How to measure the current of sensor with multimeter

Testing of throttle position sensor There are two types of throttle position sensors: one is linear and the other is switch type. The linear throttle position sensor (TPS) is a variable resistor that transmits the throttle position signal to the engine electronic control unit computer. The throttle switch signal consists of idle contact (IDL) and power contact (PSW). Modern automobile throttle position sensors are mostly composed of these two types of sensors, namely idle contact and variable resistance linear throttle position sensor. This is a very important sensor because the engine ECU computer uses its signal to calculate engine load, ignition time, exhaust gas recirculation and idle speed control. Damaged throttle position sensor will cause problems such as acceleration lag and unstable idle speed. Generally, when the throttle is closed, the signal voltage generated by the throttle position sensor is lower than 1V, and when the throttle is fully open, the signal voltage generated is about 5V. Test of throttle position sensor: generally, the signal voltage is lower than 1V at idle speed and lower than 5V at full throttle. Turn on the ignition switch, don't start the engine, slowly turn the throttle from off to on, and do it several times repeatedly to check whether the voltage value is within the required range. You can also start the MAX/MIN function key of the special automobile instrument to check whether the minimum value is 0 and the maximum value is 5V. Hall sensor test Hall sensor is an active sensor, and its output is actually the output of a switch. It is not limited by rotational speed, and the output signal amplitude at low speed is the same as that at high speed, so it is widely used in sensors such as crankshaft position and camshaft position. It consists of an almost completely closed permanent magnet and a magnetic circuit. The magnet blade rotor passes through the air gap between the magnetic poles, and notches are distributed on the blade rotor. At the groove, the magnetic field acts on the Hall element and outputs a signal; However, when the blade rotor is not in the notch position, no magnetic field acts on the Hall element, so there is no signal voltage output. Hall sensor test: Hall and photoelectric sensors are both frequency output sensors, and the frequency and DUTY ratio of the sensor can be measured with the "duty" and "Hz" gears of the automobile multimeter. The pulse amplitude of the sensor is constant and the frequency changes with the speed. Turn on the ignition switch and test the three terminals of the Hall sensor. The voltage between one terminal and the other terminal is 5V or 12V. After confirmation, connect the red probe to the other end, set the automobile multimeter to the DC voltage range, press the function switch key to select "DC" and "Hz" to measure at the same time, and the blade rotor of the Hall sensor can rotate. At this time, the frequency and voltage of the multimeter are the output signal parameters of the Hall sensor, and the frequency increases with the increase of rotating speed. Test of Magnetoelectric Speed Sensor Magnetoelectric Speed Sensor is an analog AC signal generator, and the signal it generates is an AC signal. It usually consists of coils and magnets. When the iron ring gear rotates past the sensor, the coil will generate alternating voltage. ABS wheel speed sensor is also magnetoelectric, and the amplitude and frequency of its output signal increase with the increase of vehicle speed. Testing of magnetoelectric speed sensor: The main component of magnetoelectric sensor is the coil, so the resistance and on-off of the coil should be tested first, and it should be within the specified range. Test its signal output, set the special instrument for automobile to the alternating current (AC) position, and press the function switch key to select "AC" and "Hz" for simultaneous measurement. When the iron ring gear rotates, it is observed that the amplitude and frequency of the signal increase with the increase of the rotating speed, and the decrease of the amplitude may be caused by the excessive gap of the sensor. Testing of oxygen sensor The oxygen sensor is an important feedback sensor in the electronically controlled fuel injection system. It detects the oxygen concentration in the exhaust gas, monitors whether the engine burns according to the theoretical air-fuel ratio, and feeds it back to the engine ECU computer. It consists of zirconium dioxide electrolyte which can generate electromotive force and important electrodes. When the mixture is rich, there is less oxygen in the exhaust gas, and the difference of oxygen ions passing through zirconium dioxide in the atmosphere is great, resulting in higher voltage. When the mixture is thin, there is more oxygen in the exhaust gas, and the difference of oxygen ions passing through zirconium dioxide in the atmosphere is small, resulting in lower voltage. Oxygen sensor test: start the engine, let the engine run at 2500 rpm for 90 seconds, and preheat the oxygen sensor. Set the special automobile instrument to DC mV range, and measure the output voltage of oxygen sensor. The sensor voltage should jump more than 8 times in the range of 100 ~ 900 MV, otherwise the oxygen sensor will respond slowly. The test temperature sensor generally consists of a thermistor with a negative temperature coefficient. The 5V power signal voltage provided by the temperature sensor to the engine electronic control unit computer returns a voltage signal inversely proportional to the temperature to the engine electronic control unit computer. Testing of temperature sensor: The resistance values of temperature sensors tested by various engines at different water temperatures shall conform to Table 1 (there may be differences among different models, but the error is not big). If not, it is likely to cause cold start or hot start difficulties, and the mixture is too rich or too thin. The structure type, working principle and detection method of the air temperature sensor are basically the same as those of the engine temperature sensor. As the main executive part of the injection system, nozzle test directly affects the performance of the engine. When testing the fuel injector, set the automobile meter to frequency (Hz), and press the auxiliary display key to select and trigger positive and negative pulses (ms) to test the fuel injection pulse width of the fuel injector. Fuel pump current test is helpful to diagnose some intermittent faults of fuel pump by testing the working current of fuel pump during actual maintenance. During the test, use the current gear (A) of the special automobile instrument, press the function key (SELECT) to switch to the DC gear, and connect it in series with the fuel pump circuit. When the fuel pump is working, press the dynamic record key (max/min). When abnormal fuel supply is found when the vehicle is driving, observe the maximum and minimum current recorded automatically, and compare it with the normal value to find out the cause of the failure. When testing the idle solenoid valve, the engine ECU computer generally controls its opening by controlling the duty ratio of its output. When testing, select the frequency range (duty-Hz), press the second view (auxiliary display key) to adjust the duty ratio of positive and negative pulses of auxiliary display, and check whether the data meets the standard. Simulation of sensor and driving of actuator The automobile multimeter has the function of square wave output with adjustable frequency and duty cycle. The external conversion module can drive the fuel injector, ignition coil, ignition module, engine tachometer, electronic odometer and other actuators, and can also simulate digital signals, voltage and resistance signals. (1) analog digital air flowmeter simulation: find out the signal line of the air flowmeter and judge whether the flowmeter is high frequency or low frequency. If it is low frequency, set the multimeter at 50Hz; if it is high frequency, set it at 2500Hz. Turn the switch of the conversion module to the positive pulse position on the right, insert the red probe into the signal line pulled out from the sensor, and insert the black probe into the grounding terminal pulled out from the sensor to observe the working state of the engine. Simulation of voltage-type air flowmeter: Turn the multimeter to "V" position, switch the conversion module to "V Ω" position, turn the knob counterclockwise to low position, insert the red stylus into the signal wire unplugged from the sensor, and insert the black stylus into the grounding terminal unplugged from the sensor. Turn the knob clockwise to adjust the required voltage value and observe the working state of the engine. (2) Simulation of throttle position sensor The simulation of throttle position sensor is the same as that of voltage type air flowmeter. (3) The simulated water temperature sensor of the water temperature sensor is a thermistor, and various water temperature signals are simulated by the adjustable resistor and sent to the computer. First, set the multimeter to Ω, plug in the conversion module and disconnect the power cord; Adjust the knob to make the instrument display the resistance value you need; Then unplug the module from the instrument and input it into the water temperature sensor with a red and black pen for simulation. You can also adjust the voltage to simulate. The method is: set the module to "V Ω" and the multimeter to "V"; Unplug the plug of the water temperature sensor, and insert the red and black probes on the module into 5V and ground wire on the plug respectively; Adjust the voltage knob. The lower the voltage, the higher the water temperature; The higher the voltage, the lower the water temperature, and observe the working state of the engine. (4) Simulation of the ignition signal driven by the ignition module: adjust the multimeter to the range of 10Hz, adjust the duty ratio to the range of 5%- 10%, turn the switch of the conversion module to the positive pulse range on the right, touch the ignition signal line with a red and black probe, and observe the working performance of the ignition module. 5) Simulation of ignition module driven by ignition coil: adjust the multimeter to 10 Hz- 100 Hz, and adjust the duty cycle to 5%-10%; Turn the switch of the conversion module to the left negative pulse position, and touch the negative wire of the ignition coil with a red iron pen to observe the working performance of the ignition coil. 6) Simulate the engine running tachometer, electronic speedometer and electronic speedometer sensor: most signals of the engine tachometer are taken from the negative end of the ignition coil, set the multimeter at 10Hz, and turn the switch of the conversion module to the positive pulse position on the right; Connect the red stylus to the signal line, adjust the frequency of the multimeter and observe the working performance of the tachometer. An electronic speedometer is the same as a tachometer.