Why can't a multimeter measure low-frequency and high-frequency AC voltages?

Because the multimeter is internally rectified into DC voltage, and then the measurement is realized according to the effective value ratio of DC voltage and AC voltage.

If full-wave rectification, DC voltage is 0.9 times of AC voltage; If half-wave rectifier, it is 0.45 times. Then according to the effective value of DC voltage, the measured voltage can be indirectly reflected.

Ordinary multimeter can only test voltage, resistance and current, which is not suitable for the maintenance inspection of modern electronically controlled vehicles. The multifunctional automobile digital multimeter developed according to the characteristics of electronically controlled automobile maintenance has many special test functions for automobile electrical system, such as frequency, duty ratio, pulse width and temperature.

Extended data

Method for detecting electronic control automobile electrical components by using automobile digital multimeter

1, testing air flow meter (MAF)

According to the structural principle, air flow meters can be divided into wing-plate air flow meters, hot-wire type, hot-film type, Karman vortex optical type, Karman vortex ultrasonic type and so on. According to the signal transmission type, it can be divided into digital type and analog type.

2. Test 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.