Discussing Common Issues with Using a Digital Multimeter to Test Automotive Circuits
1. If there is 12V or 5V electricity at the terminal of the measuring wire harness, it is considered that the wire harness is good
If the electrical device is not working properly, we will measure whether the power supply voltage on the connector of the electrical device is normal. For example, if the horn of a car does not sound, the maintenance technician usually unplugs the horn plug, presses the horn, and measures whether there is 12V between the power supply of the horn plug and the ground. If there is 12V, it is concluded that the horn is faulty. This judgment is too hasty. The multimeter measures the voltage of the electromotive force, and the virtual current can also measure 12V. After testing, we found that if a 10-ohm resistor is connected in series in the circuit, the voltage is 5.94V, and it is replaced by 1K ohm. resistor, the voltage is still 5. 94V. If you use a high-power test lamp to measure the power supply and observe the brightness of the bulb, you can avoid being misled by virtual electricity.
Second, as long as the buzzer sounds, it is considered that the wiring harness is good
The core of this question is when will the beep sound. After testing, it is found that when the multimeter measures a 68 ohm resistance, the buzzer will also sound, and the wiring harness must be damaged. Most cars use 12V voltage. Take a 1-ohm motor as an example. Calculate through Ohm’s law, I = U/R = 12/1, and the working current is 12A. If a 1-ohm resistor is added, the current will drop to 6A. So put a 68 ohm resistor in series and the device won't work.
3. When measuring the resistance in the ohm range, if the measured value shows the highest value, it is considered to be an open circuit
This problem will not occur when using a multimeter with automatic range selection, but if you use a meter with manual range selection and the range is not selected properly, the meter will not respond after measurement.
4. Measuring the small ohm resistance without subtracting the test lead error
The lead wire of the multimeter is very long, and there may be a small amount of resistance inside, especially when measuring small resistance values, such as motors, the motor resistance is generally between one and several ohms. Therefore, it cannot be measured directly, and it must be zeroed first, and the number displayed by the zero calibration, this value is the error value of the meter, and must be subtracted when measuring a small resistance value.
5. Measure if there is voltage on the ground wire, is the power supply short-circuited?
We will encounter such a problem when testing a certain sensor harness, unplug the sensor plug, the measured power supply has 5V voltage, and the measured ground terminal is also close to 5V voltage. At this time, we will think, how can the ground be charged? If the ground wire is connected to the power supply, won't the wire burn? In fact, it is not that this line is directly connected to the power supply, but that the grounding of this line is not good. The voltage drop between the normal grounding line and the body ground should be 0V. If there is voltage, it means that the grounding line is connected to the body. There is a large contact resistance between the grounds, and the ground wire itself has a current flowing, according to U = I * R, so we can measure the voltage drop.
6. If the resistance between the grounding wire of the measuring sensor and the vehicle body is too large, the
think there is a fault
When measuring whether the sensor grounding is normal, we are used to using a multimeter
Mu file measurement. One test lead is placed on the ground terminal, and the other is on the vehicle body. Observe the resistance value. In actual measurement, the resistance is not zero as we understand it, but has a large resistance. The reason is that the sensor is grounded inside the computer, and there is a circuit between the computer and the car body, which is not directly connected, so there will be a large resistance when we measure it, and the resistance will change greatly when the ignition switch is turned on or off.
