The multimeter is one of the indispensable tools for car repair technicians. Early multimeters were very versatile, with simple current, voltage, and resistance testing capabilities, the traditional "AV-Ω" multimeter. The new multimeter can detect speed, closing angle, duty cycle, frequency, pressure, time, capacitance, temperature and semiconductor components. With the emergence of multi-meters with different brands and different functions in the market, people often go into the misunderstanding of this important testing tool in the purchase and use, and analyze it here.
Car multimeter purchase misunderstanding
As a testing tool that is widely used in the maintenance industry, almost every enterprise needs to be equipped. However, many multimeters used by enterprises do not necessarily meet their actual needs.
1. Emphasis on functional diversity while ignoring the matching of attachments
Some users pay more attention to the variety of multimeter functions when purchasing, but it is easy to ignore the matching of accessories. The diversity of functions is naturally impossible without the necessary accessories. For example, some users want to purchase a multimeter that can detect the starting current and can detect the temperature under various conditions, but does not purchase special current clamps and thermometer probes. If you want to take full advantage of the function of a car multimeter, the matching of some accessories is very crucial. Some of these attachments are a must, and more are matched to the features of the car multimeter.
The accessories included in the car multimeter usually have the following types: Inductive pickup, battery alligator clip, SMD chip component special test clip (for testing SMD resistors, capacitors, inductors and transistors, etc., can be operated with one hand, so that More convenient measurement, current sensor, pressure probe, torque probe and IC probe. These accessories are generally less used in maintenance. For the maintenance enterprise, when purchasing the accessories, it should be considered from the aspects of the user's technical quality, the maintenance enterprise's capital status and daily use, so as to achieve the appropriate selection, in order to achieve the maximum utilization of the equipment. More exciting, please pay attention to WeChat public number: CAQC10000
2. Give up good equipment due to simple language barriers
Some users want to buy a high-end imported car multimeter, but after discovering that the interface language is English, they can easily give up more complete or high-end products. In fact, this is not necessary. The basic functions of various multimeters are similar, and many functions are identified by common symbols, which are not complicated to use. As long as you have a simple study, you can quickly master how to use it.
Maintenance personnel do not fully understand the multimeter, the multimeter is generally not idle, but many functions are not fully applied in the maintenance.
1. For some functions "because of waste"
Some repairers often do not dare to use the multimeter because they are worried about damage to the equipment. The most obvious is the current test. The current test method is not complicated. Like the starting current, the primary and secondary currents of the ignition coil, and the current test of the injector, many fault cues can be found as a diagnostic basis, but the technicians use very little in maintenance. It is understood that many repairmen are afraid to use the current test function because they are afraid of "burning the watch" or damaging the component under test due to the misoperation of the test. Therefore, it is worth considering. In fact, many multimeters are designed with special circuit devices or mechanical protection devices to protect integrated circuits. If the DY2201 multimeter is used to prevent misoperation, the current measuring jack is mechanically protected. Only when the current measurement is selected, the corresponding “mA†or “20 A†jack will be opened. Otherwise it will be blocked by the baffle, which can effectively avoid the wrong pen. Maintenance personnel can use the current test function boldly, but be sure to follow the usage process.
2. Insufficient attention to errors in the test
Any instrument will have errors in the measurement, but the error should be minimized in the measurement to avoid operational errors to ensure the accuracy of the measurement results. The large errors that multimeters often appear in use sometimes originate from the instrument and sometimes stem from some human factors. The following two aspects need to attract the attention of maintenance personnel: 1 Because the accuracy and resolution of the multimeter are not enough, if only the estimation and judgment are made, it often leads to human error. 2 Due to different test methods of various multimeters, the multimeter will have its own instrument error in different signal tests. Therefore, maintenance personnel should use the instrument with high precision when using the multimeter, and strictly follow the operation process test to the maximum extent. Ground to reduce the error.
The function of the multimeter is not fully tapped
Some of the functions of the multimeter are used less in maintenance, or their functions have not been fully utilized by users.
1. The current test function is not taken seriously
For a long time, the current test function has been neglected in maintenance. As more and more new technology is installed in the car, sometimes it is impossible to make an effective diagnosis without conducting a current test. For some difficult-to-determine leakage conditions, such as: 1 battery plate short circuit or oxidative loss caused by self-discharge and loss of electricity. 2 The electronic components are grounded to cause leakage, such as the insulation of a certain line in the circuit is aging or falling off, causing contact with other metal parts. In these cases, the multimeter's current test capability can come in handy.
(1) Method of leakage test
When conducting the leakage test, you need to use a multimeter. The specific method is as follows: turn off all the electrical equipment on the vehicle, remove the ignition key, and use the accessory current clamp of the automobile multimeter. If the current flowing through the meter is 10~30 mA, the vehicle has no leakage. . On the other hand, if the display is too large, it is considered that there is a body leakage fault. It should be noted that some systems also absorb a lot of power within 15 min after turning off the ignition switch. Therefore, if the current is measured above 50 mA, it should be tested after 15 minutes. If it is still at 50 mA or more at this time, it is clear that there is a discharge phenomenon and further diagnosis is required. When it is confirmed that the vehicle body is leaking, keep the current clamp clamped to the battery negative electrode at all times, and you can take the fuse one by one and observe the change in the current value of the meter. If a fuse is removed, the meter display does not change, indicating that leakage does not occur in the circuit. If the digital clamp type ammeter shows that the value returns to normal 10~30 mA (the normal value of some cars is below 50 mA, the specific value is related to the model), indicating that the leakage phenomenon is eliminated, indicating that the fuse control circuit has leakage, grounding or A short circuit problem exists.
(2) Rapid diagnosis of electrical equipment
With the current test function, it is also possible to quickly diagnose and detect many of the electrical equipment on the vehicle, such as headlights, sounds, oil pump relays, motors for electric windows and doors, and electrical appliances such as generators. For example, for the detection of electric fuel pump, if the working current is I=4.5A, the oil pump works normally. If there is oil breakage, it is caused by poor circuit connection; if the working current is I<4.5 A, it means there is resistance in the circuit or Poor connection, or the fuel tank is too dirty, the oil pump can not absorb oil after the impurities are blocked in the oil filter, forming no-load current; if the working current I>4.5 A, the fuel filter may be blocked or partially blocked, resulting in increased oil pump load. . In this case, oil pump misjudgment is highly prone to occur, and attention should be paid to the test. More exciting, please pay attention to WeChat public number: CAQC10000
2. The voltage drop test method is rarely used
For maintenance technicians, it is not uncommon to use a multimeter to detect voltage to diagnose faults, but it is rare to diagnose faults based on voltage drop. The use of voltage drop test to diagnose faults can play an irreplaceable diagnostic role in many cases. Experiments have shown that in all circuits, the voltage loss is at most 3% of the supply voltage, then the maximum voltage drop in a car with a 12 V supply should be 0.36 V. If the voltage drop exceeds 0.4 V in the circuit, it can be regarded as a circuit abnormality. That is, there is a high resistance.
(1) Advantages of voltage drop test method compared to other methods
The most common method of voltage drop test is to measure the starting line and charging line. For the inspection of these lines, if the method of resistance detection common to maintenance personnel is used, it has certain limitations, and it cannot find the existing hidden troubles. For example, for a wire that is "broken wire", if the resistance of the wire is measured by the electric resistance of the multimeter, the resistance value will not increase, and the seemingly problem-free measurement result will make it difficult for the technician to directly discover the true condition. If the voltage drop test method is used, the voltage test is performed on different parts when starting the engine, and the fault in the line can be found accurately and quickly.
(2) Voltage drop test failure example
A Changan passenger car has been replaced with six clutch wirings. After a few days of replacement, the clutch pedals are gradually heavy. Careful observation of the several clutch wires that have been replaced shows that the wire tube skin and some parts of the plastic part of the wire tube have different degrees of deformation, and some parts of the wire tube that are in contact with the wire core have serious scratch marks. . According to the analysis, this strain is not normal mechanical wear, but is caused by some external factors. Connect the battery negative pole with a voltmeter negative test lead, connect the engine casing with a positive test pen, measure the voltage drop on the negative side, and the voltmeter shows a voltage drop of 0.42 V when starting the engine. After checking the joints of the grounding wire, it was found that the grounding wire nut fixed on the transmission housing hanger was loose, which made the engine and the frame battery have poor grounding. When tightening the loose nut, start the engine again and test the voltage drop. At this point, the starter is powerful and the voltmeter shows a voltage drop of only 0.2 V. This car has poor contact between the engine and the frame. Due to poor contact of the negative line during starting, the current flows back to the negative pole of the battery and has to find the return route. The clutch cable acts as the second between the engine and the body. The conductor becomes a conductor with a certain resistance value. Due to its own heat, it causes different degrees of burn on the surface of the cable and the internal cable, resulting in abnormal damage of the clutch wire.
3. Other underutilized features
In addition to the current test function and the voltage drop test method, the multimeter has many functions that are not fully utilized in the maintenance, mainly as follows.
(1) Frequency test
As long as the sensor can generate a frequency signal, it can be frequency tested with a multimeter. Such as intake pressure sensor and knock sensor. Make full use of the multimeter's frequency test function, you can test the working state of the frequency signal sensor and judge the performance of the sensor.
(2) Duty cycle test
The duty cycle is a percentage of the signal's operating time to the total time. For the duty ratio test using a multimeter for the idle speed control valve and other components using the duty control method, it is possible to determine that the idle speed control valve does not operate, the operation is poor, and the operation is inaccurate.
(3) Measurement of closed angle
Measurements of the closed angle can be found for related causes of poor ignition such as distributor contact spacing and mechanical wear of the distributor. The method is as follows: 1 Insert the black test lead into the COM jack, and insert the red test lead into the corresponding jack of the closed angle. 2 Place the function switch on the desired “DWELL†range based on the number of engine cylinders tested. 3 Put the black test lead on the negative pole of the battery, and connect the red test lead to the low voltage terminal of the distributor or the negative pole of the ignition coil. 4 Start the engine and you can read the closed angle.
(4) Features should be used well
Some car multimeters have some special features, such as some multimeters with analog conversion module (referred to as conversion module), with sensor signal simulation and drive actuator function, if these functions are used properly, it can greatly improve efficiency.
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