# Mass Air Flow Sensor BOSCH HFM5 The mass air flow sensor used for measure of the air flow, spent by the engine. Signal of the air flow sensor are one of the base parameters, used engine control unit for calculation fuel delivery and optimal ignition timing.
Mass air flow sensor BOSCH HFM5.
Air flow sensor installed after the air filter before throttle valve in air flow. The output signal of the mass air flow sensor BOSCH HFM5 is DC voltage, varied in a range 0…5 V. The voltage value of the output signal depends from the value and direction of the air flow through the sensor. At the zero air consumption (the engine is stopped) output voltage of the mass air flow sensor BOSCH HFM5 is equal 1,00 V. When the engine works, through the sensor flows the air, spent by the engine, and with increasing consumption of the air, the output sensor voltage value is higher. On various working modes there can be backward emissions of air (depends on design features of the engine). The mass air flow sensor BOSCH HFM5 is capable to register backward air flows from the intake manifold to the air filter, its output voltage then is reduced to the values less 1,00 V proportionally to the value of a backward air flow. If the mass air flow sensor output signal presents, but its parameters distinguish from normal parameters, the engine characteristics are considerably worsened - increased fuel consumption, bad start of the cold engine, instability engine work on established modes, decreasing of the engine output power. Discrepancy of parameters of the mass air flow sensor output signal can be caused by the deterioration of the sensor characteristics, not tight intake manifold, the supply voltage instability. In the case if sensitive element of the sensor is dirty, decreasing speed of its response to the changing values of air flow, the value of output voltage can not match air flow value.
Checking of the output signal of the sensor. To get voltage waveform of the output signal of the mass air flow sensor BOSCH HFM5, socket of the differential oscilloscope probe must be connected to the differential analog input ¹6 of USB Autoscope III, a black "crocodile" clip of the differential oscilloscope probe must be connected to the "ground" on engine, negative (black) lead of the differential oscilloscope probe should be connected in parallel with "signal ground" of the MAF sensor, the positive (red) lead of the differential oscilloscope probe should be connected in parallel with the sensor signal.
Circuit of the connection to the mass air flow sensor BOSCH HFM5. 1 – The point of connection of black "Crocodile" clip of the differential oscilloscope probe. 2 – The point of connection of negative (black) lead of differential oscilloscope probe. 3 – The point of connection of positive (red) lead of differential oscilloscope probe.
In the program "USB Oscilloscope" choose "Control => Load user preset => HFM5". For detailed study, voltage waveform of the output signal of the mass air flow sensor should be recorded. To record waveform, in a window of program "USB Oscilloscope" select "Control => Record" before the switching of ignition. To stop recording waveform, in a window of program "USB Oscilloscope" select "Control => Record". After finishing of recording, recorded waveform can be explored in detail. Checking of the mass air flow sensor BOSCH HFM5 output signal making in three stages - measuring time of transition after the switching ignition, measuring output voltage value at zero air flow and measuring the maximum output voltage value sensor with a snap throttle. Measuring the time of transition after the switching ignition. After the switching of the ignition occurs submission of the supply voltage to the sensors and actuators of the engine control system, the mass air flow sensor too. Immediately after power onto the mass air flow sensor BOSCH HFM5 occurs warming of the sensitive element of the sensor to operating temperature, occurs transitional process of the output sensor signal.
Voltage waveform of the output signal of the mass air flow sensor BOSCH HFM5 at switching on the supply voltage. A: – voltage value at the time specified marker. In this case corresponds voltage of the output signal of the MAF sensor at zero air consumption (engine stopped) and equals 0,99 V; ΔT – time interval value between two markers. In this case corresponds to the time of transition of the output signal, after switching on power voltage to sensor and equal ~0,5 mS.
Time of transition of the output signal does not exceed mS. If the sensitive element of the sensor became dirty, the time of the transition process its output signal increasing significantly.
Voltage waveform of the output signal of the mass air flow sensor BOSCH HFM5 at switching on the supply voltage. A: – value at the time specified marker. In this case corresponds voltage of the output signal MAF sensor at zero air consumption (engine stopped) and equals 0,92 V; dT – time interval value between two markers. In this case corresponds to the time of transition of the output signal, after switching on power voltage to the sensor and equal ~70 mS.
Because of the pollution sensitive element of the sensor, the time of the transition process its output signal reaches tens of mS and sometimes hundreds of mS. Measuring of the output voltage at zero air flow. Measuring of the voltage value of the output signal of the mass air flow sensor corresponding zero air flow doing with the stopped engine and switched on ignition. For mass air flow sensor BOSCH HFM5 zero air flow corresponds to the value of the output voltage equal 1 V ±0,02 V. Measuring of the output voltage with a snap throttle. Measuring of the maximum voltage value of the output signal of the sensor with a snap throttle is made by the opening of the throttle valve for a short time (no more than one second), in condition if the transmission mode selector is set in position "Neutral" and engine is hot to operating temperature. At the time of snap throttle happens next. When the engine works at idle without a load, air is filling intake manifold, very rarefied, as inflows into the air intake manifold severely restricted throttle valve and idle actuator. Absolute pressure in the intake manifold is below atmospheric on 0,6…0,7 Bar. Intake manifold internal volume commensurate with the engine working cubic capacity, but the underpressure air mass, filling the manifold during the working of the engine at idle is insignificant. With the sharp throttle valve opening, the air sharply move through the opened throttle valve to the intake manifold and quickly fills the manifold volume until the absolute pressure did not reach values close to atmospheric. This process happens very quickly, resulting air flow through the air flow sensor at this point reached the value close to the maximum. Once the absolute pressure in the inlet manifold reaches values close to atmospheric, a flow running through the air flow sensor becomes proportional to the engine speed.
Voltage waveform of the output signal of the serviceable mass air flow sensor BOSCH HFM5 at snap throttle.
Output signal voltage of the serviceable mass air flow sensor BOSCH HFM5 immediately after the sharply opening of the throttle valve should briefly increase to the value not less than 4,0 V. In the case of significant pollution of the sensitive element of the sensor, the sensor response time decreases, and voltage waveform of the output sensor signal becomes "smoothed".
Voltage waveform of the output signal of the faulty mass air flow sensor BOSCH HFM5 at snap throttle.
Because of decreasing response time, the ability of the sensor to register rapid changes of the magnitude and direction of the air flow are deteriorating. As a consequence, voltage value of the output signal of such sensor did not reached the value 4,0 V after the opening of the throttle valve.