BMW Airflow meters

Air Flow Meters are a common problem on most European cars at the moment andi BMW are no exception. Air Flow Meters measure the amount of air passing into the engine from which the computer can work out how much fuel to add to get the perfect mixture for that specific throttle opening, the computer does this by referring to a map in its memory.

The Air Flow Meter works by seeing how much current it takes to keep a wire at a specific temperature. The fault will often not be shown on diagnostic system as the meter will still be operating within the limits the computer believes are OK, but putting the correct amount of fuel in for idle when the throttle is fully open will result in a very weak mixture and thus a lack of power.

The BMW air flow meters will occasionally fail completely which will bring up a light on the dash and the symptoms described here are only the usual ones, as we have known the fault to become apparent in other ways.
The easiest way to check if the Air flow meter is faulty is to unplug the wiring loom start the car if the car runs ok then the fault is with the airflow meter.
Luckily Airflow meters are very easy to fit and normally only take 5 minutes to fit you do not need any special tools to fit them and with a very small purchase price they are no major problem when there go wrong.

 Beware however as Lambda (or Oxygen) sensors in the exhaust can also give very similar symptoms to Air Flow Meters. Lambda sensor problems click here

Different types Of Airflow meters

The butterfly

The Butterfly type airflow meter normally called the moving vane was the early type of airflow meter.

This type of air flow meter is probably the most popular version and has been used on systems such as Bosch L, LE, LE3 and Motronic, and Ford EEC IV. This is most probably the most reliable type. Several Japanese manufacturers have also based their Systems around this tried and tested unit. The air vane meter adopts the principle of the airflow flowing into the engine and passing 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) sometimes called the electronic control unit (ECU) which then gives the correct amount of fuel for the air recorded.

The voltage output from the internal track should be linear to flap movement and this can be measured on an oscilloscope. The waveform should show approximately 1.0 volt when the engine is at idle. This 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 and drops momentarily before the voltage is seen to rise again to a peak of approximately 4.0 to 4.5 volts. This voltage will however depend on how hard the engine is accelerated, so a lower voltage is not necessarily a fault within the AFM.

On deceleration, the voltage will drop sharply as the wiper arm, in contact with the carbon track, returns 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, as this will slowly return the engine back to base idle as an anti-stall characteristic.

A time base of approximately 2.5 seconds plus is used; this enables the operator to view the AFM's movement on one screen, from idle, through acceleration and back to idle again. The waveform should be clean with no 'drop-out' in the voltage, as this indicates a lack of electrical continuity.

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, when applicable, will be via an internal air by-pass or a potentiometer, depending on the version.

The AFM can have a varying number of electrical connections, from four to seven.

This particular 12 volt supplied air flow meter (AFM) was used on early electronic injection systems and has the same operational qualities as the later 5 volt versions. The voltage should be seen to rise as the air vane is moved with no breaks or loss of continuity. The xample shown clearly demonstrates that just as the vane moves a loss of contact is seen, with the same fault occurring again as the throttle

is released and the engine returns to idle.
An AFM with this particular output would produce 'flat-spots' or 'hesitations' when driven. As the carbon tile has sustained damage the only way to rectify this problem is to change the unit for a new one.

Removal of the plastic cover will invariably show the ite plastic of the tile clearly visible through the carbon track. However this may only become apparent when the track is cleaned with a contact cleaning solvent.

HOT WIRE/FILM

The hot wire 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 ECM the quantity of incoming air.

The AFM is once again 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 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.

Hot wire type or Hot film type airflow meters

New type airflow sensors or hot film type.

The operating principles for the ‘hot film’ version are almost the same but with a greater reliability factor, due to the absence of the heated wire that is now replaced with a solid-state component.

The voltage range is similar in that the final voltage is higher than the initial voltage, due to the engine revving slightly higher as a result of the idle speed control valve operating an anti-stall factor.

The hot wire 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 ECM the quantity of incoming air.