Tip & How-To about Mercedes-Benz E-Class
Air Mass Meter (AMM)
1.1) MAF - Mass Air Flow sensor
What is it? The mass air flow sensor (MAF) is an electrical device used to monitor the amount of incoming air into the engine.
Where is it located? The MAF is located on the ducting between the airfilter/cleaner and the throttle body. It consists of either a) a large alloy or b) plastic collar next to the aircleaner/filter assembly with an electrical connection to a small unit on top.
How does it work? Early versions were called vane airflow sensors and consisted of a door flap that opened according to the amount of air that was being drawn in. The degree of flap opening rotated a shaft on the end of which was a sweeping contact arm on a carbon track (this is very similar in concept to 'throttle position sensor' internals). The output voltage signal sent to the ECU corresponded to the amount of air entering.
In a more modern MAF (AMM) the device consists of a small coil of hot wire or metal film held in the air stream. This is kept warm by passing electric current through it. Air passing over it has a cooling effect; the amount by which the MAF is cooled is proportionate to 1) the mass of air and 2) the air's temperature.
The MAF works in conjunction with the Intake Air Temperature (IAT) sensor (indicating the temperature ofthe air causing the cooling effect) and is often located with it. The ECU receives an electrical signal from the MAF proportionate to the amount of cooling and a second signal from the IAT proportionateto the measured intake air temperature. Combining these two parameters the ECU is able to calculate the total mass of air entering the engine.
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The mass air flow or MAF sensor is placed in the intake air duct between the air filter and the engine intake manifold. The MAF sensor measures the amount of intake air flow. The MAF sensor converts the air flow measurement into a voltage or frequency signal, with the voltage or frequency changing proportionally to the amount of air flow. The air flow sensor signal is monitored by the engine computer (ECM). The engine computer (ECM) uses the mass air flow signal to know the engine load and to calculate the proper amount of fuel injected. If the signal from the mass air flow sensor is out of expected range, the ECM detects a fault and sets the code P0100. For example, the mass air flow sensor signal could be higher than expected when the engine is not running or lower than expected with the engine running. A car with the code P0100 may have some driveability issues, such as stalling, lack of power, surging, hesitation, etc. In some cars, the code P0100 can cause the fail-safe mode where the engine speed would be limited to 2,500 - 3,000 rpm.
If there are other trouble codes present, they might need to be looked at first. Checking the freeze frame may help, as it may contain some important information. The freeze frame is a snapshot of the engine parameters at the moment when the fault was detected. The freeze-frame may show whether the vehicle was running or stopped, whether the air-fuel ratio was lean or rich, was the engine was cold or warmed up at the time of the malfunction. Read more about the freeze frame.
Read more about mass air flow sensor.
What could cause the code P0100:
- faulty or contaminated mass air flow sensor
- open or short in the mass air flow sensor electrical circuit
- open or short in the sensor power or ground circuit
- other electrical problem with the MAF sensor wiring, (corroded wires, bent terminals, bad ground connection, burned fuse, etc.
- vacuum leaks
- restricted air flow at or before the air filter
- wrong air flow sensor installed
- problem with ECM
P0102 - Mass Air Flow Sensor Circuit Low
The mass air flow or MAF sensor measures the amount of air entering the engine. The engine computer (ECM) uses the mass air flow signal to calculate the amount of fuel injected (read more how the MAF sensor works below). The code P0102 is set when the signal from the mass air flow sensor is lower than expected. Often a car may exhibit some driveability symptoms, such as lack of power, surging, hesitation on acceleration, hard starting, pinging (detonation), or rough idle when started cold. Often the codes P0171 or P0174 could also be stored along with the code P0102, as the incorrect air flow signal can cause the engine to run lean. The codes P0171 and P0174 indicate a lean condition. Checking the freeze frame might help in the diagnostic process. The freeze frame is stored in the ECM along with the trouble code. The freeze frame contains some recorded parameters (e.g. vehicle speed, engine temperature, fuel trim) at the moment when the fault was detected. The freeze frame can also be accessed with a scan tool.
What could cause the code P0102
- faulty or contaminated mass airflow sensor
- debris blocking the air flow at the mass air flow sensor
- unmetered air leaking past the MAF sensor (vacuum leaks)
- PCV system vacuum leaks
- restricted or collapsed intake air duct
- dirty or restricted air filter
- restricted air screen before the air filter
- improper application of aftermarket components (e.g. cold intake, modified air filter, etc.)
- wrong air flow sensor installed
- electrical problem with the MAF sensor wiring, connector, power supply or ground.
- problem with ECM
- in some cases other issues with the fuel injection system like a clogged catalytic converter or a faulty manifold absolute pressure sensor (MAP) can also cause the code P0102.
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