The voltage drop test method using a multimeter is rarely adopted
For maintenance technicians, it is common to use a multimeter to detect voltage and diagnose faults, but it is not common to diagnose faults based on voltage drop. The skillful use of voltage drop testing method to diagnose faults can play an irreplaceable diagnostic role in many cases. Experimental results have shown that in all circuits, the maximum voltage loss is 3% of the power supply voltage. Therefore, the maximum voltage drop on a car with a 12V power supply should be 0.36V. If the voltage drop exceeds 0.4V in the circuit, it can be considered as a circuit abnormality, indicating the presence of high resistance.
(1) The advantages of voltage drop testing method compared to other methods
The voltage drop test method is commonly used to measure the starting and charging circuits. For the inspection of these circuits, if the resistance detection method commonly used by maintenance personnel is used, it has certain limitations and cannot detect existing fault hazards. For example, for wires that are "connected by lotus roots and broken threads", if the resistance of the wire is measured using the resistance range of a multimeter, there will be no increase in resistance value. The seemingly problem free measurement results will make it difficult for technicians to directly detect the real situation. If the voltage drop test method is used to test the voltage of different parts when starting the engine, it can accurately and quickly identify faults in the circuit.
(2) Example of voltage drop test fault
A Changan small passenger car once had six clutch wires continuously replaced, and after a few days of each replacement, the clutch pedal gradually became heavy. Upon careful observation of the replaced clutch wires, it was found that some parts of the outer and inner plastic sleeves of the wire conduit had varying degrees of deformation, while some parts of the wire conduit in contact with the wire core also showed severe signs of strain. Through analysis, it is believed that this strain is not caused by normal mechanical wear, but rather by some external factor. Connect the negative probe of the voltmeter to the negative terminal of the battery, and connect the positive probe to the engine housing to measure the voltage drop on the negative side. When starting the engine, the voltmeter displays a voltage drop of 0.42V. After inspecting the contacts of the grounding wire, it was found that the nut of the grounding wire fixed on the transmission housing hanger was loose, causing the engine to be grounded to the chassis battery. After tightening the loose nut, start the engine again and test the voltage drop. At this time, the starter motor is strong and powerful, and the voltmeter shows only a voltage drop of 0.2V. This car has poor contact between the engine and the frame. During startup, due to poor contact of the negative pole line, the current flows back to the negative pole of the battery and has to find a return route. At this time, the clutch cable acts as a poor conductor between the engine and the body, becoming a conductor with a certain resistance value. Due to its own heating, it causes varying degrees of damage to the surface and internal cables of the cable, resulting in abnormal damage to the clutch cable.
