Operating principles

The volume of air taken in by the engine is a fundamental parameter for calculating injection time. This is because the injection ECU receives the "air volume" and processes it with the rpm to calculate the injector opening time. The result is called "basic time" because it is corrected by additional parameters, such as air temperature, water temperature and other particular engine conditions. The air volume sensor is also called a "flow meter" and consists of a floating butterfly valve withheld by a spring. The butterfly valve fastening pin is integral with the slider of a variable resistor (potentiometer). In its movement, the floating butterfly valve drags the variable resistor slider which consequently generates a voltage signal proportional to the opening position of the butterfly valve. The slider (a metallic reed with contact electrode) slides on a conductive track (resistor). Resistance, which varies according to the position of the slider, is obtained between the slider and one end of the resistor on the track (this can be verified by means of a static test, i.e. with the flow meter disconnected from the injection circuit).

 

Identification elements

Identifying the sensor is relatively easy given its particular appearance. There are different types according to manufacturing year and type of injection system. Specifically, flow meters may have a different number of connecting wires. Given the basic operating principles, additional connections include:

• connection to electrical switch for working the fuel pump
• connection to trimmer for CO regulation.

 

Electrical operating characteristics

The typical characteristic of the electrical signal "generated" by an air volume sensor (flow meter) is consequently determined by the variation of potential on the slider connection pin. The term "generated" is not proper because in actual fact the flow meter is a passive sensor. This means that the sensor is powered by the ECU to which it is connected. The component must be tested during normal use. Other types of static tests (resistance reading in Ohms) may determine false results. In practice, the most complete test, which provides the safest results, is that carried out during normal operation. An oscilloscope test is capable of detecting the operation of a component in various engine load conditions and all possible working positions. Only this type of test can ensure correct results. The electrical characteristics of this sensor follow a curve determined by the movement of the floating butterfly valve. The linear voltage variation in time, linked to the position of the butterfly valve, is the fundamental parameter for controlling the electrical performance of the air volume sensor.

 

Electrical tests

The electrical tests consist in acquiring the signal for each electrical connection of the sensor. This is because both the power and the output signal need to be tested. Normally, air flow sensors have three connection wires but other devices are commonly built into the sensor, such as the air temperature sensor, the CO trimmer, etc. Tests include:

1. potential voltage value less than or equal to 0.25 volts with connection terminals either on or off
2. power potential voltage value can be 4.5-5.5 volts.
3. Linear sensor output voltage variations
4. Test shape, stability and continuous presence of the output signal in time (oscilloscope test). This test is used to check for absence of pulse variations which are evidence of interruptions and short-circuits.   

 

Test connection

Taking the most common type of air flow sensor as an example and considering the indications illustrated above, the electrical connections of the sensor must be connected in parallel to the tool. The most correct and complete tests are carried out with an oscilloscope. A single acquisition can provide all the necessary evaluation parameters. The film shows a test sequence for all the normally used connections. The first test can be carried out on the sensor earth wire and then on the signal output wire (the continuous voltage will vary according to the position of the floating butterfly valve). Check the power voltage and the earth potential before replacing the sensor if the signal does not correspond. The problem concerns the injection system (ECU, wiring harness) and not the sensor in the case of anomalous power. Carry out all the necessary tests before replacing the sensor.

 

Typical oscillogram

The signal shown in the figure was obtained with the oscilloscope in automatic mode. In this case, the tool automatically set to measure voltage and time. The points and their physical meaning are shown in the figure:

1. Maximum voltage (butterfly valve open)
2. Minimum voltage (butterfly valve closed)

(A) Sensor interruption point (track on inner circuit)

 

Special notes

The typical anomalies of this sensor are linked to short interruptions which are difficult for the self-diagnostic system to detect because these systems only detect faults lasting for a certain time. Intermittent, short-lasting anomalies concerning this sensor indicate problems which are often difficult to re-conduct to the sensor and to false self-diagnostic system indications. Sometimes, faulty lambda sensor operation is caused by rapid fuel changes due to anomalous operation of the air flow sensor.