The multimeter pressure drop test method is rarely used
It is common for maintenance technicians to use a multimeter to detect voltages to diagnose faults, but it is uncommon to diagnose faults based on voltage drops. Skillfully using the voltage drop test method to diagnose faults can play an irreplaceable diagnostic role in many cases. Experiments have shown that in all circuits, the maximum voltage loss is 3% of the power supply voltage. Then the maximum voltage drop in a car using a 12V power supply should be 0.36V. If the voltage drop in the circuit exceeds 0.4V, it can be regarded as a circuit abnormality. That is, there is high resistance.
(1) Advantages of voltage drop testing method compared to other methods
The voltage drop test method is most commonly used 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. It cannot detect existing faults. For example, if you use a multimeter's electrical barrier to measure the resistance of a wire that is "coupled", it will not show an increase in resistance. 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.
(2) Voltage drop test failure example
A Changan passenger car once had 6 clutch wirings replaced in a row. After a few days of each replacement, the clutch pedal felt gradually heavier. After carefully observing the replaced clutch wires, we found that some parts of the outer skin of the wire tube and the inner plastic part of the wire tube were deformed to varying degrees, and some parts where the wire tube and the wire core were in contact, there were serious signs of strain. . Through analysis, it is believed that this strain is not normal mechanical wear and tear, but caused by some external factor. Connect the negative lead of the voltmeter to the negative terminal of the battery, and the positive lead to the engine case to measure the voltage drop on the negative side. When starting the engine, the voltmeter shows a voltage drop of 0.42V. After checking the various contacts of the ground wire, it was found that the ground wire nut fixed on the transmission housing hanger was loose, causing the engine and frame battery to be grounded. After tightening the loose nuts, start the engine again and test the voltage drop. At this time, the starter is strong and the voltmeter shows a voltage drop of only 0.2V. Due to the poor contact between the engine and the frame of this car, due to the poor contact of the negative electrode line when starting, the current flows back to the negative electrode of the battery and has to find a return route, and the clutch cable acts as a conductor between the engine and the body at this time. It becomes a conductor with a certain resistance value. Due to its own heat, it causes varying degrees of damage to the surface and internal cables, resulting in abnormal damage to the clutch wire.
