The mass airflow sensor provides the ECM with continuous information regarding the amount of air, or 'air mass' entering the intake manifold. A 'hot wire' is heated to a temperature of 180°C (356°F) above the ambient temperature. As airflows through the sensor, the current through the wire increases to maintain the 180°C (356°F) temperature. The Engine Control Module (ECM) uses this change in current as a means to determine the air mass entering the engine. The current is sensed by reading the voltage drop across the measuring resistor in the sensor. Changes in intake air temperature are corrected by the compensating resistor. The hot wire is mounted in an inner sleeve for protection. Each end of the mass airflow sensor also has a protective screen attached to help prevent damage to the wire. To clean the heated wire of contamination, it is heated to 1,000°C (1,800°F) for approximately one second when the engine is switched off.Digifont
This unit converts air flow to a voltage signal. Air enters and moves a spring-loaded vane which moves a potentiometer. The potentiometer modulates a voltage sent from the ECU and the return signal represents the mass of air flowing to the intake manifold. Air temperature is also measured and reported to the ECU. The air/fuel mixture adjustment is also in this unit, which is an adjusting screw that allows a certain amount of air to bypass the vane and enter the engine unmeasured. More unmeasured air means a leaner mixture.Motronic
The fuel distributor is mounted on top of a mechanical air flow sensor. Under the air duct boot is a venturi with a plate in the narrowest part of the bore. As the throttle opens and air flow increases, the plate is pushed farther up in the bore. This lifts a piston that uncovers slots in the fuel distributor to allow more fuel to the upper chamber.
The vertical position of the plate is measured with a potentiometer. A voltage is supplied to the position sensor and the portion of the signal returned to the ECU indicates the height of the sensor plate in the bore.
Testing & Adjustment
1.9L Turbo Diesel
Checking EGR & MAF Sensor Function
- Connect a proper scan tool.
- Start the engine and let it idle.
- Press '0' and '1' buttons to select Address Word 01 'Engine Electronics.'
- Press '0' and '8' buttons to select Function 08 'Read measuring value block.'
- Press the 'Q' button to enter input.
- Press '0', '1' and '0' buttons to select 'Display group 10'
- Press the 'Q' button to enter input .
- Observe display in display zone '1'.
- If the indicated specification is not obtained, check the operation of the EGR valve and system.
- If the proper specification is obtained, accelerate vehicle in 3rd gear from about 1,500 rpm under wide-open throttle.
- Press the 'PRINT' button on the scan tool (VAG 1551) when the vehicle reaches approximately 3,000 rpm.
Specified value in display field 4: 100 %
- If the indicated specification is not obtained, repeat the test and again accelerate the vehicle under WOT.
- The specified value in the display field should be 1,800 mg/H minimum (intake air mass).
- If specification is still not obtained, replace the Mass Airflow sensor.
- Use the scan tool to display the Readiness code.
- If DTC memory has been erased, create Readiness code.
- If display field '1' shows constant 550 mg/H (firm replacement value), press the '0' and '6' buttons to select Function 06 -'End Transfer.'
- Press the 'Q' button to enter input.
- Switch the ignition to 'OFF'.
- Disconnect the MAF sensor harness connector.
- Switch the ignition to 'ON'.
- Measure the MAF sensor voltage supply between the connector terminals shown in the MAF terminals and voltage chart illustrations.
- If the proper voltage specifications are not obtained, switch 'OFF' the ignition.
- Connect a VAG 1598/22 Test Box to the ECM wiring harness.
- Check the wiring for open circuit between the Test Box and the harness connector, using a wiring diagram. Check between the following points:
- Wire resistance at each check point should be 1.5 ohms maximum.
- Check the wires for shorts to one another or to vehicle ground and battery (B+).
- At each check, the meter should indicate infinite ohms.
- If the wiring is OK, replace the Diesel Direct Injection system ECM J248.
- With the ignition OFF , unplug the sensor connector and measure the resistance across the end terminals of the sensor. These are for the air temperature sensor. At 60 °F (15øC), there should be about 3000 ohms. At 80 °F (27 °C), there should be about 1900 ohms.
- The resistance between terminals 3 and 4 should be 500-1000 ohms when the flap is in the rest position.
- Connect the ohmmeter between the center terminals 2 and 3 and move the vane inside the sensor. The resistance should change smoothly and linearly as the vane is moved. There is a strong return spring on the flap but is should move smoothly with no binding.
- Remove the rubber boot from the top of the air flow sensor and lift the plate with pliers or a magnet. DO NOT loosen the bolt. The plate should move up smoothly with some resistance and should drop when released. The plate rests on a spring which allows it to move down if the engine backfires.
- To test the position sensor, make sure the ignition switch is OFF and unplug the connector. Check the resistance between terminals 1 and 2; it should be more than 4000 ohms. Check the resistance between terminals 2 and 3; it should be less than 1000 ohms.
- With the ohmmeter connected to terminals 2 and 3, move the plate up and down. The resistance should change smoothly to more than 4000 ohms at full travel.
- Turn the ignition switch ON . Connect a voltmeter between terminals 1 and 3 of the wiring connector. There should be 4.5-5.5 volts from the ECU.