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The multimeter is one of the indispensable tools for automotive repair technicians. Early multimeters had very simple functions, only offering basic current, voltage, and resistance testing, i.e., traditional “A-V-Ω” multimeters. However, modern multimeters can detect RPM, closing angles, duty cycles, frequencies, pressures, time, capacitance, temperature, and semiconductor components. With the emergence of a variety of brands and functions in the market, people often fall into misconceptions when selecting and using this important testing tool, which will be analyzed here.
Misconceptions in Selecting Automotive Multimeters
As a widely used testing tool in the repair industry, almost every business needs to be equipped with a multimeter. However, many businesses may not have multimeters that meet their actual needs.
1. Focusing on Functionality While Ignoring Accessory Compatibility
Some users pay more attention to the functionality of the multimeter when purchasing but often overlook the compatibility of accessories. Without necessary accessories, the diversity of functions cannot be realized. For example, some users hope that the multimeter they purchase can test starting current and measure temperature under various conditions, but they do not purchase a dedicated current clamp and temperature probe separately. To fully utilize the functions of an automotive multimeter, matching some accessories is crucial. Among these accessories, some are essential, while others correspond to specific functions of the automotive multimeter.
Common accessories included with automotive multimeters usually include: inductive pick-up, battery crocodile clips, SMD component test clips (used for testing parameters of SMD resistors, capacitors, inductors, and transistors, allowing for single-handed operation for convenience), current sensors, pressure probes, torque probes, and IC probes. These accessories are generally used less in repairs. For repair companies, when selecting accessories, considerations should include the technical quality of users, the financial situation of the repair company, and daily usage, to make appropriate choices aiming for maximum equipment utilization.
2. Abandoning Good Equipment Due to Simple Language Barriers
Some users want to purchase high-end imported automotive multimeters but give up on products with more complete or higher-end features simply because they find the interface language is in English. In fact, this is unnecessary. The basic functions of various multimeters are quite similar, and many functions are marked with universal symbols, making them not complicated to use. With a little learning, one can quickly master the usage methods.
Lack of Adequate Understanding of Multimeters by Repair Personnel
Multimeters are generally not left idle, but many functions are not fully utilized in repairs.
1. “Throwing the Baby Out with the Bathwater” Regarding Certain Functions
Some repair personnel often hesitate to use the multimeter during testing for fear of damaging the equipment, especially in current testing. The method for current testing is not complex; for example, testing the starting current, primary and secondary currents of the ignition coil, and the current of the fuel injector can reveal many fault clues as diagnostic evidence, yet technicians rarely use it during repairs. It is understood that many repair workers avoid using the current testing function for fear of misoperation causing “burning the meter” or damaging the tested components. This “throwing the baby out with the bathwater” is indeed worth pondering. In fact, many multimeters are designed with special circuit or mechanical protection devices to protect the integrated circuits. For instance, the DY2201 multimeter has a mechanical protection device for the current measurement socket, which only opens the corresponding “mA” or “20A” socket when the current measurement is selected. Otherwise, it will be blocked by a baffle, effectively preventing incorrect probe insertion. Repair personnel can confidently use the current testing function, but must operate according to the usage process.
2. Insufficient Attention to Errors During Testing
Any instrument will have measurement errors, but efforts should be made to minimize errors during measurement to avoid operational mistakes and ensure the accuracy of measurement results. The larger errors often encountered in multimeter use sometimes stem from the instrument itself and sometimes from human factors. The following two aspects need attention from repair personnel: ① Due to the insufficient precision and resolution of the multimeter, relying solely on estimation can often lead to human error. ② Due to different testing methods of various multimeters, there will be instrument errors inherent to the multimeter during testing of different signals. Therefore, repair personnel should try to use instruments with higher precision levels and strictly follow the testing procedures to minimize errors as much as possible.
Underutilization of Multimeter Functions
Some functions of the multimeter are rarely applied in repairs, or their functions have not been fully utilized by users.
1. Insufficient Attention to Current Testing Function
For a long time, the current testing function has been ignored in repairs. With the increasing number of new technologies and equipment appearing in automobiles, sometimes effective diagnosis cannot be conducted without current testing. For some difficult-to-judge leakage situations, such as: ① Short circuit or oxidation and shedding of battery plates leading to self-discharge. ② Leakage caused by electronic components grounding, such as a certain wire in the circuit having an aging or shedding insulation layer causing contact with other metal parts. In these cases, the current testing function of the multimeter can be very useful.
(1) Method for Leakage Testing
To conduct leakage testing, a multimeter needs to be used. The specific method is as follows: turn off all electrical devices in the vehicle, remove the ignition key, and use the automotive multimeter’s accessory current clamp. If the current flowing through the meter is between 10-30mA, it indicates that there is no leakage in the vehicle. Conversely, if the reading is too high, it indicates that there is a leakage fault in the vehicle body. It is important to note that some systems may still draw a significant amount of power within 15 minutes after the ignition switch is turned off. Therefore, if the current measured is above 50mA, the test should be repeated after 15 minutes. If it still reads above 50mA at that time, it indicates that there is indeed a discharge phenomenon, requiring further diagnosis. When confirming that there is leakage in the vehicle body, keeping the current clamp attached to the negative terminal of the battery, one can remove fuses one by one while observing the change in the meter’s current reading. If removing a certain fuse does not change the reading, it indicates that the leakage is not occurring in that circuit. If the digital clamp meter reading returns to normal at 10-30mA (some vehicles may have normal values below 50mA; specific values depend on the vehicle model), it indicates that the leakage phenomenon has been eliminated, meaning that there is a leakage, grounding, or short circuit problem in that fuse-controlled circuit.
(2) Quick Diagnosis of Electrical Devices
Using the current testing function, one can also quickly diagnose many electrical devices in the vehicle, such as headlights, audio systems, fuel pump relays, electric window motors, and generators that are not working. For example, when testing an electric fuel pump, if the operating current I=4.5A, the pump is functioning normally. If there is a loss of fuel, it could be due to poor circuit connection; if the operating current I<4.5A, it indicates that there is resistance or poor connection in the circuit, or the fuel tank is too dirty, causing the oil filter to clog and the pump cannot draw oil, resulting in no-load current; if the operating current I>4.5A, it indicates that the fuel filter may be clogged or partially clogged, causing increased load on the pump. In this case, misjudgment of the fuel pump is very likely, and attention should be paid during testing.
2. Voltage Drop Testing Method is Rarely Used
For repair technicians, using a multimeter to measure voltage to diagnose faults is quite common, but diagnosing faults based on voltage drop is not very common. Skillfully using the voltage drop testing method can play an irreplaceable diagnostic role in many situations. Experiments have shown that in all circuits, the maximum voltage loss is 3% of the supply voltage. Therefore, in a vehicle using a 12V power supply, the maximum voltage drop should be 0.36V. If the voltage drop exceeds 0.4V in the circuit, it can be considered an abnormal circuit, indicating high resistance exists.
(1) Advantages of Voltage Drop Testing Method Compared to Other Methods
The voltage drop testing method is most commonly used to measure starting and charging circuits. For these circuits, if repair personnel use the common resistance testing method, it has certain limitations and cannot detect existing fault hazards. For example, for wires that are “connected by a thread,” measuring the resistance of the wire with the multimeter will not show an increase in resistance, and seemingly normal measurement results will make it difficult for technicians to directly discover the real situation. If the voltage drop testing method is used to test voltage at different points while starting the engine, it can accurately and quickly identify faults in the circuit.
(2) Example of Voltage Drop Testing Fault
A Changan minivan had its clutch wiring replaced six times, and each time it felt that the clutch pedal gradually became heavier after a few days. A close examination of the replaced clutch wires revealed that some areas of the outer skin of the wire tube and the inner plastic part had varying degrees of deformation, and there were serious signs of pulling at certain points where the wire tube contacted the wire core. Analysis suggested that this pulling was not normal mechanical wear but caused by some external factors. By connecting the negative probe of the voltmeter to the negative terminal of the battery and the positive probe to the engine casing, the voltage drop on the negative side was measured. When starting the engine, the voltmeter showed a voltage drop of 0.42V. After checking the grounding wire connections, it was found that the nut securing the grounding wire to the transmission casing was loose, resulting in poor grounding between the engine and the vehicle frame battery. After tightening the loose nut and starting the engine again, the voltage drop was tested, and this time the starter was strong, and the voltmeter showed only a 0.2V voltage drop. This vehicle had poor contact between the engine and the frame, causing poor contact of the negative line during starting, forcing the current to find a return route back to the battery negative terminal, and the clutch pull wire acted as a second conductor between the engine and the vehicle body, becoming a conductor with certain resistance, which caused heating and varying degrees of burning on the surface and inside of the pull wire, leading to abnormal damage to the clutch wire.
3. Other Functions Not Fully Utilized
In addition to the current testing function and voltage drop testing method, many other functions of the multimeter have not been fully utilized in repairs, mainly as follows.
(1) Frequency Testing
As long as the sensor can generate frequency signals, it can be tested with a multimeter. For example, intake pressure sensors and knock sensors. Fully utilizing the frequency testing function of the multimeter can test the working status of frequency signal sensors and determine the performance of the sensors.
(2) Duty Cycle Testing
The duty cycle is the percentage value of the working time of a signal to the total time. Using a multimeter to test the duty cycle of components such as idle control valves that use duty cycle control can determine if the idle control valve is not functioning, functioning poorly, or functioning inaccurately.
(3) Measuring Closing Angles
Measuring the closing angle can reveal related causes of poor ignition, such as the distance between the distributor contact points and mechanical wear of the distributor. The method is as follows: ① Insert the black probe into the COM socket and the red probe into the corresponding closing angle socket. ② Set the function switch to the required “DWELL” range according to the number of cylinders of the engine being measured. ③ Ground the black probe or connect it to the negative terminal of the battery, and connect the red probe to the low-voltage terminal of the distributor or the negative terminal of the ignition coil. ④ Start the engine, and the closing angle can be read.
(4) Make Good Use of Special Functions
Some automotive multimeters have special functions, such as some multimeters equipped with simulation conversion modules (referred to as conversion modules) that can simulate sensor signal and drive actuators. If these functions are used properly, they can greatly improve efficiency.
The multimeter can also play many other roles in repairs, and there are many functions waiting to be explored in practice. Only by continuously expanding and adopting new application methods in repair work can repair efficiency be improved.
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