Which functions of the multimeter are not fully utilized.
1. The current test function has not been paid attention to
For a long time, the current test function has been neglected in maintenance. With the emergence of more and more new technical equipment on the car, sometimes it is impossible to carry out effective diagnosis without current test. For some leakage situations that are difficult to judge, such as: ①The battery plates are short-circuited or oxidized and fall off, resulting in self-discharge and power loss. ②Electronic components are grounded to cause leakage, such as the insulation layer of a certain line in the circuit is aging or falling off, resulting in contact with other metal parts. In these cases, the multimeter's current test function can come in handy.
(1) Leakage test method
When performing leakage test, you need to use a multimeter. The specific method is as follows: turn off all electrical equipment on the car, remove the ignition key, and use the current clamp attached to the car multimeter. If the current flowing through the meter is 10-30mA, it means that the car has no leakage. Conversely, if the display is too large, it is considered that there is a body leakage fault. It should be noted that some systems still need to absorb a lot of power within 15 minutes of turning off the ignition switch, so if the measured current is above 50mA, it should be tested after 15 minutes. If it is still above 50mA at this time, it means that there is a discharge phenomenon, and further diagnosis is required. In the case of confirming that there is leakage in the body, keep the current clamp clamping the negative pole of the battery all the time, and you can remove the fuses one by one while observing the change of the current value of the meter. If the number displayed on the meter does not change after a certain fuse is removed, it means that the leakage does not occur in the circuit. If the digital clamp ammeter shows that the value returns to normal 10-30mA (the normal value of some cars is below 50mA, the specific value is related to the model), it means that the leakage phenomenon has been eliminated, which means that the fuse control circuit has leakage, grounding or short circuit problems exist.
(2) Rapid diagnosis of electrical equipment
Using the current test function, you can also quickly diagnose and detect the reasons why many electrical equipment on the car, such as headlights, audio, oil pump relays, motors for electric doors and windows, and generators, do not work. For example, for the detection of electric fuel pump, if the working current I=4.5A, the fuel pump is working normally, if there is a fuel cut-off, it is caused by poor circuit connection; if the working current I<4.5A, it means that there is resistance in the circuit or The connection is bad, or the fuel tank is too dirty, and impurities block the oil filter, and the oil pump cannot absorb oil, resulting in no-load current; if the operating current I>4.5A, it means that the fuel filter may be blocked or partially blocked, resulting in an increase in the load of the oil pump . In this case, misjudgment of the oil pump is very easy to occur, and attention should be paid to it during the detection.
2. The voltage drop test method is rarely used
For maintenance technicians, it is very common to use a multimeter to detect voltage to diagnose faults, but it is rare to diagnose faults based on voltage drop. Using the voltage drop test method to diagnose faults skillfully can play an irreplaceable diagnostic role in many cases. Experiments have proved that in all circuits, the maximum voltage loss is 3% of the power supply voltage, so the maximum voltage drop on a car using a 12V power supply should be 0.36V, and if the voltage drop exceeds 0.4V in the circuit, it can be regarded as an abnormal circuit , that is, there is a high resistance.
(1) 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.
(2) Fault example of 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 fastening 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 of this car, the current flowed back to the negative pole of the battery due to the poor contact of the negative line when starting, so we had to find a return route, and the clutch cable acted as a link between the engine and the body at this time. The conductor becomes a conductor with a certain resistance value. Due to self-heating, it will cause varying degrees of damage to the surface of the pull wire and the internal pull wire, resulting in abnormal damage to the clutch wire.
