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ABS Speed Sensor
Air Flow Meter / Sensor
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Air Flow Meter - Hot Wire Diagnostics
(scroll for AirFlow Meter - Air Vane and AirFlow Sensor - Diesel BoschCRD)

The voltage output from the Air Flow Meter (AFM) should be linear to airflow. The voltage will rise as the engine is accelerated and air volume is increased producing an initial peak. This peak is due to the initial influx of air and drops momentarily before the voltage is seen to rise again to another peak. This voltage will however depend on how hard the engine is accelerated, a lower voltage is not necessarily a fault within the AFM.

On deceleration the voltage will drop sharply as the throttle butterfly closes, reducing the airflow, and the engine returns back to idle. The final voltage will drop gradually on an engine fitted with idle speed control valve as this will slowly return the engine back to base idle as an anti-stall characteristic. This function normally only effects the engine speed from around 1200 rpm back to the idle setting.

Technical Information

This particular form of air flow meter is, in many ways, advantageous over the conventional air vane meter as it offers very little resistance to the flow of incoming air. The mass air flow is measured by the cooling effect on a heated wire that is suspended in the air passage, and it is the air flow's cooling effect on the wire that signals to the Electronic Control Module (ECM) the quantity of incoming air.

The AFM is located between the air filter and the throttle butterfly. Inside the component are two wires, one of which is used to convey the temperature of the incoming air and the other wire is heated to a high temperature (approximately 120°C) by passing a small current through it. As the air flows across the heated wire, it will have a cooling effect on it causing a temperature change; a small circuit inside the component will increase the current passing through the wire to maintain the temperature, and it is the recognition of this current that signals to the ECM the mass air flow.




The current supplied to the heated wire will alter proportionately to the air flow. Any wire that is constantly heated will form an oxide coating; to clean the wire after each journey, a current is passed through the wire heating it to approximately 1000°C, burning off any build up, ensuring a clean wire for the next time the vehicle is started.


Air Flow Meter - Air Vane Diagnostics

The voltage output from the internal track of the Air Flow Meter (AFM) should be linear to flap movement, the voltage will rise as the engine is accelerated and will produce an initial peak. This peak is due to the natural inertia of the air vane. On deceleration the voltage will drop sharply as the wiper arm, in contact with the carbon track, returns back to the idle position. This voltage may in some cases 'dip' below the initial voltage before returning to idle voltage. A gradual drop will be seen on an engine fitted with an idle speed control valve.

The waveform should be clean with no 'drop-out' in the voltage, as this indicates a lack of electrical continuity. This is common on an AFM with a dirty or faulty carbon track. The problem will show as a 'flat spot' or hesitation when the vehicle is driven, this is a typical problem on vehicles with high mileage that have spent the majority of their working life with the throttle in one predominant position. The 'hash' on the waveform is due to the vacuum change from the induction pulses as the engine is running.

Technical Information.

This type of Air Flow Meter is probably the most popular version used and has been used on systems such as Bosch L, LE, LE3 and Motronic, Ford EEC IV. Several Japanese manufacturers have also based their systems around this tried and tested unit. The air vane meter adopts the principal of the airflow flowing into the engine passes through the metering unit, via a spring loaded flap, which in turn will move in proportion to the amount of air entering the engine. The air vane's movement is recorded by a 'wiper arm' moving across a carbon track whose output is reported back to the Electronic Control Module (ECM) and gives the correct amount of fuel for the air recorded.

The AFM can have a varying number of electrical connections, the common ones are listed below.

Four terminal units will have:- a voltage supply, an earth connection via the ECM, an output from the air temperature sensor and the output from the air vane meter. Five terminal units will have:- as above, but with an additional output from a Carbon Monoxide (CO) potentiometer.


Seven terminal units will have:- as per the four terminal unit plus an extra wire to the air temperature sensor and two terminals to the fuel pump contacts. These contacts close and complete the circuit on the fuel pump when the engine is cranked, and the incoming air moves the flap by approximately 5°. This is a typical AFM as fitted to certain Range Rover's.

The AFM will also have an internal compensation chamber that will stabilise the movement of the flap and avoid erratic movement from induction pulses. The
co-mixture content adjustment will be via an internal air by-pass or a potentiometer, depending on the version.


Air Flow Sensor - Bosch Common Rail Diesel Diagnostics.


The voltage output from the Air Flow Meter (AFM) should be linear to airflow. Voltage will rise as the engine is accelerated and air volume is increased producing an initial peak. This peak is due to the initial influx of air and drops momentarily before the voltage is seen to rise again.

This voltage will however depend on how hard the engine is accelerated, a lower voltage is not necessarily a fault within the AFM. On deceleration the voltage will drop sharply as the engine returns back to idle. The final voltage will drop gradually on an engine fitted with idle speed control valve as this will slowly return the engine back to base idle.

Technical Information

The Bosch common rail diesel system may be either turbocharged or normally aspirated, in both cases the airflow sensor can be located next to the air filter.

The airflow meter monitors the volume of air and supplies the ECM with the relevant data. The airflow meter uses a conventional method of monitoring the mass air by means of a 'hot film'. The incoming air will pass over the 'hot film' and a subsequent cooling effect will take place, thus altering the output voltage. The voltage seen at the Air Flow Meter's (AFM) output terminal will be directly proportional to air flow, with the voltage increasing as the airflow increases.

The component will have a 6 pin electrical connector (using only 5 terminals as number 4 is blank) all terminating at the ECM.