Function-test instructions, mass air flow sensor M57

Setpoint/actual-value test

This test enables you to check the mass air flow sensor for detuning of the characteristic curve. Such detuning can be caused by contamination or a faulty measuring element in the mass air flow sensor.

Preconditions for test

• EGR valve is OK.
  
• Engine is at normal operating temperature
  
• No electrical faults in the following systems may be stored in the DDE fault memory:
  
  • Charge-air-pressure control
    
  • Exhaust-gas recirculation
    
  • Mass air flow sensor
    
  • Charge-air pressure sensor
    
  • Intake air temperature sensor
    
• Intake-air duct and exhaust system are OK.
  
• All electrical loads are turned off
  

Test procedure

1. Charge-air pressure sensor is checked for indication of plausible values
   
2. Intake-air temperature sensor is checked for indication of plausible values
   
3. Exhaust-gas recirculation is automatically shut down by the DDE
   
4. Calculating and measuring the air mass in the following test aspects:
   
   • Idling
     
   • 3500 rpm
     
5. Evaluation and display of measured values
   

Calculation and evaluation of air masses

The test is based on a theoretical calculation of the air mass and a comparison with the corresponding signal from the air-mass sensor. For every operating point of the engine, the air mass that the engine needs in theory due to its displacement at a certain engine speed can be calculated. In addition to the engine speed, the following operating data is included in the calculation:

• Charge-air pressure
  
• Intake air temperature
  

In the test points, the calculated value for the air mass is compared with the measured value determined by the DDE control module from the air-mass-flow-sensor signal.

The comparison of the calculated and measured values now permits an evaluation of the air-mass-flow-sensor signal. If the read-out air mass deviates too markedly from the calculated air mass, the test module reports a fault. This fault is usually caused by a contaminated or defective air-mass sensor.

However, it must be ensured that the exhaust gas recirculation valve is tightly closed during the test. If the exhaust gas recirculation valve does not tightly, the test result is falsified.