Multimeters play a greater role in maintenance
The current testing function has not been given enough attention
For a long time, the current testing function has been neglected in maintenance. With the emergence of more and more new technological equipment in cars, sometimes effective diagnosis cannot be carried out without current testing. For some difficult to determine leakage situations, such as:
① Short circuit or oxidation detachment of the battery plate leads to self discharge and loss of power.
② Grounding electronic components can cause leakage, such as aging or detachment of the insulation layer of a certain line in a circuit, resulting in contact with other metal parts. In these situations, the current testing function of the multimeter can come in handy.
The method of leakage testing
When conducting a leakage test, a multimeter is required. The specific method is as follows: turn off all electrical equipment on the car, remove the ignition key, and use the accessory current clamp of the car multimeter. If the current flowing through the instrument is 10-30mA, it indicates that the car has no leakage. On the contrary, if the display is too large, it is considered that there is a vehicle leakage fault. It should be noted that some systems still absorb a considerable amount of power within 15 minutes of turning off the ignition switch. Therefore, if the current is measured above 50mA, it should be tested again after 15 minutes. If it is still above 50mA at this time, it indicates that there is indeed discharge phenomenon and further diagnosis is needed. When confirming that there is leakage in the vehicle body, keep the current clamp on the negative pole of the battery. You can remove each fuse one by one while observing the changes in the instrument current value. If the number displayed on the instrument does not change after removing a certain fuse, it indicates that the leakage did not occur in the circuit. If the display value of the digital clamp ammeter returns to normal 10-30mA (some cars have normal values below 50mA, the specific value depends on the vehicle model), it indicates that the leakage phenomenon has been eliminated, indicating that there is leakage, grounding, or short circuit problem in the fuse control circuit.
Quick diagnosis of electrical equipment
By utilizing the current testing function, it is possible to quickly diagnose and detect the reasons why many electrical devices in the car, such as headlights, speakers, oil pump relays, electric door and window motors, and generators, are not working. For example, in the detection of electric fuel pumps, if the working current I=4.5A, the oil pump is working normally. If there is a fuel cut-off phenomenon, it is caused by a short circuit in the circuit connection; If the working current I is less than 4.5A, it indicates that there is a resistance or poor connection in the circuit, or the fuel tank is too dirty, and impurities are blocked in the oil filter screen, causing the oil pump to not be able to suck oil and forming a no-load current; If the working current I is greater than 4.5A, it indicates that the fuel filter may have been blocked or partially blocked, causing an increase in fuel pump load. In this situation, it is very easy to misjudge the oil pump, and attention should be paid during the inspection.
The voltage drop testing method is rarely used
For maintenance technicians, it is common to use a multimeter to detect voltage to diagnose faults, but it is not common to diagnose faults based on voltage drops. Cleverly using voltage drop testing 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, in vehicles using a 12V power supply, the maximum voltage drop should be 0.36V. If the voltage drop in the circuit exceeds 0.4V, it can be considered as an abnormality in the circuit, that is, the presence of high resistance.
