Fault example of multimeter voltage drop test
A Changan passenger car once replaced 6 clutch wires in a row, and the clutch pedal felt gradually heavy after a few days of each replacement. Carefully observe the replaced clutch wires, and find that some parts of the outer skin of the wire tube and the plastic part of the inner sleeve of the wire tube have different degrees of deformation, and some parts where the wire tube and the wire core are in contact, there are serious strain marks . Through analysis, it is believed that the strain is not caused by normal mechanical wear, but by some external factors. Connect the negative terminal of the battery with the negative test lead of the voltmeter, connect the engine casing with the positive test lead, and measure the voltage drop on the negative side. When the engine is started, the voltmeter shows a voltage drop of 0.42V. After checking the contacts of the ground wire, it was found that the ground wire nut fixed on the transmission housing hanger was loose, which made the engine and the frame battery grounded. After tightening the loose nuts, start the engine again and test the voltage drop. At this time, the starter is powerful, and the voltmeter shows only a 0.2V voltage drop. Due to the poor contact between the engine and the frame, the current flowed back to the negative pole of the battery due to the poor contact of the negative line when the car was started, so it had to find a return route, and the clutch cable acted as a conductor between the engine and the body at this time. It becomes a conductor with a certain resistance value, and due to self-heating, it causes damage to the surface of the pull wire and the internal pull wire to varying degrees, resulting in abnormal damage to the clutch wire.
Advantages of the voltage drop test method over other methods
The most commonly used voltage drop test method is to measure the starting line and charging line. For the inspection of these lines, if the resistance detection method commonly used by maintenance personnel is used, it has certain limitations, and it cannot find existing hidden troubles. For example, for the kind of "broken wires" wires, if the resistance of the wires is measured with the electric barrier of a multimeter, the resistance value will not increase. The seemingly problem-free measurement results will make it difficult for technicians to directly discover the real situation. If the voltage drop test method is used to test the voltage of different parts when starting the engine, the fault in the circuit can be found accurately and quickly.
