voltage applied to the electrical load. The skillful use of the voltage drop test method for fault diagnosis often delivers irreplaceable diagnostic results.
Experiments show that the maximum voltage loss in any circuit is 3% of the supply voltage. For automobiles equipped with a 12 V power supply, the maximum allowable voltage drop is 0.36 V. A voltage drop exceeding 0.40 V in a circuit indicates an abnormality, i.e. the presence of high resistance.
Although the voltage drop test method applies to any circuit, it is most frequently used to test starting and charging circuits in automobile repair. Some technicians may wonder: if high resistance in wires, switches or connections is suspected, why not simply measure resistance with an ohmmeter? To understand the difference between resistance testing and voltage drop testing, we will analyze their similarities and differences using battery cables as an example.
For instance, if most copper strands inside a battery cable are broken and only a few strands remain connected, the resistance measured by an ohmmeter may read zero. These thin remaining strands cannot carry heavy starting current. When the engine cranks, the broken strands create resistance to current flow, causing the cable to heat rapidly and resulting in slow cranking of the starter motor. This is caused by the voltage drop at the starter.
In such cases, resistance testing shows its limitations and fails to detect hidden faults. When measuring cable resistance with the ohm range of a multimeter, no increased resistance will be shown. The seemingly normal reading prevents technicians from identifying the actual problem directly.
By contrast, the voltage drop test can quickly and accurately locate circuit faults by measuring voltage at different points while cranking the engine. The starting circuit is used below to illustrate the voltage drop measurement procedure in circuits.
